Introduction

If I could be a god there would be no flashy miracles, but there would be lots of universes. Enough universes to enable a scientist like me to conduct an ambitious experiment � an experiment that I, and I�m sure many others, dream of. I would like to identically re-create the conditions at the instant our universe came into being, run the clock forward, and examine how many of the resulting universes develop to become a home for you and me.

I would ensure that each starting point for each of my universes was identical. After 13.77 billion years, the age of our own universe, I would head to where our solar system should be in each of my experimental replicates. If our solar system was there, I would search out Earth before looking for you and me. Would we always be there, just as we are today? Or would we only be there in a few of the universes? Or in none at all? Would little green aliens with giant ears and no noses sometimes be in our place? On some occasions perhaps there would be no sign of our sun or the Earth at all, or maybe the Earth would be barren, moonless and devoid of plants and animals, circulating too far from the sun for life to flourish. In some universes life may be abundant, in others it might be rare, or even entirely absent. As far as we know, we are the only planet with life in our universe, although we have only explored a very tiny part of it.

My experiment would reveal whether the history of our universe was set at its birth or whether each replay produces a different outcome. If each play of the tape produced you and me, scientists would describe the universe as being deterministic. With hard work and a huge amount of computing power, we could perfectly predict the future of any universe from its birth until its end. Some physicists have argued that this is what would happen if the experiment I describe was run. They believe that physics will one day reveal that things we currently think are random, including the behaviour of tiny particles, will be shown to be deterministic. In contrast, if each outcome was different, as most biologists and many other scientists believe, then that means random events have influenced the development of our universe. A random difference in the history of each universe could result in very different outcomes. Perhaps if the asteroid that killed the dinosaurs had passed by Earth rather than hitting it, humans might never have evolved and an intelligent species of dinosaur might be writing a book like this instead of me.

Scientists describe universes where chance events make it impossible to accurately predict the future as being stochastic. If random events do play a role in the history of our universe and our existence, we will not be able to perfectly predict the future, but this does not mean there is no predictability. If we were to monitor 1,000 universes in my rerun experiment, perhaps intelligent life would arise 531 times but fail to evolve on 469 occasions. If you placed a wager on intelligent life evolving at the Betting Shop at the Beginning of Time, you would have just over a 50 per cent chance of winning that bet. With odds of a little less than two-to-one, if intelligent life did eventually evolve, you would win twice as much as you originally bet.

Sadly, I am not a god, and we do not have the know-how to run my thought experiment to assess whether the universe is deterministic or stochastic. Creating new universes on a whim, and studying their evolution, is beyond our technical expertise. Therefore, answering the question of whether you and I were inevitable at the birth of our universe, or if we are just incredibly lucky, requires another approach: science that we can do. This book aims to tell two stories in parallel. The story of the universe from its birth until you and me; and the story of what had to happen for you and me to exist. These stories are a 13.77-billion-year epic. They involve unimaginable violence, death and a lot of sex, and although they often read as a tragedy they are ultimately a story of success. The sleuths who have slaved to piece together the stories of why you and I are alive (and know it) are more imaginative and cunning than Miss Marple, Sherlock Holmes, Nancy Drew and Hercule Poirot combined. They count among their number Albert Einstein, Marie Curie, Isaac Newton, Rosalind Franklin and Charles Darwin, and thousands of others of whom you will likely have never heard. The story is a work in progress, and the plot is subject to edits as brilliant minds continue to solve more of the mystery of why I am me, and you are you. However, even the incomplete script is sufficiently awe-inspiring to be a story that everyone should know.

I find it humbling that scientists have been able to piece together so much of the story from the Big Bang to you and me. Perhaps even more humbling is earning my living as a scientist who conducts research to identify tiny new bits of this story, so that I can add this new understanding to the remarkable insights so many other scientists have made. I am so impressed by this story that I decided I wanted to try to tell it in a way that makes it accessible to everybody, and not just other scientists. I want to do this because I have met so many people who are unaware of the remarkable progress of science and, because of this unawareness, are distrusting of it. Science can be difficult and intimidating, the language used is often technical, and the concepts can be hard to grasp. Although I personally enjoy reading texts on the chemistry of marine hydrothermal vents, or models of the first microseconds of our universe�s existence, I frequently have to look up the meaning of words despite three decades of working as a professional biologist. In this book, I want to explain things in words that the reader doesn�t have to look up.

The Scientific Method

Thales of Miletus has a claim to be the world�s first scientist. He was also the first of the seven sages of ancient Greece, men famed for their achievements in philosophy, law and politics who were revered for their wisdom and knowledge. Thales lived 2,600 years ago in the city that takes his name and, if everything written about him is true, he led a remarkable life and was quite a celebrity.

Piecing together the life of Thales of Miletus is not easy, as many of the achievements attributed to him were recorded decades after his death. Accounts of his accomplishments from the time of his life are scarce, and historians have failed to find any texts he wrote. But he was held in high regard by ancient Greek historians such as Herodotus and Eudemus, who list some of his most remarkable achievements as measuring the diameters of the sun and the moon, working out that the year is 365 days long, improving naval navigation by using the stars, predicting solar eclipses, setting the summer and winter solstices, diverting the flow of the River Halys so an army could cross it, developing numerous mathematical theorems, inventing economic futures trading and determining that the Earth is spherical. Evidence for some of these deeds is more compelling than for others, but one universal theme runs through these histories: Thales sought understanding via hypotheses he could test.

Scientists, from Thales of Miletus to the modern day, strive to discover facts about the world. They do this using something called the scientific method. It is not complicated, but it is incredibly powerful, and it is the way that all the facts described in this book have been identified.

Science is about explaining why a particular observation happens and then predicting future occurrences. The simplest observations are of things that always happen, such as jumping into the air and falling back to Earth. When we jump up from the ground, we never float off into outer space, and we have gravity to thank for this. Gravity always pulls us down, but other observations are more fickle, sometimes occurring and other times not. Ice, when left alone, will sometimes become liquid water, and other times will remain as ice. When the temperature is below freezing (when you�re at sea level on Earth, at least), ice remains ice, but when it is above freezing, ice will become liquid water. Science aims to make sense of observations, and to identify the circumstances when particular ones occur. Jumping into the air, and ice melting or not, are relatively simple observations that we can endlessly repeat.

Rarer events are harder to study. Observing that we exist and working out whether you and I were inevitable at the universe�s birth is a much harder observation to understand and predict because it has only happened once, Nonetheless, scientists have been chipping away at the problem. Events that happened long ago are also difficult to study. The further back we look in time, the less information is typically available. We know much more about the life of modern celebrities, and often way more than we might like, than we know about eminent people in ancient Greece. Yet we know more about the inhabitants of ancient Miletus than we do about those of �atalhoyuk, the world�s first city, which reached its zenith about 9,000 years ago. Knowledge gets even scarcer when we go even further back in time. The fossil record that contributes to our understanding of the history of life is sparse, and usually patchy. Very few ancient individuals of any species became fossils after death. To date, palaeontologists have unearthed thirty-two adult Tyrannosaurus rex fossils. T. rex were large animals with massive bones and teeth, the parts of animals that are most likely to become fossils. You might think that thirty-two sounds like quite a lot, and compared to some species it is, but this fearsome and massive dinosaur species roamed the Earth for 2.5 million years, and palaeontologists have estimated that a total of 2.5 billion T. rex individuals may have lived. If these rates of fossilization translated to humans alive today, the remains of only about 100 of us will survive as fossils 66 million years hence. That translates to 0.00000128 per cent of people becoming a fossil.

For an animal (or plant) to become a fossil, it must die under the right circumstances in the right environment. If you want to maximize the chance of becoming fossilized, your best bet is to be rapidly buried in sediment from a flash flood or ash from a volcanic eruption, and for your corpse to lie undisturbed for at least 10,000 years, yet this is still no guarantee. I would like to become a fossil � my children argue I already have � although I�d rather have a more peaceful end than being buried alive by flood or eruption. I�d like to become a fossil because it would help future scientists interpret the history we are creating today. If future palaeontologists were to discover my fossil, I may prove more useful to humanity after death than during my life.

The history of organisms alive in the past is partially written in impressions in rocks, but these etchings are rare and are often little help when working out part of the story of how you and I came to be. Despite this, palaeontologists studying fossils have uncovered remarkable insights about the history of life.

In this chapter I want to explore how science works. The scientific method consists of several techniques that allow us to find out things about the universe in which we live. Contrary to the belief of some populist politicians and social media gurus, repeatedly saying fictitious stuff over and over again is not how knowledge is created. It is a very outdated way of trying to make sense of the world. Prior to knowledge gained via the scientific method, people would explain observations of the world around them with stories. Many of these are great yarns, but they are often nothing more than myths.

Given how much knowledge the scientific endeavour has created, I am surprised and disappointed at the high-profile role that myths and myth-creation continue to play in today�s society. I have met conspiracy theorists who believe that the royal family is a race of alien lizards, that Elvis is still alive and in hiding, that Donald Trump won the 2020 US election, that the moon landing was faked, and that Keanu Reeves is immortal and may in fact be a vampire. Some of the more nonsensical theories may be laughable, but when people embark upon a path of rejecting facts for unsupported nonsense, they can do harm. Science has convincingly shown that neither homeopathy nor distance healing works, and that ginger cannot cure cancer. Yet many people cling to these misguided beliefs. Science and medicine cannot cure all diseases, but they have made astonishing progress in treating many. Turning one�s back on modern medical treatments because a mystic you have never met claims to be praying for you in Timbuktu risks shortening your life. Trust in science. It produces evidence to support or refute hypotheses and to explain the unexplained. Myths and conspiracies are not based on evidence. Our civilization is advanced because of science, not because illuminati are acting as master puppeteers, pulling strings from behind the scenes. The facts of why you came to exist are more inspiring than any myth or conspiracy theory can ever be, and they can also be verified.

The Beginning

I have always been an avid reader. My parents would have to tell me to stop reading and to go outside to play, so the opposite of any normal kid, and I would hide under my covers with a torch and a book after being told to go to bed. I can�t remember the first science book I read, but I would go to the mobile library that visited our village and borrow books on any aspect of science I could find. The best reads were often about physics, and particularly those that revealed secrets of the universe, although books on dinosaurs came a close second. I remember hoping scientists might even discover dinosaurs living on other planets.

Although I loved reading about physics at home, it was taught in a fabulously dull way at school. The curriculum required us to learn about the differences between potential and kinetic energy for what seemed like weeks on end, yet we were never taught about exciting things like supernovae. We would work out the angle a ball would bounce off a wall, but we never learned about particles such as quarks or photons. At home I would read about how time slowed down as objects neared the speed of light, but in the classroom the most exciting thing we did with light was to build a pinhole camera.

My teachers strove hard to make the lessons exciting, but they fought a losing battle, for whoever had designed the curriculum was on a mission to make an interesting subject boring. Physics is the science that explains why the universe behaves like it does. It describes how fundamental forces work, how energy and matter (anything that can be weighed and takes up space) are related, and why particles interact with one another in the way they do. It now seems remarkable that my classmates and I weren�t wowed with mind-bending knowledge. I was left nonplussed but undeterred. I continued to read about the exciting bits of physics at home, and soon came to realize that the inanimate world could be fascinating, and even kinetic and potential energy could be interesting. Nonetheless, for those readers who suffered the same physics curriculum as I did, that is the last you will hear of these types of energy in this book.

For you and me to exist, the universe had also to exist, but how did it come to be? We take it for granted that our universe has the right characteristics to harbour life, but was it inevitable that the universe would turn out this way at its birth? Could a universe have formed in which it was impossible for life to evolve? Scientists have found that if some of the characteristics of our universe were just a little bit different, then atoms, stars, planets, you and I could not exist. The first steps in our history will take us from a minuscule point of intense energy from which our universe grew, to fundamental particles, to the first atoms, and then to the first stars. There is still much to learn about the early days of our universe, but scientists have an astonishing understanding of the particles, energy and forces that determine how our universe behaves, and how these laid the foundations for life on Earth. Before we start this journey, I will introduce a few facts about our universe.

Today our universe is truly massive. No one knows quite how big, but scientists have estimated that it may well be over 7 trillion light years across. It would take a beam of light 7,000,000,000,000 years to travel that vast distance. One light year is just under 6 trillion miles � that�s how far light can travel in a year � which makes the universe 42 trillion trillion miles across. From Earth, we can see only a fraction of this, and scientists call this the observable universe. It is a mere 93 billion light years across, but it is getting bigger. Later in the chapter I explain why, along with why we can�t see all of the universe.

Distances that are measured in billions of miles don�t make much sense given our daily experience on Earth. One of the longest non-stop flights you can take is from London, England, to Perth, Australia. It takes 17 hours, and the plane travels 9,000 miles. That is 0.0000000015 of a light year. Light can make the journey in less than 1/20^th of a second. The Boeing 787-9 Dreamliner that makes the London�Perth journey has a top speed of 690 miles per hour, which is very sluggish compared to light. It is also slow compared to our fastest machines. NASA�s Parker Solar probe is the fastest object humans have produced. It achieved 0.05 per cent of the speed of light when it hit a speed of 330,000 miles per hour as it passed close to the sun, but that is still only 1/2,000^th of light speed. Light is very fast, and our universe truly vast.

To understand how our enormous universe behaves it is necessary to examine the behaviour of the tiny particles from which you, I and all matter are made, along with how these particles interact with one another. These particles, as best as we can measure them, have a diameter of about a thousand trillionth of a millimetre. Scientists refer to these particles as fundamental because they cannot break them down into smaller parts. These particles are the building blocks from which all things are made.

The behaviour of large objects like rocks, cars, aeroplanes, asteroids, the moon and other planets in our solar system is familiar to us. These are known as macroscopic objects because they are large compared to fundamental particles. Everything we experience in our daily lives, from other people, through books, to a glass of water, is macroscopic. Macroscopic objects move through time and space just like us. They do not exist as a blur, nor do they disappear from one location to mysteriously appear in another, as some fundamental particles, such as electrons in an atom, can do. In contrast, on the spatial scale at which fundamental particles interact, the microscopic scale, particles behave very differently. They are hard to pin down to a location, and this makes the behaviour of these particles appear deeply peculiar. For some of this chapter and the next, when I describe the behaviour of fundamental particles it helps to suspend disbelief a little. Reality as we know it is not reality at the scale of fundamental particles.

Up until the first half of the twentieth century, most scientists assumed the universe was constant in size, was unchanging, and had always existed and always would. But as telescopes increased in power, it became apparent that the universe was not constant but was expanding, and this suggested it must have had a beginning, and at that beginning our universe must have been very small. It has grown and changed a lot since its birth as the tiny dot of intense energy that it must have been.

Our universe started out tiny but is now vast. In every direction we look from Earth we see universe. We cannot see the universe�s edge, but there is still a lot to look at. We can visualize what we can see by considering a hypothetical space voyage. Imagine being an astronaut on a spacecraft that is 400 kilometres from the surface of our planet � that�s the orbit of the International Space Station. You look out of the window, and you can see our beautiful home world. Let us now start a journey away from Earth. As our planet recedes into the distance, the next object we encounter is the moon, which, like the space station, orbits the Earth but further away. We then start to encounter other planets in our solar system. We see a total of eight planets orbiting the sun � Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune � along with a dwarf planet called Pluto, and millions of smaller rocks. As we leave the solar system, the light from the sun dims the further we travel, until other stars shine brighter than our own. We will eventually start to pass some of these other stars, each with its own set of orbiting planets and rocks. In due course, the stars start to become less frequent, and we realize that our sun, now lost in a vast cloud of other stars, is just one of at least one 100 billion stars that are spinning around a central point. All these stars are in our galaxy, the Milky Way.

There are about a trillion galaxies in the part of the universe we can see. When we look at galaxies beyond the Milky Way through telescopes, we discover something interesting: most of them are moving away from us, and the further away a galaxy is, the faster it tends to be moving. If we were to reverse time, something that cosmologists have done in computer simulations of the universe, we discover that all the matter and energy that is now spread across the vast tracts of the cosmos must once have been compressed into a single point called a singularity. The singularity would have been smaller than the smallest of fundamental particles and would have been extremely dense and hot, and from this point emerged the universe in which sentient life evolved at least once, here on Earth. Physicists have shown that key to the way our universe behaves, including why it is expanding, are four fundamental forces that are central to our story. These forces determine how particles, atoms and larger objects including planets, stars and galaxies interact with one another. Because these forces are so fundamental to our existence, before considering how they work it is helpful to introduce them. They are the stars of the book, as without them you and I would not be here, and nor would the atoms from which we are made.

All objects in the universe are made up of atoms. Each atom consists of a central blob called a nucleus and electrons that whizz around the nucleus at various distances from it. Atomic nuclei are made from smaller particles called protons and neutrons. Each atom of an element has the same number of protons and electrons, and, except for one type of hydrogen that has no neutrons, each atom also contains one or more neutrons, with the number depending upon the element. Hydrogen atoms always have one proton and one electron. However, in nature, hydrogen nuclei can have either zero, one or two neutrons. These different types of hydrogen are called protium, which has no neutrons, deuterium, which has one, and tritium, which has two. These three types of hydrogen atom are known as isotopes of hydrogen. Neutrons and protons are built from quarks. The quarks are held together by a force called the strong nuclear force. This force also holds together the protons and neutrons to make atomic nuclei. The strong force prevents neutrons and protons from drifting apart, while also preventing quarks from separating from one another. If the strong nuclear force did not exist, neither would atomic nuclei, nor life.

The next force to introduce is called the weak nuclear force. It also operates within atomic nuclei, and it is the reason that isotopes of some elements are radioactive. Most atomic nuclei are stable and one of their neutrons doesn�t suddenly morph into a proton, or vice versa, creating an atom of a different element. However, some isotopes of some elements have an unstable configuration of neutrons and protons, and this can cause a proton to become a neutron, or a neutron to become a proton. When this happens, a type of radiation is emitted. Not all radioactive elements decay like this, but ones that do, like tritium (the isotope of hydrogen with two neutrons), which decays into helium, do so because of the weak nuclear force. You might think that the weak nuclear force has little to do with daily life, but without it the nuclear fusion that powers our sun could not happen. Nuclear fusion is caused by the weak nuclear force, and in the sun it is responsible for creating helium from hydrogen isotopes, and this generates a lot of energy in the form of heat and light. Without the weak nuclear force, our universe would be dark. No weak force, no life.

Protons and neutrons are often depicted in diagrams as tiny balls that stick together to form atomic nuclei, and that is a reasonable analogy, even if it is not quite accurate. A blur, or fuzz, of electrons whizzes around the nuclei in a type of orbit that chemists call an orbital. The electromagnetic force keeps the electrons in place. If electromagnetism did not exist, electrons would drift away from atomic nuclei and atoms could not exist.

Electromagnetism does more than this. Atoms come in different types, determined by the number of protons, neutrons and electrons. The number of protons and electrons in an atom is equal, and this number determines the element. For example, lithium has three protons and three electrons, while carbon has six of each. When a chemical reaction occurs, atoms share or exchange electrons with one another thanks to the electromagnetic force. When they do this, they form molecules. Water is an example of a common compound made of molecules. Each water molecule consists of two atoms of hydrogen and one atom of oxygen bound together by the way they share electrons via the electromagnetic force. Carbon dioxide, methane and rust are examples of other molecules. Molecules occur in an extraordinarily diverse array of forms. but each is made of atoms, and each atom is made of protons, neutrons and electrons. No electromagnetic force and no atoms or molecules.

The final force to introduce is gravity. We rarely think about the fact that we can walk around on Earth without floating off into space. Gravity is responsible for anchoring us to our planet. It is a force that attracts objects towards one another, and it means that things with mass, including atoms, have a tendency to clump together. Gravity impacts large objects as well as small ones. It keeps the moon orbiting the Earth, the Earth orbiting the sun, and the sun orbiting the centre of our galaxy. You will know that it takes one year for the Earth to go around the sun. It takes 27 days for the moon to complete an orbit of Earth, and about 250 million years for our sun and solar system to journey around the centre of the Milky Way. We are on a rock orbiting a star that is orbiting the centre of our galaxy. Gravity is the force that keeps this dance going and, without gravity, life would not exist, as there would be no sun, no Earth and no moon.

These four forces are all central to life in that they determine how our universe behaves. Understanding the forces is consequently an important step in the narrative of why we exist. We are made out of particles that have combined to make atoms and molecules that in turn have combined to form you and me. Our next step is to investigate how the four fundamental forces interact with, and impact, matter.

Particles can combine and interact in multiple different ways. The rules of how they interact are constant across the universe, but the outcome of the interactions depends upon the way that matter is spread out across space. Part of the reason for this is that some forces operate across very short distances, while others operate over much larger ones. For example, the strong nuclear force operates at the smallest scale, followed by the weak nuclear force, with electromagnetism and gravity working up to very large scales.

The way that the strong and weak nuclear forces and electromagnetism work is very similar, with quarks and electrons continually exchanging things called force carrier particles with one another. The force carrier particles for the strong and weak nuclear forces are respectively referred to as gluons and bosons, while the force carrier particle for the electromagnetic force is called the photon. Scientists suspect gravity works via a force carrier particle too, but they have been unable to detect it. They call the hypothetical particle the graviton. Detecting gravitons would require more energy than is available to us on Earth. Some estimates suggest that we would need to build a machine like the LHC that was the size of our solar system, that we would need to position it next to a very dense star, and that even then we would observe only one graviton every decade or so. If they exist, gravitons are elusive. Fortunately, force carrier particles that determine the other forces are easier to detect, and they have all been observed. Each force carrier particle is associated with a force field, with each force having its own field.

These force fields permeate the universe in every direction and at every point, even in deep space between galaxies. The easiest way to imagine these fields is by considering the force field around a magnet. We can see this magnetic field by sprinkling iron filings on top of a sheet of paper placed above a magnet and gently tapping the paper. The iron filings trace arcs from one pole of the magnet to the other. What you see when you do this is a trace of the electromagnetic force field. The other force fields are not as straightforward to observe, but they are everywhere all the time.

Each force field consists of energy, and there are continuous fluctuations in this energy that result in the force carrier particles appearing and disappearing. They do not exist for long, only a fraction of an instant of a second, before disappearing again. Force carrier particles are sometimes referred to as virtual particles, although they are, in fact, very real. These virtual particles pop in and out of existence more frequently close to matter than further away, but despite this, even in parts of the universe a long way from any matter, energy is morphing into these particles and back again.

Although these virtual particles are small and cannot be broken into smaller components, there may be things happening on an even smaller scale. Some physicists argue for a theory, called string theory, that hypothesizes that there are tiny, vibrating, string-like structures that determine all of matter. The frequency at which each string vibrates determines the type of particle we observe. At the scale of strings, space is thought to look very different to the way we perceive it. We think of space as being smooth, and not being divided up into discrete chunks. As we walk or drive, we do not jump from one location to a neighbouring one, but rather we move through space smoothly. On the extremely tiny scale where vibrating, string-like structures are thought to exist, scientists believe that space loses this continuity and becomes divided up into minuscule volumes, a little like an infinite stack of miniature square boxes. Physicists refer to this scale as the Planck scale, and the Planck length (the side of one of these boxes) is the smallest length that exists. It cannot be subdivided. The idea of half a Planck length makes no sense. At the Planck scale, tiny strings might be vibrating, and this would make space look like bubbling foam if we could observe it at such a minuscule scale. These conclusions are reached through the analysis of mathematical models because we do not have the technology to see space at the Planck scale.

The Planck scale is not the only aspect of the universe that remains elusive to physicists. Our universe consists of matter (including something called antimatter; more on that later), in the form of fundamental particles; energy, in the form of heat and light; something called dark energy, which must exist but that scientists are yet to fully understand; and virtual particles that are continually being produced and destroyed that allow other particles to interact. Each of these building blocks of the universe must be linked, but quite how dark energy links in is currently unclear. Nonetheless, physicists know how matter is related to non-dark energy in the form of heat and light via the most famous equation in science: Einstein�s E = mc². The E stands for energy, the m for mass and the c for the speed of light in a vacuum, where a vacuum is space containing no matter. The equation links energy to matter, showing how they are related. The amount of energy tied up in matter is very large, because the speed of light in a vacuum (c) is a big number � 299,792,458 metres per second. The square of this number, 299,792,458 � 299,792,458, is an even bigger number. What this means is that if matter can be coerced into releasing this energy, as happens inside stars and nuclear bombs, a lot can be produced. Conversely, when matter is created from energy, it locks up a lot of energy.

At its birth, 13.77 billion years ago, the universe was a point of hot energy before some of this energy transformed into quarks and electrons that did not then instantly morph back into energy as virtual particles do. Instead, they survived to allow us to exist. They stuck as matter, and scientists are not entirely sure why. What they do know is that these fundamental particles soon started to interact with one another, and when the universe had cooled to a temperature of 1 billion degrees Celsius, the quarks started to join to form the protons and neutrons found in the nuclei of atoms.

Neutrons and protons are collectively known as nucleons. Each nucleon consists of three quarks, with the quarks that form nuclei coming in two different types called up and down. Protons consist of two up quarks and one down quark, while neutrons consist of two down quarks and one up quark. These quarks are bound together, or constrained, into a small space by virtual particles called gluons, with each pair of quarks continually exchanging gluons with one another. As quarks are moved apart, the strength of the containment increases as more virtual particles appear and disappear between the quarks, pulling them back towards one another. Each gluon that is exchanged exists for only a tiny fraction of a second as it moves from one quark to the other, but this movement strongly binds the quarks together.

Each quark has several characteristics that scientists use to describe them. Some of these characteristics are colour, mass, spin and electric charge. An up quark has a 2/3 positive electric charge, while a down quark has a 1/3 negative electric charge. When they combine to form protons, the two 2/3 positive charges and the 1/3 negative charge add up to give a positive charge of 1. In a neutron, the 2/3 positive charge and two 1/3 negative charges combine to give an electric charge of 0. Colour charge is conceptually similar to electric charge, but comes in three types, red, green and blue, rather than the positives and negatives of electric charge. These colours should not be taken literally, they are simply a tool that physicists use to distinguish the three types of colour charge. When quarks interact by exchanging gluons, they also change their colour charge.

The strong nuclear force not only binds quarks together to form nucleons, it also binds protons and neutrons together to form the nuclei of atoms. Two particles with the same electric charge repel one another via the electromagnetic force, much as magnetic poles of the same type push against one another. Because all protons have a positive charge, the electromagnetic force acts to push them away from one another. The strong nuclear force is sufficiently powerful to prevent the positively charged protons being pushed apart by the electromagnetic force, and it does this by binding protons and neutrons together. It does this binding not with gluons but with another virtual particle called a meson. For reasons I won�t elaborate on, mesons are not defined as force carrier particles although they do play a key role in the way the strong nuclear force operates. Nonetheless, like gluons, mesons operate on only a very small scale that is slightly larger than a nucleon. Their binding ability is impressive. The binding strength of mesons is sufficient to keep multiple positively charged protons together within the nuclei of atoms, bound up with neutrally charged neutrons.

The weak nuclear force is weaker than the strong nuclear force, and much like the strong nuclear force it operates on a small scale. As with the other forces, there is a weak nuclear force field that pervades the universe, and the force is carried by force carrier particles called W and Z bosons. There are in fact two types of W boson, but we will not consider the details here.

When the weak nuclear force turns protons into neutrons and vice versa a particle called a beta particle is emitted. Beta particles are very high-energy particles and this makes them radioactive. The elements from which we are made, including hydrogen, oxygen, carbon, nitrogen and calcium, do not decay into other elements. They are stable, which is probably just as well, because if they did decay, we would quickly fall apart.

The first atomic nuclei to form in the universe were those of hydrogen and helium. These nuclei persisted in a hot plasma, along with unbound electrons, particles that have an electric charge of −1, until the universe had cooled sufficiently for the first atoms to form. At this point electromagnetism began to play a major role in the development of matter.

Electromagnetism is a hundred times weaker than the strong nuclear force, but it operates over much greater distances than either of the nuclear forces. Along with gravity, it is the force that we experience most in our daily lives. It is the force that makes compounds solid, liquid or gas, and it is the force that allows atoms to bind together in the form of molecules. Much as there are force fields throughout the universe for the strong and weak nuclear force, with gluons and W and Z bosons being the particles that carry the forces, there is also an electromagnetic force field. The particles that carry the electromagnetic force, and which allow protons and electrons to interact with one another, are called photons. These are also the particles from which light is made. Quarks exchange gluons when they interact, while electrons and protons exchange virtual photons when they interact. Virtual photons are popping in and out of existence as protons in atomic nuclei and electrons in orbitals around these nuclei interact.

The weakest force of all is gravity. It is 10 raised to the 38^th power weaker than the strong nuclear force. Despite being so weak, when objects get large the pull of gravity is strong. The Earth is large enough to stop us floating off into space. Unlike the strong and weak nuclear forces, but in a similar manner to electromagnetism, gravity can operate over very large distances. Gravity and electromagnetism impact our daily lives more than the other two forces because we are very large compared to the scale at which the nuclear forces operate.

Up quarks, down quarks and electrons are not the only fundamental particles. My discussion so far has focused on only one of three groups of fundamental particles. Physicists refer to these groups as generations. I described the first generation, and that is the one all the matter we observe today is made from. I also omitted to mention a particle called the neutrino when describing the first generation. Neutrinos are extremely abundant, but they rarely interact with atomic nuclei or electrons. Perhaps somewhat obviously, you can only detect something if it interacts with something else, and not all particles are affected by all four forces. Neutrinos only interact via the weak force and gravity; they are unaffected by the strong force and electromagnetism. Every second, millions of neutrinos emitted from the sun pass through the Earth without interacting with any of the matter from which it is made. Nonetheless, a very tiny proportion of the neutrinos do interact with atomic nuclei. I did not discuss neutrinos above because they are not part of us, but they do need mentioning now to help appreciate how remarkable is the understanding of the universe that physicists and cosmologists have developed.

The first generation of particles contains four particles: up and down quarks, electrons and neutrinos (or, more accurately, electron neutrinos). Heavier versions of these particles make up the second generation, while even heavier counterparts constitute the third. As an example, the second generation of matter is made up of charm and strange quarks, the muon (rather than the electron) and the muon neutrino. �Charm� and �strange� are names rather than meaning that one of these particles is alluring and the other peculiar. The strange quark is heavier than the down quark, its sister particle in the first generation. Particles in the second and third generations of matter are not stable, and when they are formed at high energies they quickly decay to create the particles in the first generation.

Table of the Fundamental Particles

The four particles in each of the three generations combine to make a total of twelve fundamental particles that are the building blocks of matter. I have also described the four force carrier particles, the gluon, photon and W and Z bosons, which allow the other twelve particles to interact. Ignoring gravitons, which are hypothesized to exist but not observed, we are up to sixteen particles. There is one more particle to introduce, and it is called the Higgs boson, taking the total number of particles to seventeen. Without the Higgs boson, no particles would have mass, and stars, planets and you and I would not exist. The story of the discovery of the Higgs boson reveals just how well physicists understand the dynamics of the three generations of matter and the force carrier particles.

The Standard Model is a theory developed by physicists to describe how the seventeen particles listed above interact and behave. The full equation of the Standard Model is mathematically daunting. It cannot be described as a particularly elegant-looking equation, but it is an amazing achievement and provides astonishingly accurate predictions on how particles interact via electromagnetism and the strong and weak nuclear forces. Although complicated, the Standard Model can be broken down into four parts. The first part describes the four forces, while the second captures how the four forces act on each of the fundamental particles in the three generations of matter. The third part explains that the Higgs boson determines the mass of each of the fundamental particles, while the fourth part describes how the Higgs boson goes about doing this. The problem was, up until quite recently, no one had ever detected a Higgs boson. The Standard Model depended upon its existence, but what if it didn�t exist? Physicists would have to go back to the drawing board and derive a new model as their understanding of the universe would be flawed.

Detecting the Higgs boson was never going to be easy. It would require creating conditions similar to those of the early universe, and that requires a huge amount of energy. The boson would need to be detected very rapidly after it was coaxed into appearing before it decayed into other particles. Physicists built the world�s largest machine to search for the Higgs boson, a machine I have been fortunate enough to visit.

My eldest daughter has always been an avid reader and, like me, she went through a spell when she devoured books on physics. Like father, like daughter. The curriculum she was taught had thankfully been significantly reworked since I was at school, and she liked physics so much she went on to study it at university, and then to work for a physics start-up company in Oxford, the town where my wife Sonya and I live. As Sophie�s eighteenth birthday drew near, I asked her what she wanted as a present, and she said she�d like to visit the LHC, the machine that physicists built to detect the Higgs boson.

One of the great joys of working at a university is you get to meet experts in all sorts of subjects, and I happened to know a physicist who splits his time between Oxford and CERN. I asked him when was a good time to visit, and he told me to come when it was shut down for maintenance. I looked confused, and he said that if we came then, he could arrange for us to go underground and see the heart of the machine. Sophie and I duly headed to Geneva, where my friend took us to visit part of the machine called the ATLAS detector. As a field biologist who spends a fair amount of my time in wild parts of the planet, I was used to colleagues in other subjects being a little envious of where I work. I very rarely wish I had chosen a different career path, but I did suffer physics envy when we visited the LHC. It is one of science�s, and humanity�s, greatest achievements.

At the LHC, beams of protons, and occasionally other particles, are accelerated to very close to the speed of light before being collided with one another at one of four locations on the 27-kilometre underground particle speedway that is the core of the machine. At each of these locations there is a huge bit of complicated equipment specifically designed to detect the particles that are created from high-speed proton�proton collisions. The particles produced by these collisions do not last for very long, before they decay into other particles giving off energy, or interact with protons, neutrons or electrons. The ATLAS detector is at one of the eight locations on the LHC, and it was one of two detectors that observed the Higgs boson in 2012. The ATLAS detector measures 25 metres by 45 metres and is housed in an underground cathedral-sized cavern. The detector is designed to measure the masses, trajectories and lifetimes of the particles created when proton�proton collisions occur.

The LHC and its four detectors constitute not only the world�s largest machine but also its most technologically advanced. Particles are accelerated and steered around the loop using super-cooled high-powered magnets before being crashed into one another with remarkable precision. The LHC has significantly advanced our understanding of particle physics, but the discovery of the Higgs boson has been its greatest success.

The Standard Model is almost as remarkable as the LHC. It is a mathematical formulation of how quarks, electrons and neutrinos (and higher generations of particles) interact with one another via electromagnetism and the strong and weak nuclear forces. It accurately describes what happens in the physical world around us, and it predicted the existence of a particle that had not been observed. When physicists looked for it, it was there, just as the Standard Model predicted. Thousands of physicists and engineers collaborated to design and build the world�s biggest machine to detect the particle. It cost �3.8 billion to build, and costs about �1 billion each year to run, but in my opinion the cost was well worth every penny. We now have a good understanding of how the universe works. Except that not all physicists are happy. There is something missing, and this led to some scientists arguing that we need a complete rethink of the Standard Model. Any new version must perform as well as the existing model, but it will be bigger and better.

The reason the Standard Model is incomplete is it does not include gravity. It explains how the seventeen particles it describes behave in the absence of gravity. One of the fundamental forces is missing from the model. A huge challenge in physics today is linking our understanding of gravity with the Standard Model.

Physicists have a theory of gravity that is remarkable, and as impressive in its predictions as the Standard Model. The theory is called the theory of general relativity, and it was developed early in the twentieth century by Albert Einstein. The theory of general relativity is an extension of an earlier theory that Einstein also developed called special relativity, and together the two theories describe how gravity, energy, space, time and mass are linked, and how the speed of light is central to the association between energy and mass. Einstein built on fundamental work on gravity that was begun by Isaac Newton, another colossus of science.

The universe has a maximum speed limit, which is the speed of light in a vacuum. The speed limit, as I explained above, is 299,792,498 metres per second, and this is the speed at which photons travel through empty space. If you were to approach the speed of light, some strange things would happen. First, time would slow down, and second your mass would get greater and greater. It is impossible for something with mass to reach the speed of light as its mass would become infinite, but if you were able to, time would stop. Photons can travel at the speed of light because they have no mass. Each photon is a packet of energy zipping through space.

We must exercise our imagination and forget day-to-day common sense when considering Einstein�s theories of relativity. As an example, if we measure how long it takes light to travel to Earth from the sun, we will record a time of eight minutes and twenty seconds. That is our perception of time. From the perspective of the photons that constitute the beam of light travelling through the vacuum of space, each photon is simultaneously at the sun and on Earth. From the perspective of the photon, there is no time, because time has stopped. Einstein named his theory appropriately, because reality is relative to the speed at which you are travelling.

Because speed and time are linked, regardless of how fast you are travelling, if you were to switch on a torch, you would always measure light moving away from you at 299,792,498 metres per second. Regardless of whether you are standing still, or travelling at just 1 kilometre per hour slower than the speed of light, if you turned on a torch and measured how quickly the beam of light accelerated away from you, you would always record the same number: 299,792,498 metres per second. It doesn�t matter how fast you are going; you would always see the same thing, light racing away from you extremely quickly. This is because one hour lasts for much longer when you are travelling close to the speed of light than when you are stationary. The speed of light is a constant regardless of how quickly you are moving.

Such behaviour is very different from our experience on Earth. If you are travelling in a car at 95 mph and another vehicle overtakes you at 100 mph, it does not appear to be travelling much faster than you. In contrast, if you are travelling at 100 mph, a car passing you at 200 mph moves away from you at a much faster speed. At these low speeds, we do not notice time running at different rates as a function of how fast we are travelling, but even at these velocities time does run at ever so slightly different speeds.

The link between speed and time has some unusual consequences. If one twin was to head off on a space journey at 99.99 per cent of the speed of light aged twenty years old, returning to Earth after what would seem to her to be one year later, she would be twenty-one years old but her twin sister would be ninety. On Earth, we think of time as a constant, always ticking away at the same rate, but this is not true when we start to move faster. Time is relative.

As I teenager I was fascinated by the thought of travelling at light speed, even though I knew it could never happen. I wondered what the universe would look like out of my spaceship�s window as it hit light speed. Because time had stopped for me, but not objects travelling at less than the speed of light, would I be able to see all the future locations of all the objects in the universe? Would Earth�s orbits of the sun appear like a smear across space? In imagining this, I had made an error of logic, but I learned an important lesson. My mistake was forgetting that space and time are linked. As speed increases towards the speed of light, and time slows, space contracts, and this means the universe would appear to get smaller and smaller until, at light speed, it would appear as a very bright but infinitesimally small dot. The lesson I learned is that science proceeds by trial and error. What might be a good idea may prove to be wrong. Science is about explaining patterns in data by ruling out hypotheses that turn out to be incorrect. In forgetting that space contracts as speed increases, I had arrived at a flawed hypothesis but learned something important: that science proceeds one disproved hypothesis at a time.

Einstein got it all squared away. He showed not only that space and time are changed by speed but also that they are impacted by gravity. His general theory of relativity includes space, time, speed and gravity. The stronger gravity is, the slower time flows. It is even possible to measure this effect on Earth, with time running faster at the top of a mountain than at sea level. The effect we see on Earth is very small but is exactly what Einstein�s equations predict.

Gravity bends space as well as slowing time. As a beam of light travels through space, if it passes by an object with a large mass, its trajectory bends. A widely used analogy to describe the effect of gravity on space, but in two dimensions rather than three, is to imagine a trampoline with nothing on it. Let us assume that the surface of the trampoline is perfectly flat, such that if you were to roll a marble across it, it would continue in a straight line until it fell off the other side. Now repeat the experiment but having placed a weight in the centre of the trampoline. The weight is sufficient to cause the trampoline to sag in the middle. If your marble is travelling fast enough, its trajectory across the trampoline will now be curved. The gravity of an object curves space much in the same way the weight has changed the surface of the trampoline from a flat plane to being depressed in the middle. And if the heavy object is very heavy, your marble�s trajectory will be so sharply curved it�ll go into orbit around the heavy object.

The greater the mass of an object, the stronger gravity becomes. The sun, for example, has a much stronger gravitational pull than the Earth because it has over 333,000 times more mass. The sun bends space to a greater degree than the Earth, and time runs slower the closer to the sun you get. But even the sun�s gravitational pull pales in comparison to that of black holes. Around each black hole is something called an event horizon. It is the point at which gravity becomes so strong that even light cannot escape its attraction. The gravity in black holes folds space in on itself and stops time for any object that crosses the event horizon.

Einstein�s theory of relativity has repeatedly been tested and has always been found to be remarkably accurate. It describes how (and why) the moon orbits the Earth and the Earth orbits the sun, and why we do not float off into space; it has been used to age the universe and to reveal why it continues to expand. It also reveals that there must exist another type of matter. Without this matter, galaxies would not exist.

When scientists discovered galaxies spinning at such a speed that solar systems towards the edge should not be held in their orbits by the gravitational pull of the rest of the galaxy, they had a problem. The speed at which the galaxies spun meant the solar systems towards the galaxy�s edge should be jettisoned into space. Cosmologists resolved this contradiction by devising a hypothesis that particles exist that do not interact with other particles via the strong and weak nuclear forces and electromagnetism. They called them �dark matter�. According to the theory, they only interact via gravity. This makes them extraordinarily hard to detect and, so far, scientists have not been able to observe dark matter particles, even with the LHC. Nonetheless, dark matter particles must be very common given Einstein�s theory appears to be correct.

Dark matter is not the only stuff that scientists hypothesize must exist if Einstein�s theory is true. The universe is expanding quickly, and its rate of expansion is increasing. Physicists have calculated that something other than dark and observable matter must exist to explain this expansion. On very large scales, something is pushing galaxies apart. Scientists have termed this mysterious stuff dark energy briefly mentioned earlier in this chapter. Dark matter and dark energy must be extraordinarily abundant given the observations of the universe we have made. Estimates suggest that 5 per cent of the universe is made up of energy and matter we can see, 27 per cent is dark matter and a whopping 68 per cent is dark energy.

Although scientists know little about dark matter particles or where dark energy comes from, there are many measurements that suggest they must exist. The fact that particles and energy exist that we know so little about is not entirely bonkers. Before the discovery of the Higgs boson, you might have argued that the Standard Model must be wrong because a particle fundamental to it had not been measured. The fact we have not yet been able to directly measure dark energy and see dark matter does not mean it does not exist. In addition, as you will have gathered, not all particles in the Standard Model are influenced by all four of the fundamental forces. In fact, it is only the quarks that feel the strong and weak nuclear forces, electromagnetism and gravity. Electrons are not impacted by the strong nuclear force, and neutrinos are so hard to study because they only interact with other particles via the weak nuclear force and gravity. If dark matter, as is hypothesized, does not interact with the electromagnetic or nuclear forces, the particles that constitute it will be hard to directly detect, requiring levels of energy well beyond those generated by the LHC.

Let us briefly recap where we are at before returning to our story of the history of the early universe. Scientists have one theory, the Standard Model, that is astonishingly good at describing how three of the fundamental forces interact. They have a separate theory that explains how the fourth force, gravity, works. But they have a problem in that merging the two theories into a theory of everything is mathematically extraordinarily challenging. The Standard Model breaks down when gravity distorts space and time, and it cannot be extended to incorporate gravity using the mathematical tricks that were fundamental to its development.

Despite this, there are theories, including string theory I mentioned earlier, M-theory and quantum loop gravity, that are proposed as ways to unify gravity with the other three fundamental forces. Currently we have no way of testing the predictions each theory makes, and this means a widely accepted grand theory of everything does not exist. A general theory of everything would help us further probe the history of the early universe, but even without such a theory, there is quite a bit that scientists have been able to discover about the youthful universe. They have also been able to run computer simulations where they change the strength of the four forces and see whether atoms, and stars, would form. Not all values of the four forces would result in a universe where life could evolve, so we should be grateful they take the strengths they do. We do not know why the forces have these strengths, but I�m glad they do. If they didn�t, you wouldn�t be reading this book.

Insights from the Standard Model, the theory of general relativity, collisions between tiny particles in the world�s most complicated machine, computer simulations, and observations of galaxies across the heavens have allowed scientists to piece together the history of the early universe. With these insights we can step from energy to quarks and electrons to atoms of a whole host of elements, the first necessary parts of the universal history of you and me. What happened is what I summarize next. The history of the universe as it developed from a singularity to the stars we see when we gaze at the night sky.

Before our universe�s birth, scientists speculate there was a nothingness. When I say nothingness, you might imagine the emptiness of space, perhaps like the space between the Earth and the moon, or between the edge of our solar system and Alpha Centauri, our closest star system. But this space is not empty. It contains energy and force fields and on the Planck scale might look like a bubbling foam. Scientists assume that before the universe formed, there was nothing. Nothingness means no energy and no force fields. Somehow from this nothingness our universe materialized, and scientists do not know how. Some researchers have put forward theories, but currently there is no way of collecting data to test them. For instance, some mathematicians have argued that the nothingness from which our universe was born was unstable, and it collapsed to form the singularity that was our very early universe, but in truth we have no way of knowing whether this hypothesis is true.

The singularity that appeared 13.77 billion years ago and from which our universe grew may have been over a billion billion degrees Celsius and was very, very dense, with all the mass and energy of today�s universe contained in a single point that was smaller than the smallest particle seen in the universe today. The four fundamental forces of physics that now permeate our universe did not exist at its birth, and the force (or forces) at play in the first instant of our universe may have been simpler than those that govern our lives. In a fraction of a fraction of a second after the singularity appeared, there was a brief period of what scientists refer to as inflation that lasted less than a billionth of a second, during which time the universe expanded astonishingly quickly. During this period the universe grew by 1 followed by 78 zeros times. Even after this period of rapid expansion the universe would have only been somewhere between the size of a grain of sand and a basketball. In the remainder of the first fraction of a second of the universe�s life, the four fundamental forces emerged.

As the young universe expanded it began to cool, and as it did so the fundamental forces we see today began to reveal themselves. First to appear was gravity, followed by the strong nuclear force. Cosmologists do not agree whether the strong nuclear force separated from the electroweak force � the combined weak and electromagnetic forces � immediately before or after inflation, but its emergence filled the universe with a hot soup of quarks and gluons. Very shortly after the emergence of the strong force, the electroweak epoch ended, when the two remaining forces � electromagnetism and the weak nuclear force � appeared and electrons were formed. All this happened extremely quickly, and by the time the universe was 0.000001 seconds old, it had cooled sufficiently that neutrons and protons were able to form.

The hot soup of protons and electrons was opaque, and energy, in the form of photons, could not travel through it as easily as they traverse the universe today. The early universe by was consequently dark. Cosmologists study the universe by examining different types of electromagnetic radiation: gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves and radio waves. These different types of electromagnetic radiation travel across the universe as photons that are not only particles but are also waves (I explain this in the next chapter). Electromagnetic radiation lies on a scale defined by the distance between the peak of consecutive waves. Radio waves have the longest wavelength, and gamma rays the shortest. None of them could travel through the early universe, as the photons that carry energy would continually interact with the quark�electron soup.

By the time the universe was 380,000 years old, it had cooled sufficiently that the protons, neutrons and electrons could combine and form atoms of hydrogen and helium. At this point, the universe became transparent, and electromagnetic radiation could travel through it. The oldest radiation we can detect on Earth today has been travelling through the universe since it was 380,000 years old, a journey taking over 13.7 billion years. It is called the cosmic microwave radiation.

The cosmic microwave radiation is everywhere we look, and it has very similar properties whether we look towards the centre of our galaxy, or away from it. The model that ran time backwards from now towards the singularity at the birth of our universe predicted that such radiation should exist. Discovering it helped prove that the universe started with a singularity and a big bang.

Since the 1960s, when the cosmic microwave radiation was discovered, many research projects have mapped it, with a very high-resolution all-sky map being published in 2013. The map revealed the radiation was not completely identical in every direction but instead exhibited very small variations. There are points where it is slightly warmer, and others where it is a bit cooler. Because this radiation permeated the universe from its birth, this patchy temperature variation reflects slight differences in the distribution of energy and matter in the very early universe. Scientists believe that this variation early in the universe�s life is what resulted in the formation of galaxies and the large-scale structure of the universe we see today.

The detailed map of the cosmic microwave background also provided compelling support for the period of inflation, with the small fluctuations being caused by tiny differences in the distribution of matter during the universe�s very early history. The map also allowed cosmologists to calculate the age of the universe to a precision of 13.772 billion years old, plus or minus 59 million years.

Because the universe is so staggeringly vast, light that arrives on Earth may have been travelling through space for millions, or even billions, of years. When we look at an object in the night sky, we do not see what is happening to it now, but rather we see what it was doing when the radiation that we see left the object. Light departing the moon takes 1.3 seconds to reach Earth. Light from Alpha Centauri takes a little over four years and three months to get to Earth. If something catastrophic were to happen to the Alpha Centauri system, we would not know about it for over fifty-two months. When we gaze into space, we are consequently looking back in time, a bit like a geologist digging down through layers of time in rocks. The geologist is looking at a still picture of the past, while the astronomer sees a movie. If alien astronomers orbiting a star 70 million light years away are at this moment pointing their immensely powerful telescopes at Earth: they are eyewitnesses to tyrannosaurs charging across the Cretaceous landscape.

The further away an object is, the longer it has taken the electromagnetic radiation to reach us. By looking at objects that are different distances away, scientists can consequently piece together the history of the universe. The observable universe has grown from the small local neighbourhood we were in when the universe became transparent. The cosmic microwave background is the first light from the bit of the universe our current neighbourhood was in when the universe became transparent. All the galaxies we can see spread out over billions of light years are the matter that was packed into a small locality when the universe was 380,000 years old.

One way to visualize this is to imagine putting a dot of ink on an uninflated balloon. The surface of an uninflated balloon is a bit like the whole universe, but in two dimensions. When the universe became transparent and light could start to travel through it, our neighbourhood was like the ink dot on the uninflated balloon. Now imagine blowing the balloon up. As the surface of the balloon stretches, the ink dot becomes bigger. Inflating the balloon mimics the universe expanding. The stretched ink dot is like what we can see now, the observable universe. There is a lot of the balloon we can�t see, and that is equivalent to the unobservable universe. The cosmic microwave background can be thought of as the light that was in the ink dot on the uninflated balloon. The galaxies represent all the matter that existed in the ink-dot region of the uninflated balloon. Cosmologists know the universe is still expanding and the rate of expansion is accelerating. This is like the balloon being blown up at an ever-increasing rate. The �universe as a balloon� analogy is useful, but not perfect. The surface of an expanding balloon is curved, while scientists have shown that the universe is not curved but flat. The balloon analogy has other limitations too. Over-inflated balloons pop. Let�s hope the universe doesn�t eventually do the same.

By the time the universe became transparent it had expanded quite a lot from the singularity it started out as and was about 1.4 million light years across. The reason we cannot see the edge of the universe is that the observable universe we see today was not on the edge of the universe when it became transparent. When the universe was 380,000 years old the bit that became the universe we observe today was surrounded by matter and energy in all directions. We have no idea whether we are closer to the edge or the centre of the universe, and perhaps we will never know.

Once the universe had become transparent and hydrogen and helium atoms were common, gravity then began to work its magic. The first atoms and molecules were attracted together until they formed the first stars. These stars first burned when the universe was about 100 million years old, and they were many times larger than the sun, but their lifespans would have been shorter.

Stars burn bright because the force of gravity pulls atoms of hydrogen and helium closer together, heating them up and making them energetic enough that they start to collide. These collisions can break the bonds between nuclei and electrons, eventually forcing the nuclei together, where they can form the nuclei of heavier elements such as nitrogen and oxygen.

Brighter stars use up all their hydrogen and helium fuel more quickly than those that are smaller, but all stars above a certain size turn their fuel into nuclei of heavier elements such as nitrogen, oxygen, carbon and iron. As these stars near their end, collapsing in on themselves, gravity becomes so strong it pushes numerous protons and neutrons together to form the nuclei of the even heavier elements such as uranium and plutonium. It is fascinating how atomic nuclei of heavier atoms can be formed from hydrogen via the weak nuclear force and gravity.

In the centre of a star, hydrogen nuclei are turned into helium nuclei and something called a helium core forms. The nuclear fusion in a star is triggered because gravity pushes atoms closer together, making them highly energetic. The fusion generates a lot of energy in the form of heat and light that exerts pressure, preventing the outer layers of the star collapsing in on themselves. For most of a star�s life, the effects of gravity pushing atoms towards one another, and the pressure created by nuclear fusion emitting energy, are approximately in balance. Nonetheless, towards the end of a star�s life things become unstable. Exactly what happens at this point depends upon the mass of the star. In the case of the sun, it will continue to get steadily hotter and larger over the next 5 to 6 billion years until a time when it will go through cycles of expansion and contraction. At its largest size it will extend to the orbit of Jupiter, before contracting again to be smaller than it is today. These pulsing contractions and expansions will lead to the helium core becoming heavily compressed and being raised to a temperature of 100 million degrees Celsius. The helium is converted into carbon and oxygen and other heavy elements, generating huge amounts of energy that cause the sun to expand to a vast size once again. Eventually these cycles of expansion and compression cease, with the heavier elements created in the sun�s death and any unburned hydrogen forming a cloud of dust. The Earth will be swallowed and burned up during one of the sun�s dying expansions and our planet will cease to be.

When stars that are greater than about eight times the mass of our sun run out of fuel, gravity causes them to collapse until they form supernovae � the most violent explosions in the universe. The star that goes supernova explodes and as it does so it produces elements heavier than iron. The elements, dust and debris from a supernova permeate space and can form nebulae from which the next generation of stars is made. Heavy elements are quite abundant on Earth and elsewhere in our solar system, providing evidence that our sun cannot be a first-generation star. In fact, scientists know that the universe was already over nine and a quarter billion years old when our solar system formed. The Earth may be formed from the debris of a second-, third- or fourth-generation star � we do not know. What we do know is that the heavier elements made from the death of earlier generations of stars were necessary to produce our home planet and the emergence of life.

At this point in our story we have gone from a singularity to our neighbourhood in the universe where some of the hydrogen and helium formed earlier in the universe�s life has been converted, via a process known as nucleogenesis that happens in the heart of stars, to heavier elements. Our next step on the journey is to investigate how these heavier elements combine to form molecules like water, carbon dioxide and methane � groups of atoms of different elements that have combined via the electromagnetic force.

In this chapter we have covered how the following things had to happen for you, and me, to exist. 13.77 billion years ago a very hot, very dense, singularity consisting of large amounts of energy had to form. The universe began to expand and started to cool, with its expansion and cooling continuing to this day. As the universe started to age, but still within the first blink of an eye, the four fundamental forces emerged, and energy was converted to matter in the form of the fundamental particles. The link between energy and matter is critical to our universe�s functioning, and to you and me. Quarks soon started to form protons and neutrons and then the nuclei of the lightest elements. As the universe continued to cool further, the nuclei combined with electrons to form the first atoms. These were eventually pulled together to form the first stars, creating local pockets of intense heat that created heavier elements, which eventually, in our neck of the cosmic woods, combined to create our sun and solar system. I wish I�d been taught this at school, as every child should know what remarkable knowledge physicists have of how and why atoms of the periodic table came to be. Chemistry is the next step in our history, and in particular why chemical reactions happen. You and I are made of complex chemistry, so understanding how and why atoms interact as they do is crucial to figuring out why we are here.

Molecules

There were only a few bits of chemistry I liked at school, and those were the bits that resulted in explosions. Such violent reactions were few and far between, mainly because my teachers were all good chemists who made sure to give us chemicals that would not blow up when mixed. I did have a chemistry set at home, but it was hard to generate much chaos with that either. One of the reasons, as I eventually learned, is that the chemicals that are most explosive need to be kept in unreactive liquids, and for this reason were not included in chemistry sets and were kept locked up at school. Metals like sodium are kept in oil because as soon as they encounter air, they start to react with the oxygen in it. However, even the reactions of the most volatile chemicals are tame compared with the death of stars described in the last chapter, or the consequences of mixing matter and antimatter. Most of this chapter will be about reactions due to the electromagnetic force, and how electrons behave, organize themselves around atomic nuclei and form various types of atomic bonds. But before we start to focus on electrons, there is one other type of interaction to briefly consider as it too is central to our existence.

Shortly after our universe�s birth, when quarks, electrons and neutrinos came into existence, so too did particles of antimatter. These are called anti-quarks and positrons, and the anti-quarks can combine to form antiprotons and even antineutrons. Some of the most impressive explosions in the universe would have occurred when matter and antimatter collided, as they destroy one another and generate vast amounts of energy. For example, if you collide a negatively charged electron with its antimatter equivalent, a positively charged positron, they emit two highly energetic photons. Antimatter is rare in our universe because shortly after an antimatter particle is created it collides with a matter particle and they annihilate one another, creating energy. One unsolved mystery of science is why matter won out over antimatter early in our universe. Theory suggests that matter and antimatter should have been produced in equal amounts, and they should have instantly completely annihilated each other, leading to a very premature end to our universe. The theory must be lacking in some way because there was ever so slightly more matter than antimatter, and that led to our universe existing as it does today. If antimatter and matter had been produced in equal amounts, we would not exist.

Antimatter was not only formed at the birth of the universe. It is produced, sometimes in the most unexpected of places. The bananas in your fruit bowl create a steady stream of positrons, with each banana producing one positron every hour and fifteen minutes or so. Bananas contain an element called potassium, including a rare isotope of the metal named potassium-40, which very, very slowly decays via the weak nuclear force to calcium-40. If you had a gram of potassium-40, just under half of it would decay to calcium-40 in a billion years. We cannot predict when any particular atom of potassium-40 will decay via the weak nuclear force to calcium-40, but we know that on average it takes 1,290,000,000 years. Because this is an average, some potassium-40 atoms will decay much more quickly, while others may last for several billion years before decaying. Given that one atom of potassium-40 decays in each banana approximately every seventy-five minutes, this suggests that there must be a lot of potassium-40 atoms in each banana. There are. Just one gram of potassium-40 contains 15 followed by 21 zeros atoms. An average banana contains only about 0.015 g of potassium-40, but that is still a very large number of atoms. When a potassium-40 atom decays it releases a beta particle � in this case a highly energetic positron � which is annihilated when it encounters an electron. Our eyesight is not good enough to see the annihilation, but it does occur, and it releases energy. If we were to scale things up by annihilating 1 gram of matter with 1 gram of antimatter, it would produce the energy equivalent of the atomic bomb dropped on Hiroshima. We are fortunate that antimatter is not produced too rapidly in our fruit bowls. If it was, making a banana smoothie could be lethal. Antimatter plays no further role in the story of why you and I exist, but be thankful it is no longer very common in our universe. If it was, you and I would not be here.

Chemical reactions do not involve the annihilation of matter and antimatter. Instead of being driven by the weak nuclear force, they are determined by electromagnetism. Despite this, some chemical reactions can still be astonishingly violent. 1-diazidocarbamoyl-5-azidotetrazole, or azidoazide azide as it is sometimes called, is one of the most volatile molecules known to humanity. It is close to impossible to store, as a small change in pressure, exposure to light or even a tiny shift in temperature can cause it to explode. It is so unstable that standard instruments used to measure how volatile a chemical is cannot be used to measure it, as attempts to use them make it explode. At the other end of the spectrum, there are molecules that are very difficult to engage in chemical reactions. Helium, for example, is very stable and tends not to get involved in reactions with other substances. It consists of two protons, two neutrons and two electrons. This does not mean that helium is not useful; it has many uses. It is lighter than air such that helium balloons float, and it is also amusing to take a breath of helium and experience your voice jump up in register. What is it that makes some chemicals more reactive than others?

In the previous chapter I introduced the four fundamental forces but focused most on the nuclear forces. I described how the life cycle of stars fused hydrogen nuclei into helium nuclei, and helium nuclei into those of heavier elements. These nuclei then joined with electrons to create atoms of the heavier elements. The next key part of our history is the advent of molecules caused by elements joining together to form a huge array of chemicals. They do this via the electromagnetic force, and it is time for this force to take centre stage. The force drives all chemical reactions, from simple interactions between small molecules to the way that some drugs stop some complex molecules from doing their jobs inside your cells.

Although I didn�t particularly enjoy chemistry at school, it did play an important role in my early life. While I was growing up, my parents were drug dealers. They ran a chain of chemist shops in Cambridgeshire, dispensing prescriptions and selling over-the-counter treatments for minor ailments, illnesses and injuries. I did briefly toy with the idea of going into the family business, but I�d never enjoyed helping out in the shops as a teenager. It wouldn�t have been the right job for me. Given my parents were pharmacists, they were probably a little horrified by my chemistry reports from school. Nonetheless, as with my extracurricular physics reading, I spent time at home reading around the subject, but not in quite the same way as I did with physics. I failed to develop a good grasp of electromagnetism, but I did learn how molecules could treat and prevent disease. My parents had a large book listing endless ailments and diseases and the drugs that could be used to treat them. It was fascinating reading, and remarkable to learn how many different ways there were to become sick, and ways in which drugs can relieve symptoms. The problem was that every time I got a temperature, spot or rash, I would self-diagnose, always fearing the worst. Being pharmacists, my parents were very responsible with drugs and, apart from the occasional aspirin, my requests for drugs to treat scurvy, Guinea worm, elephantiasis or the bubonic plague fell on deaf ears.

At about the age of ten or eleven I read about a disease that put the fear of God in me because it had no cure and a terrible end: rabies. Each year we would head to France on holidays, and for a couple of these trips I hardly slept. I was convinced every cat, dog, cow or sheep I saw was rabid and out to get me. I imagine that my intense anxiety meant Mum, Dad and my sister, Fiona, didn�t enjoy the trips much either.

I did at one point think I might have caught rabies. During the time I spent in Zimbabwe in my late teens, I did get bitten by a dog, and for a brief spell my fear of rabies returned. I went to the local clinic and learned that I could get vaccinated, but it would require seven unpleasant injections into the stomach that could only be administered in the capital city, Harare, which was not an easy journey from where I was based. The dog bit me because it had been asleep and I had woken it up by stumbling, a little worse for wear, out of a bar and trodden on it. I also discovered there had been no cases of rabies in the area for months, if not years. I decided the risk of death-by-rabies was low and I wouldn�t get the jabs. Nonetheless, I did spend the next few weeks trying to assess whether I was turning mad or starting to develop a raging thirst. I did read up about rabies, and when I learned of a reliable vaccine being available I made sure I was vaccinated ahead of work trips that might result in my coming into contact with wild animals.

My fixation with disease, vaccines and prescription drugs helped me learn a little about just how remarkable the behaviour of some molecules can be, and I continue to be impressed with the breakthroughs chemists make. For example, the mRNA vaccines, such as the Pfizer jab that was developed during the COVID pandemic, are based on a remarkable understanding of the chemistry of our immune systems. Artificial chemicals pervade our lives, from curing sickness and preserving food to killing pests and dyeing our clothes, and they all operate via the electromagnetic force. It has been estimated that as many as 10 million new chemical compounds are synthesized by chemists globally each year, with 2,000 of these being approved annually for commercial use in the US alone. Elements can combine in a vast array of ways to form an infinite number of substances. Some of them are beneficial to us, others harmful. The chemical weapon carbonyl chloride which was used in both world wars is one of the most poisonous synthetically made chemicals. As little as 200 parts per million in the air can kill a person. That�s bad, but not as bad as the botulinum toxin produced naturally by the bacterium Clostridium botulinum. Just 1 gram has the potential to kill as many as a million people. Some people have tiny amounts of it injected into their bodies, as it is a key ingredient of Botox.

Chemistry works because atoms of different elements join together to form molecules, and different molecules can combine in a huge variety of ways. You and I consist of vast numbers of molecules, some simple such as water, which is a small molecule, to other more complex ones such as DNA, which is thousands of times larger. If elements could not join to form molecules the universe would be a much less interesting place, and we could not exist. The next bit of our history involves investigating why elements join together to form molecules, and why some of these molecules, such as DNA, are very stable, while others, such as azidoazide azide, are less so. The periodic table helps us understand how and why elements differ.

Chemists organize the elements in the periodic table. The table orders them starting with hydrogen, the lightest element, made of atoms containing a single proton and electron, at the top left and working through the elements in increasing order, row by row, all the way to oganesson, whose atoms have 118 protons and 118 electrons. Rows and columns of the table organize elements by the way their electrons are arranged, something I describe in detail below. The last and heaviest element in the table, oganesson, does not occur naturally as it is highly unstable and quickly decays, but chemists have created five, and possibly six, isotopes of the element in the lab. It is one of twenty-four human-produced elements. I find it amazing that chemists can create elements in the laboratory that are not found in nature. Is Earth the only planet where these elements have been created, or are there aliens elsewhere in the cosmos who have also made oganesson?

The periodic table separates elements that are metals, such as lithium and sodium to the left, from non-metals like chlorine and oxygen to the right. Each column groups elements by their properties, such as how they interact with other elements. Metals on the table want to donate electrons when they react with other atoms, while non-metal elements to the right want to receive them. In contrast, those in the middle of the table are more likely to share electrons. Chemists now have a good understanding of why elements behave as they do, as well as the number of protons, neutrons and electrons different atoms of different elements have, but it took a long time to achieve these insights. Exactly what constituted an element was unclear for thousands of years. Answering the question of what things are made from is hard.

People have been interested in elements that make up nature since the time of ancient Greece, and probably even earlier. Nearly 2,500 years ago Empedocles claimed that there were four fundamental elements: earth, fire, air and water. Half a century later Democritus of ancient Greece and Kamala of India independently argued that matter could be broken down into indivisible parts, but it was unclear what these parts were. The Greeks referred to them as atoms but realized they were too small to be seen.

Throughout the Middle Ages, the focus on chemistry turned from identifying elements to making useful substances, and in particular to the possibility of turning cheap and abundant metals like iron into rarer and more expensive elements such as gold. Observation had shown that new compounds could be made by combining specific ingredients, and a primary goal of alchemy was to turn one metal into another. Alchemy remained a respectable science through Isaac Newton�s lifetime and was one of his interests. These days we think of alchemy as quackery, but it was of major interest to one of humanity�s greatest thinkers, one of the two people who gave us calculus, a key dialect of the mathematical language of science.

By the eighteenth century, scientists and philosophers were beginning to turn their backs on alchemy, and the era of modern chemistry began. Throughout the 1700s, various elements and compounds, both gases and metals, were isolated and named. Hydrogen was isolated in 1766 and named �inflammable air� by the British scientist Henry Cavendish. The Swedish chemist Carl Wilhelm Scheele discovered oxygen in 1773, with it being independently discovered a year later in England.

The first chemistry text describing elements was published by the French chemist Antoine Lavoisier in 1789 and it listed hydrogen, nitrogen, oxygen, phosphorus, sulphur, zinc and mercury as substances that could be broken down no further. Other chemicals that we now know are not elements were also listed, but despite these errors Lavoisier�s book was an important summary of chemical knowledge and theory towards the end of the eighteenth century. In the early nineteenth century new elements were discovered at a faster rate, with the English chemist Humphry Davy discovering sodium, potassium, calcium, boron, magnesium, strontium and barium primarily through using a technique known as electrolysis, which uses electricity to break down a compound into its constituent elements.

By the late 1860s, nearly a century after the discovery of hydrogen, sixty-six elements were known, and had their weights estimated. Mendeleev, a Russian chemist, arranged them into the first version of the periodic table in 1869. He revised his table in 1871, and in so doing revised the weights of some elements and predicted the existence of three elements today known as scandium, gallium and germanium. The discovery of gallium and scandium in the late 1870s, and of germanium in 1886, led to Mendeleev�s periodic table becoming widely accepted. Chemistry had become a predictive science, and Mendeleev�s legacy as one of the greatest chemists of all time was guaranteed.

At the same time, Mendeleev was developing his periodic table, the first theories of how elements combined to make compounds were proposed, and work began in trying to understand the way atoms joined with one another via chemical bonds. The electron was discovered by J. J. Thomson in 1897, with key breakthroughs in radioactivity made by the Polish chemist Marie Curie and the New Zealand physicist Ernest Rutherford. Quantum mechanics, of which more soon, was formulated in the first half of the twentieth century, and by its second half, chemistry had become a mature science. Humanity had a good understanding of how and why chemical reactions worked, and they could even predict many of them.

The development of chemistry is a fabulous example of the application of the scientific method, but it also reveals that progress can take generations. Early observations were that some substances could react, for example with iron and silver seen to oxidize when exposed to water and air, and others could be broken down into constituent parts. The hypothesis that many things could be broken down into elements became refined, and experimentalists were able to identify many of the different types of elements. Mendeleev realized that many elements behaved in similar manners, and he was able to discover a way of grouping them together. This grouping suggested some underlying principles of the ways elements interacted, and chemists and physicists developed hypotheses of how these might work. Some of these hypotheses, such as the existence of phlogiston, an element found in all compounds that could burn, or the plum-pudding structure of the atom, where electrons were embedded in a soft, spongy, cake-like nucleus similar to a Christmas pudding, were proved wrong. But other hypotheses were supported, and ever more elegant, and technologically challenging, experiments were designed and conducted.

As with physics, mathematical equations were developed to capture chemistry and the workings of atoms and molecules. The electromagnetic force became well understood, and this allowed it to eventually be linked to the strong and weak nuclear forces in the Standard Model. Physics is arguably the most advanced of the sciences, with chemistry following a close second. There are still open questions in both subjects, and new breakthroughs and applications will continue, but major breakthroughs will likely happen at a slower pace than in the sciences that focus on life. We will discover new uses for molecules, and new molecules to solve humanity�s problems, and we will continue to unearth new nuances in the way that electromagnetism works, and hopefully link it to gravity as part of a theory of everything. But that�s for the future. Let�s now return to what we currently know, describing why atoms interact to form molecules that are essential for life.

Protons and neutrons, regardless of which element they form, are all made up of up and down quarks, so the nuclei of atoms of every element are made up of the same fundamental particles. Protons always have a positive electric charge, while the electrons that make the outer parts of atoms always have a negative electric charge. The difference in charge between protons and electrons means they are attracted towards one another. Despite each element being made up of the same type of positive and negatively charged particles, each element is different. It is difficult to get atoms of some elements, such as argon, to combine with other atoms via the electromagnetic force. In contrast, atoms of other elements, such as oxygen, are highly reactive, and will interact with atoms of other elements to form all sorts of compounds.

Not only do atoms of different elements behave differently when they encounter other atoms, but the physical properties of the elements also differ. Hydrogen and oxygen are gases at room temperature, mercury is a liquid, while lithium and iron are solid. When hydrogen and oxygen react to create water, the two gases combine to form a substance that is liquid. Unless, that is, the temperature is below zero degrees Celsius, in which case solid ice is formed, or above 100 degrees Celsius, when steam forms. These temperatures change with pressure, which is why at higher altitudes, where the air pressure is lower, steam is formed at temperatures lower than 100 degrees Celsius.

You are a complex form of chemistry, and to understand why you came into existence it is necessary to understand some chemistry. Millions of chemical reactions occur within each of your cells every second, and these reactions require atoms of elements that are reactive. If all elements were inert, like helium and neon, life could never have got started. The next step in our journey from the Big Bang to you is from atoms to molecules. If you found any of the concepts in the previous chapter strange, such that the universe has a maximum speed limit, or that gravity can slow time, or that the universe would look like a bubbling foam if it could be observed at the Planck scale, things continue to be peculiar at the quantum scale at which atoms share or exchange electrons.

In the world we experience, things do not suddenly disappear from one place and reappear in another. My dog, Woofler, does not suddenly vanish from the bedroom before instantaneously appearing in the garden, however much he might like to do that. Similarly, objects can only be in one place at a time. I cannot simultaneously be in the office and down the pub, however hard I try. The world of the very small inhabited by fundamental particles and atoms is rather different.

All electromagnetic radiation, such as visible light, travels in waves. Yet such radiation is made up of particles called photons. The world of the very small is strange because photons, and indeed other particles such as electrons, individual atoms and even small molecules, are simultaneously both particles and waves. Being two things at once doesn�t make much sense given our experience of always only being one thing: a body. However, believe it or not, you, the trees in the countryside, the cars on the road, the planets, and indeed the entire universe, are also both solid objects and waves. The waves these big objects form are billions of times smaller than the objects themselves and they do not influence our everyday lives, so we can ignore them. Things get peculiar when an object�s wave is bigger than the object itself, and this is what happens to tiny objects like photons, electrons and atoms. Objects with waves that are larger than they are exhibit what scientists refer to as quantum behaviour. One way to demonstrate this, a phenomenon also called wave-particle duality, is with a neat experiment.

Scientists have conducted the two-slit experiment numerous times, and they always get the same result: atoms can appear to simultaneously be in two places at once. To do the experiment you first need a device to fire individual atoms towards a screen that can record where they collide with it. The device and the screen are in fixed positions. Next, take a sheet of material with two, close together, parallel vertical slits cut into it and place it between the gun and the screen. The slits are wide enough for the atoms to pass through them. You now fire individual atoms at the sheet and record where they appear on the screen.

Let us start by envisaging what we would expect given the classical world in which we live. Imagine scaling this experiment up so you are firing tennis balls from a gun through slits in a sheet and on to a Velcro screen. Each ball will go through one slit or the other before sticking to the Velcro. You end up with two vertical lines of tennis balls on the Velcro screen that are determined by the positions of the gun and each of the slits.

In the quantum world, some of the atoms you fire appear to behave like scaled-down tennis balls. They will look as if they leave the end of the gun, go through one slit or the other in the sheet and will make a mark on the screen used to detect them. The mark, a slit and the gun will all lie on the same line. It turns out it is just luck that a few atoms behave like this, and most atoms will not behave like the tennis balls. Instead, other vertical lines of collisions between atoms and the screen will appear. A set of many parallel lines, separated by areas where no atoms are recorded, appears on the screen. Which slit did the atom go through when the screen records a mark that is not on a straight line with the gun or either of the slits? The answer is both: each atom goes through both of the slits at the same time. That is a truly mind-bending conclusion given our existence in the classical world where a tennis ball can go through only one slit or the other.

Each atom, and indeed any object, has waves associated with it. Each wave can be described with a mathematical expression called a wave function. The wave function for any object�s location describes all possible places where the object could be, along with the probability that the particle is in that part of the wave. For large objects with very small waves, the object will always appear in the same place. For small objects with large waves, the object can have a large number of places where it could be. For an atom, the wave can be viewed as describing all possible futures if it were to be forced to reveal its whereabouts as a particle. Objects can appear to be in two places when the wave function is bigger than the particle. When I mention a fuzz of electrons around an atomic nucleus, I am describing the wave function of the electrons.

In the film Men in Black 3, Michael Stuhlbarg plays a character called Griffin, an alien from the planet Archanan. These aliens are described as multidimensional beings who can see an infinite number of alternate timelines and possible futures. They are unable to predict which specific future will occur, but as particular events are realized some possible futures disappear while new ones emerge. Griffin uses this ability to steer the men in black to the only future that saves the world, and while doing so he admits that the ability to see so many options at once is a bit of a burden. I imagine it would be. Griffin�s power is a bit like seeing wave functions for the world around him. The wave functions he sees change as time passes, and so too do the possible futures, with some becoming more likely and others less so. A particle�s wave is a little bit like that. However, things in the quantum realm are even more bewildering.

There are various things you can measure about an atom, or an electron, or other particles, including their location and how fast they are moving. Each of these properties has a wave function associated with it, and the various wave functions are linked. What this means is that the more you find out about where, say, a particle is likely to be, the less you know about its speed. If you know where a particle is, you have no information about its velocity; and if you know how fast it is travelling, you have no idea where it is. The inability to know both of these things at once is called Heisenberg�s uncertainty principle, after the German physicist who first described it. The quantum world is based on uncertainties that scientists can describe with probabilities. The quantum behaviour of tiny particles and atoms is probabilistic, and ultimately it all boils down to their wave functions being larger than they are.

The wave function that describes where the atom could appear in the two-slit experiment creates what is known as an interference pattern, and it is this that creates the multiple vertical lines on the screen separated by areas where no atoms hit. One way to envisage an interference pattern is to think about the behaviour of water. Imagine a wave of water flowing towards two slits. It hits the slits and emerges on the other side. Semicircular expanding waves emanate from each slit. Each of these waves will move away from the slit before the waves collide with waves formed from the other slit. The collision generates an interference pattern of peaks and troughs in the watery waves. You can see something akin to this when the wakes from two boats meet. The water becomes choppy, with highs and lows. The same phenomenon happens with the wave function of the atom fired from the gun. The wave function of the atom is choppy, with highs describing where the particle is most likely to be, and lows where it is unlikely to show itself. The vertical stripes that form on the screen in the two-slit experiment are these highs, while the gaps between them are the lows. The two-slit experiment is a very clever way of using lots of atoms to reveal what an individual atom�s wave function looks like.

As an atom is fired from the gun it forms a wave which passes through the slits before the particle is coerced to emerge from the wave at a particular location on the detector screen. The slits must be close together for the experiment to work, and the width of each slit must be close to the wavelength � a property of the wave function � of the particle for an interference pattern to form. When these conditions are met, the experiment is repeatable, and the effect is always observed. All this raises the question, what makes the particle emerge from its wave and appear in one location on the screen?

When the atom in the two-slit experiment hits the screen, its wave function is said to collapse, and it reveals itself at a particular location. It would be a bit like you simultaneously leaving a building via both the front and back doors before magically appearing in one place in the street outside. The wave function describes an object�s wavelength. The bigger an object is, the smaller its wavelength becomes and the object appears in one location.

As an aside, large objects can be made to behave like small particles if the wavelengths of all their constituent atoms can be aligned in a particular way, but doing this is very difficult for anything larger than a single molecule. An approximate analogy for aligned wave functions is a line of metronomes ticking in perfect synchrony. In most cases, the atoms in an object are not synchronized like this but are more like a line of metronomes each doing its own independent thing. That is what happens in you. Your wavelength is about a billionth of a billionth of a billionth of a centimetre because the wave functions of your atoms are not in sync. Because you are much, much bigger than this, you behave as a solid object and we can ignore the wave function. If you were able to line up all the wave functions of each atom in your body, you would behave like a wave. You could behave like a quantum particle, leaving a building by multiple exits and appearing in the street. You are unable to line up your atoms� wave functions, and this means the two-slit experiment won�t work for you, because you cannot fit through the extraordinarily narrow slits that would be required for the experiment to work. It would be like threading a camel through the eye of a very tiny needle. The two-slit experiment only works for objects that have wavelengths that are larger than they are, and these tend to be particles and atoms. For example, the wavelength of an electron is a couple of thousand times larger than the particle itself.

When an atom passes through a slit in the two-slit experiment and encounters the detector screen, it forms chemical bonds with the detector, becoming joined to the much larger object. The atom and screen have a wavelength, but because the wavelengths of the atoms in the screen are not aligned, the atom now appears in one place as part of the screen. If the atom were to be freed from the detector, it would regain its original quantum properties. As atoms join to form molecules, and molecules join to form large objects we can see, their wavelengths usually become unimportant for their behaviour, and they behave like objects we are familiar with in our everyday lives rather than like particles in the quantum world.

When I described the early universe, I discussed the cosmic microwave background, the oldest radiation in the cosmos, and how fluctuations exist in this radiation. Cosmologists think this variation dates to the very early universe, when it was so small its wave function was larger than it was. Highs and lows in this quantum wave function were amplified as the early universe rapidly expanded, resulting in variations in the distribution of matter. In time, these differences in the density of matter resulted in galaxies containing billions of stars and areas of star-free space between them. The large-scale structure of our vast universe is thought to have resulted from its quantum behaviour while it was still extraordinarily small.

The Two-Slit Experiment

The quantum behaviour of atoms is deeply peculiar, but I have not yet explained how it helps us understand the way in which atoms interact. Thinking back to my chemistry lessons at school, I was unable to find the answer to this question. We weren�t taught it. In fact, I realized only three things remain with me from these lessons. First, I recall a kid called Gary managing to fill the classroom with an acrid-smelling gas that made everyone cough and led to the whole science block being evacuated. Gary spent the rest of his school chemistry career attempting to repeat the feat, but failed. Second, I remember telling my friend Mike that if he attached the Bunsen burner to the water tap it would shoot a powerful jet of water that would probably go as far as the teacher about fifteen feet away. Not believing me, he attached the Bunsen to the tap, but he didn�t have the nerve to try to drench the teacher. Instead, he just turned the water tap on with the burner sitting on the bench. The water hit the ceiling and we got dripped on for the rest of the lesson. Third, we were taught that atoms are a bit like the solar system, with the nucleus at the centre, being orbited by particles called electrons that behaved like planets. There would have been better things for me to remember than the first two, and the third �fact� we were taught is largely wrong.

One aspect of the solar system analogy that is correct is that most of an atom is empty space, but that�s about as far as it goes. The simplest atom is that of hydrogen, consisting of one proton and one electron. If we were to expand the proton to be the size of the sun, the electron�s most likely closest position would be ten times further away than Pluto is from the sun. That�s a very long way. Mercury, the closest planet to the sun, would be closer to the nucleus than where we would expect to find any electron if the solar system were to be shrunk down to the size of an atom.

Talking in terms of probability by saying things like �the electron�s most likely closest position� sounds odd, particularly when we think of planets. It would be much easier to write that the electron is ten times further away than Pluto is from the sun, rather than that is its most likely position. Unfortunately, though, in the same way that we do not know where an atom is when it leaves the gun and before it hits the screen in the two-slit experiment, we do not know where the electron is with respect to its atomic nucleus. We just know where it is more or less likely to be, and this means we have to talk in terms of chance. Given this, we cannot describe an electron as being in orbit around an atom�s nucleus. Its wave function is smeared around the nucleus and the electron can appear anywhere within this wave function if coerced to show itself. To describe this complexity, chemists use the word �orbital� rather than �orbit�. Electrons form orbitals around their nuclei, and there is a probability that they are at any point in their orbital. If planets formed orbitals rather than orbits, they would appear as a mist smeared across the night sky. Some bits of the mist would be thicker than others, with the thick bits denoting where the planet would most likely be. Probability adds complexity, but we do all sometimes think in terms of probability in our everyday lives.

Mine is not a family that tracks each other on our mobile phones, so as I write this in my office I cannot tell you where my wife, Sonya, is, nor the whereabouts of my three children Sophie, Georgia and Luke. I can put odds on Sonya being in her office at work, out shopping or having a coffee with friends, but to spare my marriage I won�t list those chances. Luke will likely be at college, but if the weather is nice and he doesn�t have a lecture he could either be at the skatepark or out taking pictures for his photography project. Electrons live less exciting lives than members of my family, but chemists can assign them odds of being at any location around their nucleus much in the same way I can assign where my family members are likely to be.

A typical atom of hydrogen consists of one proton that forms the nucleus, and one electron. The wave function that describes where the electron could be can place it anywhere around the nucleus, but says it is much more likely to be between a range of intermediate distances, rather than being close in or further out. The electron�s energy level determines its wave function, its orbital around the nucleus, and consequently the distance the electron is likely to be from the proton. Electrons with more energy are found in more distant orbitals than those with less energy.

Image of a Hydrogen Atom

Electrons are attracted to the nucleus, and they like to be in the closest possible orbital to it. To do this, they need to have as little energy as possible. An atom of hydrogen consisting of a proton and a highly energetic electron in a distant orbital loses energy, and the electron disappears from the distant orbital and appears in a closer-in one. The energy the electron produces is carried away from the atom by a photon. As this happens, the electron vanishes from the outer orbital and appears in a more inner one.

Electrons have a property called spin, and this creates patterns in how multiple electrons arrange themselves in atoms of elements with more than one electron. Spin is all about the way an electron moves, but its definition is mathematically quite complicated. There are two types of spin � spin up and spin down � and a good analogy is to think of a spin up electron as spinning clockwise and a spin down electron as spinning anticlockwise. But like �charm� and �colour� in quantum theory, �spin� doesn�t have its ordinary English meaning. Electrons don�t really spin like a cricket ball.

Each orbital can contain a maximum of two electrons, but two electrons with the same spin cannot share an orbital. Orbitals are arranged around atoms in structures called shells. The shell closest to the nucleus contains one orbital, the second contains four orbitals, the third nine, the fourth sixteen, the fifth twenty-five and the sixth thirty-six. In terms of numbers of electrons, shells one through six can respectively hold a maximum of two, eight, eighteen, thirty-two, fifty and seventy-two electrons.

Hydrogen has one electron in its first shell, while helium, the second-lightest element, which consists of a nucleus of two protons and two neutrons, has two electrons in its first shell � one spin up electron, and one spin down. Lithium, the next-heaviest element, has a nucleus of three protons and four neutrons, and consequently has three electrons, because each atom has the same number of protons and electrons. It contains two electrons in its first shell, and one in its second. Oganesson has two electrons in its first shell, eight in its second, eighteen in its third, thirty-two in its fourth, thirty-two in its fifth, eighteen in its sixth, and eight in its seventh. At least, that is what chemists predict. No atom of oganesson has existed for sufficiently long for its electron configuration to be accurately determined.

The lighter elements fill up their inner shells completely before moving on to the next-closest shell to the nucleus. Heavier elements fill up their shells in more complicated ways. For example, oganesson�s 118 electrons are distributed across seven shells but could fit into six. Heavier elements fill up their shells in more complex ways because shells further away from the nucleus are split into subshells, and the amount of energy needed for an electron to inhabit an orbital differs between subshells. Sometimes it takes less energy for an electron to be in a lower-energy subshell in a shell further away from the nucleus than in a higher-energy one closer to it. Having written all this, I can see now why my school chemistry curriculum glossed over the details and ran with the inaccurate solar system analogy. Atoms are complicated. Yet all this complexity allows chemists to predict how atoms of different elements will react with one another, and these reactions are fundamental for our existence.

Atoms are most stable when they have as little energy as possible. An atom in this ground state has each electron inhabiting the lowest-energy orbital available. If an atom is energized such that one electron is pushed into a higher-energy orbital, it will revert to the ground state as soon as possible by releasing an electron. A hydrogen atom with its one electron in the second shell is not at the ground state, and the electron will want to lose energy by releasing a photon, enabling the electron to disappear from the outer shell and to appear in the innermost one. The photon that is released has a wavelength determined by its energy level. In the case of an electron in the second shell of a hydrogen atom, the photon released as the atom reverts to the ground state is in the ultraviolet spectrum that we cannot see with our naked eyes. Less energy is released when an electron moves from a hydrogen atom�s third shell to its second shell, and the photons that are released when this move happens are consequently less energetic. They have a longer wavelength and appear to us as being red.

Even if atoms are at their ground state, they can still be reactive. Atoms are least reactive when shells, then subshells, and then orbitals are full of electrons. A lone electron in a shell makes some elements highly reactive. Hydrogen, lithium, sodium, potassium, rubidium, caesium and francium are all elements with a single electron in their outermost shell when they are in their ground states. They are found on the left-hand side of the periodic table. Hydrogen is a gas at room temperature, but all these other elements are solid, and collectively they are called the alkali metals. If an alkali metal donates the electron in its outermost shell to another atom, it becomes a positive ion. What this means is it has more protons in its nucleus than electrons in its shells and this gives it a positive charge. A positive ion is called a cation, and it will be attracted towards negative charges.

Fluorine, chlorine, bromine, iodine, astatine and tennessine are each one electron short of having a full subshell or outer shell and are known as the halogens. They are found on the right-hand side of the periodic table. Unlike the alkali metals, rather than wanting to lose an electron, they want to gain one, but when they do this they become an anion with a negative electrical charge. They are attracted to positive charges. What happens when atoms of alkali metals and halogens meet? An electron is donated from the metal atom to the halogen atom, and a compound is formed. The compound is formed via a type of chemical linkage known as an ionic bond, a strong form of electrostatic bond. Strictly speaking, compounds formed solely by ionic bonds are not molecules, although this distinction is frequently ignored in everyday English. Sodium chloride is a compound, but you will be more likely to know it as salt.

Sodium chloride can be made by mixing the elements sodium and chlorine together. Sodium atoms donate the sole electrons in their outer shells to fill up the outer shell of chlorine atoms. The sodium atom becomes positively charged, while the chlorine atom becomes negatively charged, and they are attracted to one another. When lots of ions of sodium and chlorine ionically bond, salt crystals are produced. If you make salt in the laboratory from chlorine gas and sodium metal you often need to provide a bit of energy to get the reaction started, but once it is underway and pairs of sodium and chlorine atoms transfer an electron, the reaction generates energy that can be seen as a bright yellow light. The amount of energy required to get a chemical reaction started is called its activation energy.

Ionic bonds are only one type of chemical bond. Another way of joining atoms is via something called covalent bonds, where atoms share electrons. Water molecules consist of two atoms of hydrogen and one atom of oxygen that are covalently bonded. The bonds result in each of the three atoms filling their outer shells, albeit by sharing electrons. The more electrons that are shared between atoms, the stronger the covalent bond. Carbon atoms illustrate this nicely as pairs of these atoms can bond by sharing either one, two or three electrons. It is much harder to prise apart carbon atoms when they share three electrons than it is when they share two, and it is harder to prize double-bonded carbon than single-bonded carbon atoms. When atoms of elements form covalent bonds, they produce molecules.

Chemists categorize covalent bonds into two types, polar and nonpolar. Bonds between pairs of carbon atoms are described as nonpolar when the electrons are shared equally between the two atoms. The wave function that describes where an electron is likely to be is symmetrical between the two atoms. If we could force the shared electron to appear, half the time it would be closer to one of the atoms, and the other half of the time closer to the other. Nonpolar bonds describe cases where you are on average more likely to find an electron closer to one of the atoms than the other. Water is such an example. Each shared electron is more likely to be closer to the oxygen atom than a hydrogen atom.

Nonpolar bonds mean that some parts of molecules tend to have positive or negative charges. These charged areas are called dipoles, and oppositely charged dipoles can attract, forming weak electrostatic bonds. Within water, the oxygen end tends to be a negatively charged dipole because it tends to hog the shared electrons, while the hydrogen atoms tend to be a positively charged dipole. The positive charge from the hydrogen atom allows it to form weak bonds with negative dipole oxygen, and these types of bonds are called hydrogen bonds. Although weak compared to ionic bonds, hydrogen bonds are relatively strong for an electrostatic bond. They are also extraordinarily important for life. If chemistry didn�t work like this, you and I would not exist.

Hydrogen bonds are the reason that ice floats in water. When water is liquid the atoms within it are constantly moving. They have sufficient energy that hydrogen bonds do not form for long and the molecules are frequently colliding with one another. As water cools, and energy is lost from the liquid, it starts to form a lattice. This happens because the hydrogen atoms in one water molecule form hydrogen bonds with oxygen atoms in other water molecules, resulting in a twelve-sided lattice forming. The hydrogen bonds hold the water molecules apart, resulting in a solid with lots of gaps between molecules. The large amount of empty space in ice makes it less dense than water. Without hydrogen bonds ice would not float in water, or in your gin and tonic. It is relatively rare for compounds to behave like this. Most of us don�t spend much time thinking about the behaviour of water, but it is surprisingly unusual.

The final property of an atom that determines how it reacts is its size, as this influences how easy it is for an atom to share, donate or accept an electron. The more protons and electrons an alkali metal has, the more easily it will donate electrons. Potassium is more reactive than its lighter counterpart sodium because potassium can give up its sole electron in its outer shell with less effort. In contrast, the smallest halogens most readily accept an electron, and the most reactive halogen is fluorine. Because it is small, and all its electrons are close to its nucleus, it is very good at accepting electrons from other compounds, and this makes it more reactive than its cousin chlorine, which is heavier.

Fluorine is so reactive it is found in two of the most dangerous chemicals on the planet. The world�s strongest acid is fluoroantimonic acid, which is a mixture of two molecules that contain fluorine: antimony pentafluoride and hydrogen fluoride. The strongest acid we are likely to come across in our everyday lives is sulphuric acid, which itself can be pretty nasty and capable of burning skin. Fluoroantimonic acid is close to 10 quadrillion times stronger than sulphuric acid (that�s 1 followed by 16 zeros). It reacts violently with almost anything and is extraordinarily difficult to store, requiring containers made of compounds like Teflon that cannot be corroded.

Chlorine trifluoride is a second fluorine-based compound that is highly reactive. It is made from three atoms of fluorine and one of chlorine that are covalently bonded. Its electrons are arranged in a way that makes it highly unstable. You can use it to burn asbestos, a compound that was widely and effectively used to prevent fires in the middle of the twentieth century. It can even set alight ash made from burning wood. Although highly reactive, the compound is used in rocket fuel and in the computing industry in small doses. In everyday life, it is considered safe at levels of 0.1 parts per million, which is very dilute. There was therefore considerable risk when 900 kilograms of pure chlorine trifluoride were produced in the 1950s. While being moved across a warehouse, the chemical was accidentally spilled, burning through 30cm of concrete and 90cm of gravel before all the molecules had reacted and the fire burned out.

Highly reactive molecules are rare in nature because they quickly react with other molecules to form something more stable. Life does not use highly explosive compounds like chlorine trifluoride, 1-diazidocarbamoyl-5-azidotetrazole or fluoroantimonic acid. Although life requires energy, and highly reactive molecules can easily react, releasing it, life has found much more manageable ways of using chemistry.

If physics is the universe�s way of turning energy into atoms, then chemistry is the cosmos�s way of transforming elements into life. Life is picky with which elements it uses. Iron makes up just less than a third of our planet�s weight, but less than 0.01 per cent of your body�s. Oxygen is the second most abundant element on Earth, making up 30 per cent of its mass, but it is even more abundant in the human body, making up 65 per cent of your mass. Silicon, magnesium, sulphur and nickel are some of the most abundant elements found on Earth, yet life uses them sparsely, with each making up only a fraction of a per cent of you. We might be made from star dust, but life has been selective with which bits it uses.

Carbon is an element that is absolutely essential to life on Earth, and likely to life elsewhere in the universe if it exists. By weight, you are 18.5 per cent carbon. A carbon atom has six protons and six electrons, and anywhere between two and sixteen neutrons. The majority of carbon atoms have six neutrons and come in the isotope known as carbon-12, with only two other isotopes, carbon-13 and carbon-14, found in nature. Carbon-14 is weakly radioactive, decaying into nitrogen-14, a property that archaeologists use to age ancient artefacts. All other isotopes of carbon have only been made in laboratories and are unstable.

When carbon is in its ground state, it has two electrons in its inner shell, and four in its second shell. It can consequently make up to four covalent bonds with other atoms by sharing electrons. Being a small atom, the covalent bonds that carbon forms are quite strong, allowing it to form stable compounds. Each atom can combine in several ways to create compounds with very different properties. For example, at high pressure, each carbon atom can form single bonds with four other carbon atoms in a triangular pyramid structure, a tetrahedron, to produce diamonds. In contrast, at low pressure (and the right temperature) graphite forms. In this molecule, which is grey-black in colour, each carbon atom joins with three other carbon atoms via a complex electron-sharing arrangement. A sheet of linked carbon atoms is created, and these sheets are held together in a stack with electrostatic bonds, structured in a similar way to a ream of paper.

Carbon atoms not only join with other carbon atoms but can also form covalent bonds with atoms of other elements, including some metals. In the natural world, carbon often bonds with nitrogen, hydrogen and oxygen atoms, and because it can form multiple covalent bonds it frequently forms the large molecules that are so central to life.

There is no molecule more synonymous with life than deoxyribonucleic acid. A single molecule of DNA can contain billions of atoms. Each DNA molecule contains some of the genetic code to assemble organisms, be they bacteria, plants, fungi or animals. These molecules are stable, and DNA does not react with other molecules to change its atomic structure, which is a good thing, because if it did, the self-assembly manual to build you and me would be rapidly lost. Many other of life�s molecules are similarly quite stable. We store energy in the form of molecules of fat that can be kept until they are needed to free up energy. They are stable in our cells until we need energy, and they can then be broken down. Proteins, which are the workhorses of life, are also stable, being able to repeatedly facilitate other reactions. The chemistry of life is all about controlling reactions to either break down molecules to generate energy, or to use energy to build cells and structures like bones and brains.

Like all chemical reactions, those needed for life have an activation energy. Molecules that easily react with one another form reactions with very low activation energies, while other reactions, such as splitting water molecules into their hydrogen and oxygen components, require more energy. Once a chemical reaction is underway, it can either give out energy or take it in from the environment. An energy-generating reaction releases heat and light and is termed exothermic. In contrast, one that uses heat is endothermic. If you burn something, you are creating an exothermic reaction, while photosynthesis, the chemical reactions that plants use to live and grow, is an example of an endothermic reaction because it uses photons, the energy carrier particles, to power the production of sugars that the plant needs to grow.

Life uses both exothermic and endothermic reactions, and it uses a neat trick to reduce the activation energy of many of them. It does this using a class of chemicals called enzymes. Enzymes are nearly always proteins, molecules that consist of long strings of smaller molecules called amino acids that are folded into large, complex structures. Trillions of enzymes are active throughout your body at any one time. They are responsible for joining molecules together to build your body, and also to break molecules apart when burning food to provide the energy you need to go about your daily business. Enzymes are remarkable, and without them life could not exist.

Enzymes are large and complex, with complicated three-dimensional shapes. They often have valleys and channels and odd-shaped protuberances. A small number of amino acids within these structures form something called a binding site, which is specific to each enzyme and is where the chemical reaction it facilitates occurs. The binding site allows the enzyme to form electrostatic bonds with a specific chemical that is key to the reaction the enzyme catalyses.

A catalyst is any compound that can speed up a chemical reaction, and they do this by making it easier for the chemical reaction to start by lowering its activation energy. Enzymes are the catalysts that life most frequently uses, but catalysts are also important in inorganic chemistry. The molecule that an enzyme breaks apart or builds is called the substrate, and the enzyme alters the substrate�s shape. Some enzymes manage the chemical reaction alone, while others use other molecules called cofactors. The enzyme binds to the substrate, lowers the activation energy of the reaction, triggers the reaction, and then releases the end products that might be new building blocks for your body, fuel to run each of your cells, or molecules to be disposed of. The enzyme itself is not chemically altered by the reaction and survives to repeat the reaction it catalyses again and again and again.

The substrate can only bind to the active site of the protein in one way. In some enzymes, the substrate fits snuggly into the site, a bit like a 3D jigsaw. In other cases, either the substrate or the enzyme, or both, temporarily change shape a little to enable a tight bind. The binding is usually caused by hydrogen bonds, or other types of weak electrostatic bond. However, some enzymes form temporary nonpolar, and occasionally polar, covalent bonds. Once the active site and the substrate are bound, various processes can occur to start the reaction. Some of these processes involve complex chemistry that is beyond what I can cover here, but others are more easily understood.

When the substrate is forced into a different shape by the enzyme, this puts stress on the covalent electron bonds in the substrate, making them easier to break either by a cofactor, or another part of the protein that can donate or steal protons or electrons. In other cases, the enzyme orients the substrate in such a way it is easier for other molecules involved in the reaction to align and react or join with the substrate. Regardless of the process, the activation energy for the reaction is reduced and the reaction is speeded up.

The increase in the rate of the chemical reactions that enzymes catalyse can be quite remarkable, and sometimes they speed things up by several million times. This is nicely demonstrated by some work on an ancient fossil. Sixty-eight million years ago a female Tyrannosaurus rex died. We don�t know how, but it was one of only a very small number of animals that became fossilized. Remarkably, even some of its muscles and tendons survived. In 2007, Mary Schweitzer, a molecular palaeontologist at North Carolina State University, analysed the tissue and exposed a little of it to an enzyme called collagenase. The enzyme broke down molecules of collagen in the soft tissue in a matter of hours.

Collagen is a tough protein that is central to building a body. It is found in skin, cartilage, tendons and bones. Without collagenase, it is difficult to break collagen down, which is one reason why cartilage and tendons can remain tough and stringy even after meat is cooked, and why these parts of animals are some of the slowest to be broken down by microbes following death. Despite collagen�s known tough properties, Schweitzer�s findings of microscopic remains of soft tissues lasting millions of years were controversial when published, and they remain so. Some independent researchers have been able to replicate some of her findings, while others have not. Science often proceeds like this. Claims that challenge current understanding need multiple independent tests before they are accepted. By no means all researchers in molecular biology or palaeontology are ready to accept Schweitzer and colleagues� findings, but my reading of the subject is that the pendulum is swinging in her favour. A growing number of scientists, including me, are becoming convinced.

I chose to describe these T. rex findings here, despite their being less strongly supported than the rest of the science described in this chapter. If these findings stand the test of time, they will show that an enzyme can make a reaction happen to a substrate that has been stable for nearly 70 million years. Prior to Schweitzer�s findings, collagenase was known to accelerate the breakdown of collagen by many tens of times, but the T. rex findings suggest it can be even more potent.

Once an enzyme like collagenase has catalysed a reaction it must release the product. Any temporary chemical bonds between the enzyme and the products that have been produced must be broken. Sometimes the rearrangement of charge on the modified substrate may be enough for them to separate and disperse away, but in other cases something more is required. Following the reaction, the enzyme can return to its original, pre-binding shape, helping expel the new product. The enzyme is once again primed to conduct the reaction with a new substrate molecule.

Chemical reactions are governed by the electromagnetic force. If the electromagnetic force was substantially stronger, or weaker, electron shells and the positive and negative charges of cations and anions would be different, and life may not be possible. If atoms held on to their electrons more or less strongly, many chemical reactions would not occur or would require much higher temperatures and pressures to take place. We owe our existence to chemistry, and chemistry works as it does because of the workings of the electromagnetic force.

Although I have only scraped the surface of chemistry in this chapter, I have touched upon how atoms behave, how they share electrons, and why chemical reactions occur and molecules form. These processes are fundamental for you and me to exist. The chemistry of life is not as flamboyant as some of the reactions we encountered that involve highly reactive and explosive compounds, but it is nonetheless astonishing. Scientists have a very deep understanding of how all four forces of nature work, and their hard-won insights into the electromagnetic force means they can predict the outcome of very many reactions before they mix the compounds involved together. Nonetheless, there are aspects that are not well understood.

Chemists cannot yet easily predict many aspects of organic chemistry, the chemistry underpinning life. For example, we do not understand how many reactions that happen within living organisms are controlled and prevented from continually happening. Another thing we do not know, and this is true for all four forces of nature and not just electromagnetism, is why the forces take the strengths they do. Why is the electromagnetic force such that electron shells are spaced as they are, and not closer, or further, from atoms? Is it inevitable that the electromagnetic force is the strength it is, or is it a fluke of our universe? What we do know is that the electromagnetic force defines the chemistry we experience, and that, without chemistry, life could not exist. But it is not the only force that life required to be just right. We needed the weaker force of gravity to allow our solar system to form, and the Earth within it. I now turn to the role of gravity in our existence.

Our Locale

Our universe is truly magnificent, yet one aspect of it is deeply frustrating. It is so large that the prospects of us exploring beyond our immediate backyard are slim. The space probe Voyager 1 was launched in September 1977 when I was nine years old, and it has been travelling for over forty-five years. It is the most distant human artefact from Earth, being about 24 billion kilometres away. A beam of light fired from Earth could reach Voyager 1 in just less than a day. On the scale of the universe, 24 billion kilometres is very close by. Alpha Centauri, the closest star system to our sun, is 4.37 light years away. Voyager 1 is not travelling towards Alpha Centauri, but if it was, at its current speed it would take about 80,000 years to get there.

Our sun is a star, and it, along with the planets around it, forms a star system. Our star system, the solar system, has just one star, the sun. Many star systems have more than one star, and they orbit one another in elaborate dances. The Castor star system, for example, has six stars.

Star systems, such as the solar and Castor systems, are grouped together into galaxies, with our home galaxy being the Milky Way. It contains at least 100 billion stars, and perhaps as many as 400 billion. Each star we can see twinkling in the night sky with our naked eyes is within our galaxy. It is possible to see our nearest galaxy with the naked eye on clear, moonless nights, but it is not possible to distinguish stars within it without a powerful telescope. There are estimated to be about 1 trillion galaxies in the observable universe, each separated by vast tracts of outer space. That is a lot of stars, and a lot of cosmic real estate that we can�t visit or communicate with. If there are intelligent aliens out there, they would have to be cosmically close by to know of our existence.

Guglielmo Marconi sent the first radio message � �Can you hear me?� � about 125 years ago, in 1897. If an alien civilization was listening out for radio messages and heard this and was to immediately reply, �Yes, we can hear you,� and we were to pick up their reply, they would have to be within 62.5 light years of Earth. Radio waves travel at the speed of light, so Marconi�s signal from Earth would have reached the most distant aliens who could have heard it sometime in 1959, and their response would be arriving back on Earth around now. There are about 15,000 stars within 62.5 light years of Earth, and we�ve heard nothing intelligent from any of these systems. Conversations with aliens on their home worlds will at their fastest take generations.

There is one radio signal astronomers have detected that may have originated from an extraterrestrial civilization. It is called the �Wow!� signal, and it was recorded in 1977 by a radio telescope belonging to Ohio State University. The astronomer Jerry Ehman wrote �Wow!� on the paper printout that contained the signal, and the name stuck. The signal sounds nothing like Marconi�s first radio message and instead is in the form of a type of wave nothing the like of which has been detected since. In 2012, humanity responded to the Wow! message by sending 10,000 tweets towards the constellation Sagittarius, the direction from which the signal came. If the signal was from aliens, let�s hope they don�t think we�re trolling them. Personally, I will remain doubtful that the Wow! signal is evidence of extraterrestrial life until we hear the same or a similar message again and can rule out other potential causes.

The reason I find the vast distances between star systems frustrating is I would dearly love to know how common life, and in particular intelligent life, is in our universe. The fact we have not encountered aliens does not mean they are not out there. Even if on average each galaxy contained only one planet that evolved intelligent life during its entire history, that would still equate to 1 trillion planets with civilizations in the observable universe, yet we would never know. It seems I am not destined to visit with aliens on other planets, but fortunately there is much to see on our own, and the history of our planet is amazing and has generated some breathtaking places to visit.

One of my favourite places on Earth is the Bungle Bungle mountain range of Western Australia. I became fixated on this corner of our planet as an undergraduate student. It was the late 1980s, and my friends and I were avid watchers of the Australian soap opera Neighbours. One character, Helen Daniels, announced occasional trips to the Bungle Bungles to paint, presumably whenever the actress who played her, Anne Haddy, needed a break. I didn�t believe there could possibly be a place called the Bungle Bungles so headed off to the library to learn more. After spending way more time than I had expected, and time I could ill afford, given my looming exams, I learned that the Bungle Bungles was an ancient mountain range consisting of beehive-shaped striped mounds and canyons carved by wind, water and sand over millions of years. The area had been first seen by Europeans in 1983 when a film crew flew over it, and four years later it was designated a National Park. The local Gija people had been visiting Purnululu, their name for the range and what it now should be referred to as, for over 20,000 years, and possibly much longer.

It took me thirty years to make the trek to Purnululu, with my wife Sonya, and my children Sophie, Georgia and Luke, visiting for my fiftieth birthday in 2018. To get there, we flew on progressively smaller planes from London to Singapore to Darwin to Kununurra before the final leg on an eight-seater propeller plane which deposited us on a sandy airstrip that is grandly named Purnululu airport, close to the range. Although it took me three decades to visit this UNESCO World Heritage-listed site, I was told by someone who had a dream job of working in the park that I had fallen for an Australian marketing campaign. Having gazetted the national park in 1987, the Australian Tourism Board wanted to attract visitors to it, and although I have been unable to independently verify this, embedding the Bungle Bungles into Helen Daniels� lines was apparently part of the campaign.

The beehive-shaped mounds of Purnululu are striped orange and black, and they sit in front of mountains that today reach a height of only 575 metres above the surrounding plains. These mountains are criss-crossed by canyons carved over the course of millions of years. The history of the mountain range dates to a time when amphibians were the dominant animals, well before mammals had evolved. Between 350 and 375 million years ago, multiple layers of sediment were laid down in the bed of a river or lake before they were eventually compressed into layers of sandstone. Geological activity caused by the shifting of huge slabs of rock called tectonic plates that form the crust of our planet then pushed the sandstone upwards to form a towering mountain range. Over time, the range was eroded by rainfall, streams, wind and fluctuations in temperature, and Purnululu was formed. The orange and black stripes of the mounds have faced the brunt of winds blowing off what is now the Simpson Desert, and they are the most eroded part of the ancient range. The stripes are thought to be due to differences in the clay content of the sedimentary sandstone layers from which the range initially formed. The more clay that was in the sediment when it formed, the higher the water content of the rock, and these wetter layers are home to cyanobacteria, a species of photosynthesizing microorganism. The stripes are only a couple of millimetres thick because the cyanobacteria need light to survive and it does not penetrate far into the rock. The bacteria are also unable to survive in drier layers of the rock. The stripes are consequently indicators of fluctuations between wet and dry periods that took place over a 25-million-year period nearly 400 million years ago in the ancient lake where the sediment was laid down.

The Purnululu range is unique to Earth, and Earth itself is unique. In the topics we have covered so far in this book, we have not yet had to focus on local events. Gravity, electromagnetism and the weak and strong nuclear forces operate throughout the universe in the same way (or at least physicists assume they do because they emerged when the universe was very tiny and are thought not to have changed, and there is no evidence to suggest they vary from one galaxy to the next). Matter is not spread evenly throughout the universe due to quantum variation when the universe was very young, and that difference in its distribution resulted in gravity pulling together matter into stars and galaxies. The four fundamental forces are responsible for our planet and the solar system in which we reside, but to understand how our little corner of the universe came to be we must now think a little more locally. The details on how matter has been distributed in our neck of the woods become important as we consider how our solar system and planet formed. Was it inevitable that a star the size of our sun developed where it is today, or that it would have planets tracing the orbits we observe across our night sky? Each star is a little bit different, as is each solar system and galaxy. For you and me to exist, we needed the right combination of events to occur locally so that a planet formed where life could evolve and develop into an intelligent species. In this chapter, I largely focus on what we know about how our local conditions came about, and gravity is the force that takes centre stage. But before I get on to that, there is a gap to fill.

The fundamental forces, the first stars, heavier elements and the first molecules were all in existence before the universe was a few hundred million years old. The universe formed 13.77 billion years ago, yet our solar system formed only 4.6 billion years ago. What happened in the intervening billions of years?

Galaxies formed about 13.6 billion years ago, and they exist in groups and clusters across the heavens. The Milky Way is one of at least fifty-four galaxies in a cluster called the Local Group, which in turn is one of a hundred or so groups that form the Virgo supercluster of galaxies, which itself is a neighbourhood of the Laniakea supercluster of 100,000 galaxies. The smallest of these galaxies contain only a thousand stars or so, while the largest contain 100 trillion or more.

Galaxies move. All galaxies spin around a central point because of the way they formed from vast clouds of hydrogen gas being pulled together by gravity. On average, galaxies are also moving apart as the universe expands. However, galaxies within clusters like the Local Group can move towards one another, and they can collide. Astronomers have discovered 30,000 or so stars in the Milky Way with orbits that go in the opposite direction to that of most of its stars, and they have identified these as being part of another galaxy which crashed into the Milky Way many eons ago. Calculations of the trajectories of the Milky Way and the Andromeda Galaxy, a close neighbour only 2.5 million light years away, suggest they will collide in 4.5 billion years� time.

Although all galaxies began to form at about the same time, they don�t all die at the same age. The Hubble Space Telescope, a remarkable device orbiting Earth that has provided many insights into our universe, has identified six galaxies that died by the time the universe was 3 billion years old. For reasons unknown, they had used up all their hydrogen and could no longer form new stars. In contrast, most galaxies are still producing stars, although the rate at which they do varies. Some galaxies contain lots of heavy elements like iron, having burned through many generations of stars, while others contain few heavy elements, being on star generation one or two. Within the Milky Way, some areas are more metal and oxygen rich than others, revealing that the history of different parts within a galaxy can also vary.

Most of the first stars in the Milky Way have died, but many new stars continue to be born. Parts of our galaxy consist of dust and molecules from the death of past stars, along with vast amounts of hydrogen and helium created in the early universe but not yet captured by stars. Sometimes these vast clouds are shaken into action by the explosion of stars several times more massive than our sun. Such supernovae are some of the most violent events in our universe, and they can result in dust and gas clouds forming nebulae, the birthplaces of new stars. The Helix Nebula is Earth�s closest star-forming nursery, with light from it taking seven centuries to reach us. In nebulae such as these, gravity causes cold gas and dust to start to clump together, becoming warmer and eventually forming new stars. These processes occur throughout the galaxy where there is sufficient hydrogen, with small, medium and large stars being produced, many with planets, but a few without.

Averaging across the star systems studied to date, each star has between one and five planets. Each of the star systems and each of its planets is unique. Although their formation has been driven by gravity, variation in the density and composition of the dust in the nebula in which they were born, and the strength of the star�s gravitational pull at the location where each planet formed, determine the planet�s size and composition. Each star and each planet has its own history, so now to turn to the history of the solar system.

Between 4.9 and 5 billion years ago a supernova triggered the formation of our solar system. If the nebula in which our sun formed was like those we observe elsewhere in the galaxy, many other stars and solar systems formed within it. Once young stars and solar systems form in nebulae, the spin of the Milky Way sets them off on their first orbit around the centre of our galaxy. At the centre of the Milky Way is an enormous black hole weighing about 4 million times the mass of our sun. We call the black hole Sagittarius A*, and the stars and solar systems of our galaxy orbit this black hole, at least until they are sucked into it, or they burn out, having used up all their hydrogen fuel. Although large, there are some black holes that are an order of magnitude larger than Sagittarius A*: in early 2023 astronomers estimated that a black hole called TON618 is ten times larger.

Our solar system is 25,640 light years from the centre of the galaxy. Two hundred and fifty-six centuries ago, humans built the first settlement, consisting of huts built of rocks and mammoth bones. Light that left the sun at the time of these innovative ancestors of ours will only now be arriving at the centre of the Milky Way. Given how far it is to the centre of the galaxy, it is not surprising that it takes a long time for our solar system to make a complete orbit of the galaxy. So long that scientists got fed up with dealing with such large numbers and instead defined a new measure of time to describe the orbit: a galactic year. Each galactic year is between 220 and 230 million years (it�s hard to measure), and the Earth is about twenty galactic years old. The first life evolved about 16.9 galactic years ago, while the first humans made their appearance approximately a galactic fortnight ago.

The Sun�s Position in the Milky Way

By 4.6 billion years, or 20.4 galactic years ago, our sun had formed, with huge amounts of hydrogen being pulled together by gravity until nuclear fusion began. The sun is by far the biggest in the solar system and constitutes 99 per cent of its mass. Although tiny compared to some of the objects I�ve discussed in this chapter such as massive black holes, it is, to us, huge. It has a diameter that is eleven times larger than Jupiter�s, the next-largest object in the solar system, and 109 times larger than that of Earth�s.

Other stars that formed in the same nebula as the sun set off on different orbits around the galaxy, with some moving faster and others more slowly. The closest stars to us did not form in the same nebula as Earth, and we know this because they vary significantly in age. For example, Barnard�s Star, the second-closest star system to Earth, is about twice as old as our sun, while the Luhman 16 system, the next-closest system, is only 600�800 million years old. Because different star systems orbit the galaxy at different rates, Alpha Centauri, the closest star system to us today, will likely not be our closest neighbour a billion years hence.

Much as the trajectory of galaxies within galaxy clusters reveals each galaxy pursuing its own journey across the universe, each star system within our galaxy follows its own orbit around Sagittarius A*. Some of these orbits mean that some stars repeatedly pass close by one another. Every galactic year or so, our sun will fly by star systems it has encountered before. Some scientists have argued that such close encounters may result in meteoroids and asteroids being displaced from their orbits in the outer reaches of our solar system, putting them on new trajectories that take them closer to the sun, potentially putting them on a collision course with Earth. There is evidence that asteroid and meteorite impacts peak and trough on galactic year timescales, although not all scientists are persuaded. Collecting more data to confirm this happens will take a very long time.

The terminology of space rocks can be confusing. I have mentioned asteroids, meteoroids, meteors and meteorites, but how do they differ? An asteroid is a rocky object that has a diameter of greater than a metre and that is smaller than a planet. A meteoroid is also rocky but is less than a metre in diameter. When a meteoroid enters the Earth�s atmosphere it burns up, producing a meteor that appears as a shooting star across the night sky. If the meteor hits the ground it is classified as a meteorite. As an asteroid approaches a planet, it often breaks up into meteoroids, which become visible in the night sky as a meteor storm. The impact that killed the dinosaurs was an asteroid that didn�t disintegrate completely into meteoroids, and which resulted in a large lump of rock crashing into the planet. In contrast to an asteroid, meteoroid, meteor, meteorite or space rock, a comet is made of ice and dust rather than rock. As comets in our solar system near the sun, they partially melt, leaving a trail of water and dust. Light reflecting off the melting comet makes it appear as a bright light travelling across the heavens.

We do not have enough data from other star systems to know whether our solar system is typical or atypical. Perhaps solar systems such as ours are two a penny, or perhaps ours is the consequence of a set of highly specific circumstances that have never been repeated. The study of exoplanets � planets orbiting other stars � reveals that planets are common, and planets of a similar size to Earth that orbit around stars at a distance where water, a key chemical for life, exists as liquid, are likely not particularly rare. Many Earth-like planets may exist, but that does not mean we are not unique. Earth�s history is important on the journey from the Big Bang to you and me, and to study it we have to piece together our planet�s history using all the tools of the scientific method. At this point, I take a slight detour from the history of our planet to explain the next bit of my history of becoming a scientist.

The Emergence of Life

The focus of the book until now has been on the physical, chemical and earth sciences and summarizes what happened, and what had to happen, to create a planet where life could emerge. The next few chapters move on to focus on the life sciences, on how life arose and diversified, and how some species became very complicated. DNA is a chemical that is central to this story. Its first remarkable property is that it can make copies of itself. A second, even more remarkable property of DNA is its ability to encode instructions to make complex objects such as proteins, cells and bodies that aid its replication. Some of these objects are staggeringly complicated, and some of them help DNA codes replicate better than others. Evolution is the natural process that selects the DNA codes that are most effective at replicating themselves. What had to happen for you to exist is DNA had to come into existence, and evolution had to select DNA codes that result in you and me. To understand why this happened we need to know not only how life emerged, but also how the genetic code works, and how evolution invented things like multicellularity, sex, sentience and cooperative behaviour. The next few chapters consider this part of the journey from the Big Bang to us.

The first step on our journey is to explore why life emerged, and it is one of science�s most challenging problems. Although the problem is difficult, it can be quite easily articulated: how did life self-assemble the first self-assembly manual that describes how to build an organism? All living organisms contain an instruction manual � a genome � that describes how to go about making itself. How did the first genome that provides the instructions on how to build an organism arise? It was not designed, but instead it came about via the interaction of molecules somewhere on the Earth when it was young. All organisms alive today are descended from this first primitive organism.

All life uses a chemical called deoxyribonucleic acid, or DNA for short. The complete collection of an individual�s DNA is referred to as its genome, and every living organism on Earth, from the tiniest bacteria to the largest blue whale, has a genome. Each genome carries instructions to develop an organism. Your genome codes for a set of steps that enabled you to develop from a single cell formed by the fusion of an egg from your mother and a sperm from your father into a thinking, talking, eating, book-reading, intelligent ape. Similarly, my dog Woofler�s genome codes for a set of steps to develop from a single fertilized egg an animal which many people describe as cute despite his keen desire to chase down squirrels and his irresistible urge to roll in anything that smells terrible. These self-assembly manuals are not written in words but in genes: stretches of DNA that carry instructions to make molecules called proteins that are fundamental to life. The purpose of any life, including yours (from a biological perspective at least), is to produce offspring, because those offspring carry your genes, and these genes help your offspring develop, hopefully into biologically successful adults. Life is all about making copies of itself. Every genome in every organism contains instructions to make proteins that are necessary for the genome to make copies of itself. But which came first? The genome or the proteins? Biologists are dealing with the �which came first� problem, the chicken or the egg, but at the level of molecules.

The emergence of life from non-living chemicals is called abiogenesis. Research into the mechanisms underpinning abiogenesis is highly technical, involving very complicated chemical reactions. Scientists studying abiogenesis have worked out how and where many of the key molecules central to life formed, but they have not yet been able to explain how these key molecules self-assembled into the first, simple, singled-celled organism. Part of the reason for this is that even the simplest single-celled organism alive today is staggeringly complicated, containing over 500 genes producing hundreds of different proteins. These simple organisms are hugely more complicated than our most complex machines, including the LHC at CERN. I am not going to describe the chemical mechanisms that emergence-of-life researchers have uncovered, but I will instead focus on what had to happen for life to emerge.

Life got going relatively quickly after our planet formed, suggesting that when conditions are right, it can evolve without too much difficulty. The oldest non-controversial traces of life are found in the rocks of the Pilbara in North-western Australia, and date back approximately 3.4 billion years. These traces are in the form of fossilized stromatolites, layers of sediment formed by simple microorganisms called cyanobacteria. Stromatolites are still around today, having possibly changed very little over the last 3.4 billion years, and they can be seen at Shark Bay, just a few hundred kilometres from the Pilbara. I visited them in 2022 � it was a bit of a pilgrimage for me � and they look like shrunken-down versions of the beehive-shaped mounds of Purnululu I described earlier, in that they are often dome-shaped and striped. However, they usually contain limestone rather than the sandstone from which Purnululu are formed, and they only grow to about half a metre tall. These days stromatolites are a rarity �the only other place they are found is the Bahamas � but in the past they were more widespread. The cyanobacteria that created ancient stromatolites were descended from simpler life forms, with the earliest ghosts of life thought to have appeared sometime in the preceding 500 million years. Rocks from Quebec and Greenland have tantalizing evidence of this earlier life in the form of specific chemical signatures, although scientists do not agree on the origins of these chemicals. Exactly when the first life appeared remains unclear, but it probably evolved when the Earth was just a few hundred million years old. You, me, the vegetables in your salad, the worms in your garden and Woofler are all descended from early forms of life, and that is why understanding how life arose is central to the story of how and why we came to exist.

The species I have just listed � even the salad ingredients � are all extraordinarily complicated and built of vast numbers of cells. Yet even the simplest single-celled form of life alive today is very complex. The simplest bacterium did not simply and suddenly spring into existence. It evolved from something simpler. These simpler forms of life are long extinct, and fossil evidence of them is lacking, so knowing what they were poses a scientific challenge. The divide between the mix of organic compounds that were the precursors of life and even the simplest bacteria is so vast that understanding the journey life has made is daunting. Scientists are yet to see life emerge out of beakers of chemicals in the laboratory, and this means there is inevitably some speculation about how life began, and how the earliest forms behaved. Nonetheless, early life would have been subject to the forces of evolution that govern it today, and so some inference as to the processes that had to happen for life to evolve can be drawn from contemporary observations.

Distinguishing between the mix of organic molecules that led to life and the first living thing is not straightforward. There is a continuum from the set of chemicals from which life emerged to you and me, and deciding where to divide this continuum into living and non-living is subjective. Scientists have long argued over where the life�nonlife break in this continuum should be because any discussion of life requires a definition. Although not really romantic enough for something as beautiful as life, I will define it as a set of naturally arising, replicating, controlled chemical reactions within a membrane that is maintained by an external energy source. Try putting that into a song lyric.

It is worth discussing this definition a little. �Naturally arising� means that life arose from simpler chemical reactions and was not designed, produced or kick-started by some higher being. �Replication� refers to the production of descendants. All living organisms strive to make copies of themselves via clonal or sexual reproduction. Clonal offspring are genetically identical to their parent, while those produced by sexual reproduction share half their DNA with each of their two parents. Biological and evolutionary success is measured in the number of offspring you leave behind, with each carrying replicate copies of some parental DNA. From a historical perspective Pope Innocent III and Genghis Khan were both influential leaders during their time, yet their biological legacies are very different. Unlike some popes, such as Sergius III and Alexander VI, who fathered children during their tenures as pope, Innocent III had no children or descendants, while one in 200 men alive today is estimated to be descended from Genghis Khan. He will also have many female descendants, but the way our genetics works (and more on that later) means that they are harder to count. No one knows how many children Genghis Khan had, with only nine formally recognized, but some estimates suggest as many as a thousand.

Returning to life�s definition, the next part states �a controlled chemical reaction�. This is a reaction that can be speeded up, slowed down or stopped. At any point in time, there are millions and millions of chemical reactions going on inside your body. If you are healthy and tumour-free, each is controlled and operates within bounds. Evolution has found ways of slowing or stopping reactions when the compounds produced are sufficiently abundant for living organisms to run smoothly, while initiating and speeding up others when other molecules are scarcer than life would like. These chemical reactions are powered by �external energy� in the form of light, reactive chemicals or food, and the majority of these reactions occur either inside or on the outer �membrane� of cells that form the basic building blocks of all forms of life.

Each cell, regardless of whether it belongs to a bacterium or a multicellular organism like you, is complicated. Countless chemical processes and reactions occur continually within each cell, including the reading of the genetic code and the production of proteins and other large and complex molecules. Even in the simplest free-living bacterium these chemical processes are the product of hundreds of genes. There are some simpler parasitic species of bacteria that have fewer genes, but only because they rely on the hosts they invade to do some of the hard work of living for them. In this chapter, and the next, I consider how life began and spread to conquer the planet, but before discussing life�s origins it is worth considering just how complicated some key aspects of it are, as that will help frame the enormous challenge scientists face when considering how even the simplest forms of life alive today came about.

Life Conquers All

In the previous chapter I described how and why competition between autocatalytic molecules would have resulted in the evolution of replicating systems of chemicals in the environments where the first life formed. I doubt the description upset many readers. In contrast, when evolution is discussed with respect to more complex forms of life, and particularly humans, some people challenge its primacy in determining our existence. Yet the evolutionary processes that produced the first living organism are the same as those that produced humans nearly 4 billion years later. Evolution is as real as gravity, electromagnetism and the strong and weak nuclear forces, and without it we could not exist. Biologists understand evolution to about the same extent that physicists understand the strong and weak nuclear forces and chemists understand electromagnetism. They know how and why evolution creates new species. Evolutionary biologists like me have a clear definition of evolution and we know how to measure it. We understand how genetic differences arise and how natural selection operates on this variation to generate new forms of life. We understand how changing environments result in evolution, and in some cases biologists can predict how natural selection will change animals and plants. If you accept science as a way of finding out how and why the universe works as it does, you should accept that evolution is as real as the four fundamental physical forces discussed earlier.

There are two reasons why evolution can be a challenging topic. First, some people deny evolution because they find it hard to accept that humans evolved from simpler life forms such as single-celled bacteria and, more latterly, chimpanzees. Yet the comparisons of genomes between species as diverse as bacteria that cause disease, bananas, sea squirts, worms, fruit flies, dogs, gorillas and us provide overwhelming proof that all forms of life on Earth are related to one another. To generate this astonishing biological diversity, evolution needed billions of years, and during that time it invented death and sex, tiny structures in cells called mitochondria and chloroplasts, thousands of new types of molecules including forms of carbohydrate, protein and lipid, numerous cell types, and multicellular organisms such as the silver birch tree, Woofler and you. This chapter is about what had to happen for animal, plant and fungi species to become complicated. There are thought to be about 8.7 million species of plants and animals alive on Earth today, along with an unknown number of fungi species, and hundreds of millions of species of single-celled microbes. Evolution has invented a vast number of ways for organisms to make a living. We will encounter some of these as we think about what happened for the first mammals to evolve.

The second reason that evolution can be challenging is it is a quite complicated concept. Some readers will find it harder to understand than the workings of the four fundamental forces. Evolution is difficult because it involves several things happening at once. On their own, each of these things is easy to grasp, but when combined things can become complicated. I shall try to briefly summarize.

All organisms need resources such as food, water and shelter to live, and sexually reproducing species such as humans need to find a mate to reproduce. However, there are many things in the environment such as predators, or disease, or large numbers of competitors, that can make finding resources, including mates, difficult. Individuals that are good at finding, acquiring and using resources while avoiding the threats that the environment imposes upon them tend to have particular attributes such as the ability to deter things from eating them or the ability to mount an immune response to an infection. These attributes are determined, at least in part, by an individual�s genes � its DNA code. Those individuals that succeed in acquiring resources and avoiding threats tend to produce the most offspring, and this means their genes are represented in more individuals in the offspring generation compared to individuals who did not have these particular attributes. These genes are likely to produce the desirable attributes in offspring that made their parents successful. Evolution proceeds as genes that determine attributes that enable individuals to acquire resources while avoiding threats increase in frequency within a population. Over short periods of a few generations, evolution happens quite slowly and not a huge amount of change is observed. Over many hundreds of thousands or even millions of generations, populations can significantly genetically diverge, and new species can evolve. Evolution is consequently a process driven by which individuals are best at surviving and reproducing, but evolutionary change over a few generations is usually quantified by looking at how populations change over time or by comparing genetic differences between separate populations of the same species. On longer timescales, evolution is quantified by looking at genetic differences between individuals of different species. And there are a lot of species to compare.

When my children were young and I asked them to name their favourite species they would say lion, dolphin or dog. I failed to persuade them of the merits of species like Candidatus Prometheoarchaeum syntrophicum strain MK-D1, a species of single-celled organism found in the depths of the ocean. Each new MK-D1 cell starts life as a blob and develops by growing tentacle-like projections. These thin structures search out bacteria that acquire their energy from hydrogen molecules, and having found them, MK-D1 then gets the energy to power its metabolism from their waste products. MK-D1 lives off the poo of species of bacteria that do not use oxygen, a gas we often think of as essential for life.

Strain MK-D1 is a form of life you might be unfamiliar with. Biologists divide all life on Earth into three domains called the Bacteria, the Archaea and the Eukarya. Bacteria and archaea are always single-celled organisms, and these cells are relatively simple. In contrast, eukaryotes have more complicated cells, subdivided by internal membranes. Some species, like yeast, are single-celled organisms, but eukaryotes can also be multicellular species such as seaweed, fungi, trees and animals. Each of these three domains of life can be subdivided again and again, into hierarchical levels of organization called kingdoms, phyla, classes, orders, families, genera and species, until each species is eventually classified. I remember this with the pneumonic �Do Keep Pigs Clean Or Farm Gets Smelly� with the D in Do representing domain. Our full classification from the species up is Homo sapiens, in the genus Homo, which sits in the Hominidae family, which in turn is classified as being in the Primate order, in the Mammalia class, in the Chordata phylum, in the Animalia kingdom and, finally, in the Eukarya domain. In contrast, the microbe species Escherichia coli that can sometimes make you sick is classified as: genus Escherichia, family Enterobacteriaceae, order Enterobacterales, class Gammaproteobacteria, phylum Pseudomonadota, and domain bacteria. I�ll spare you the full classification of strain MK-D1, simply stating that it is a member of the Archaea domain, a group of single-celled organisms that biologists once thought were bacteria but have more recently realized are not, and are instead more closely related to animals, plants and fungi.

Species in the same genus are closely related to one another, typically having shared a common ancestor within the last few million years. Humans and chimpanzees are not in the same genus but they really should be, and our common ancestor lived between about 6 and 10 million years ago. As we move up through the classification system to family, order, class, phyla, kingdom and domain the species we encounter become progressively less closely related to us. MKD1 and humans are in different domains, and we shared a common ancestor over 3 billion years ago.

The reason I like strain MK-D1 is not only because it lives an unusual life but because it does so very slowly. It takes almost a month for a single cell to grow and divide into two cells. You might think that a month isn�t very long, but in the world of single-celled species it is an eternity. E. coli, for example, divides every twenty minutes. In the time it takes for one MK-D1 cell to become two individuals, if E. coli had access to unlimited food, one cell could grow to produce a colony of two raised to the power of 2,190 bacteria, a number so astronomically large that my computer returns infinity when I ask it to calculate it. Another example of the power of exponential growth.

Strain MK-D1 is also much more biologically interesting than a lion, dolphin or dog. It has genes that, until its discovery, were thought to be found only in eukaryotes, leading scientists to hypothesize that MK-D1 can shed light on how the complicated cells that eukaryotes such as you and I are built from evolved from the much simpler cells of archaea. As an example, single-celled eukaryotes are able to do something called endocytosis, a feat beyond bacteria. Endocytosis is the process these species use to envelop small objects such as particles, viruses and sometimes even smaller cells in order to bring them across their cell membrane. It is a process that helps bring nutrients inside a cell for it to use. Cells do this by bending their membrane to form a bubble around the object to be brought into the cell, before pinching the bubble off to create a small membranous sphere in the cell�s interior that contains the object. It is the cellular equivalent of you swallowing a morsel of food. MK-D1 produces a protein called actin that is necessary for endocytosis, and which is found in all eukaryotes but in no bacteria. Strain MK-D1 provides insight into the common ancestor of archaea and eukaryotes by providing clues into how life made a major jump in complexity, and biologists think it may also help us understand how endocytosis evolved. However, gaining this knowledge was difficult because it is very hard to keep MK-D1 cultures alive outside of its ocean habitat. It is only in the last few years that a group of biologists in Japan succeeded in keeping a population of MK-D1 alive in their laboratory. Lions, dogs and dolphins are not only much easier to keep in captivity but could never have existed if species similar to MK-D1 had not evolved genes that produce actin, a gene central to all complex life.

Strain MK-D1, E. coli and you all use the same genetic code. We all use twenty amino acids to build proteins, and these amino acids are coded by the same set of nucleobase triplets. Thymine�thymine�thymine codes for the amino acid phenylalanine not only in these three species, but in all species of life on Earth. The genetic code is universal to life on our planet. Although every species uses the same genetic code, different species have different genetic sequences, allowing them to make different proteins. By comparing how similar genetic sequences are between species, biologists can work out how closely related different species are. Because humans and chimpanzees shared a common ancestor only a few million years ago, the genetic sequences of the two species are much more similar than the genetic sequences of humans and MK-D1.

Despite the ubiquity of the genetic code, the very earliest life would not have used it, but it didn�t take long for life to adopt it. Inferring exactly what happened and when 4 billion years ago is not easy, but the genetic code we use today was probably in operation only a few million, or perhaps tens of millions, of years after life appeared on Earth. Although we do not know exactly how long ago the genetic code evolved, we do have a name for the organism that first used it: LUCA, or the Last Universal Common Ancestor, a term that scientists have used since the 1990s. You, me, the trees outside, the birds that sit in them, along with the bacteria in their guts, the invertebrates that parasitize them and the microscopic fungi that can cause them disease, are all descended from LUCA. There is a direct line of descent from LUCA to you, from LUCA to MK-D1, and from LUCA to E. coli. You might feel fortunate you have a direct line of descent from LUCA, but so too does every organism alive on Earth today, from each cockroach, to Woofler, to you. LUCA has very many and very diverse descendants. In this chapter, I explore how and why complex animals such as humans evolved. Why are we so different from dogs, E. coli and strain MK-D1? What had to happen in the 4 billion or so years since the genetic code evolved for our genetic self-assembly manuals to arise? Evolution is the process that has moulded life on Earth, and to understand it it is necessary to turn to the workings of DNA, phenotypic traits, and natural and sexual selection, terms I define in the coming pages.

The tree of life starts from LUCA and describes how new species evolved from existing ones. It is not complete, and there are many gaps, particularly the further back in time we go, because species alive in the distant past are extinct, and for many of them no fossils have been found from which they can be described. Nonetheless, it is clear from even the incomplete tree of life that different now-extinct species had very different fates. Some produced many descendant species that are still alive, while others left none. Anchiornis huxleyi was one of the first species of birdlike dinosaurs, evolving about 160 million years ago, and even though it is long extinct all of the ten thousand or more species of bird alive today are thought to be descended from it. Tyrannosaurus Rex is at the other extreme, leaving no descendant species, as its lineage went extinct following the meteor impact that brought the Cretaceous geological epoch to an end 66 million years ago.

We share nearly all of our line of descent from LUCA, and most of our DNA sequences, with chimpanzees and bonobos. The common ancestor of our species and of these two great apes walked the Earth up until 6 to 10 million years ago. If we accept an estimate of 3.9 billion years since LUCA lived, then humans, chimps and bonobos had identical lines of descent for 99.8 per cent of the time that life has been using the genetic code. For 90 per cent of those 3.9 billion years, humans and fish had identical lines of descent, and for approximately 75 per cent of them we shared a direct line of descent with bananas, rice and tulips. For between 15 per cent and 20 per cent of those 3.9 billion years we had the same line of descent as bacteria like E. coli. We share progressively less of our genetic sequences the further back in time we have to go to find a common ancestor with another species. Our DNA is consequently more similar to that of species of fish than it is to species of plants. What the tree of life reveals is that life has been diversifying and proliferating for a very long time. You are descended not only from apes but also from fish, jellyfish-like beasts, archaea like strain MK-D1 and bacteria.

The tree of life that biologists have drawn is remarkable. I can spend hours looking up species that I was unaware existed, and that in many cases no one has studied. As one moves forward in time from LUCA we first encounter bacteria, which then threw off a new branch, the archaea, which in turn split into two to produce the eukaryotes on a new branch. There are then endless splits that encompass yeast, plants and animals, until eventually we find Homo sapiens at the end of one of the branches. Sitting next to us on neighbouring ends of branches are chimps and bonobos.

It seems unlikely biologists will discover a new domain of life, so I expect these three major groupings to remain, but scientists are continually discovering new extinct and extant species in each of the three domains and will continue to do so as we catalogue more of life�s remarkable diversity. Despite being unfinished, at the time of writing, the tree of life contained over 2.2 million species with nearly all of these organisms consisting of only a single cell. We are unsure quite how many species there are still to be discovered, but as we find them, they will fit somewhere into the tree of life.

Each species alive today differs from others not only in their genetic sequences but also in what they can do. Strain MK-D1 is able to live off the waste of hydrogen-gobbling bacteria, but I cannot. In contrast, I can just about run a mile, while strain MK-D1 cannot. I have a very different body shape to Woofler, or the bird singing in the silver birch tree outside, and these body shapes map to things we can do. I can hold conversations, climb a tree and peel a banana. Despite my attempts to stop him, Woofler can effectively chase down pigeons and squirrels and expertly dispatch them in a way I cannot. Neither Woofler nor I can suck nectar from flowers and we would be unable to repeatedly migrate 3,000 miles from Alaska to Mexico and back each year, a feat the 3.5-gram rufous hummingbird achieves in each of its three to five years of life. Each species is different, and each is adapted to a particular way of life.

Woofler can effectively hunt small prey, I can peel a banana and hold a conversation, and a rufous hummingbird can fly the length of the North American continent because we have attributes that enable us to do these things. I have dextrous hands, a voice box and a large brain; Woofler has sharp teeth, keen eyesight and a very strong hunting instinct; while rufous hummingbirds have long bills that allow them to feed on high-energy nectar deep inside flowers and wings that let them hover and migrate. Each species has characteristics that allow it to do a specific set of tasks that enable it to find food, survive and reproduce. Biologists call these characteristics phenotypic traits, and they are defined as the attributes of an organism you can measure. Phenotypic traits range from specific molecules that individuals produce through to an individual�s body weight or aspects of personality.

Phenotypic traits differ between species. Dogs look different to people because they have a different set of phenotypic traits. Different individuals within a species can have different values of the same phenotypic trait. For example, you may be taller than me, or shyer, or have different-coloured eyes. Differences between species in the phenotypic traits expressed, and differences between individuals within a species, are the result of genetic differences. If we have different blood types from one another it is because we have different alleles at one or more of our genes. When individuals with different phenotypic trait values have different chances of surviving and reproducing because they differ in their ability to detect, acquire and utilise resources such as food, natural or sexual selection occurs, and this can result in different generations having different frequencies of specific alleles. Such genetic change across generations is evolution.

An individual�s DNA sequence contains the code to produce proteins that determine how an organism develops, and the phenotypic traits it expresses. Phenotypic traits are important because they determine what an individual of a particular species can do, while the DNA sequence determines which phenotypic traits an individual of particular species will develop. The study of phenotypic traits has been the focus of much of my career post-Ph.D. Given my graduate studies at Imperial College London had focused on how animals impact the distribution of trees by dispersing and consuming seeds and seedlings, it might seem surprising that I switched tack and started studying phenotypic traits and evolution as I got older. I am glad I did, but the route I took was not carefully planned.

Woofler�s Taxonomy

As I neared the end of my Ph.D., the realization that I would need to get a job dawned, and so I started to think about next steps. I knew I loved science and wanted to understand why I existed, but I didn�t know the best way to achieve this. Was it to become an academic, working on one small part of the problem while reading up about other aspects, or should I take another job that might give me flexibility to do the reading I needed to do? I didn�t really know. The first job I applied for was a postdoctoral researcher position at the Institute of Zoology, the research arm of London Zoo, where a postdoc is the next stage in academia after completing a Ph.D. Postdocs are typically employed on contracts that last for between one and five years with the job description outlining a particular scientific challenge that should be addressed. They are usually advertised by a university faculty member who has succeeded in securing funding from a government grant or a charity following a highly competitive peer-review process. As well as this job, I also sent off for forms to become a weatherman at the BBC, and for the civil service fast track, where postgraduates are trained to become scientific advisers within government departments. The detailed description for the weatherman job revealed I would need to be more interested in the weather than I was, and the civil service fast-track application procedure was so long and drawn out I could never motivate myself to start filling out the form, so the only job I ended up applying for was the postdoc position at the Institute of Zoology. I was amazed when I was offered it, as the interview was disastrous.

On hearing I had an interview, two of my housemates, Ana and Jane, insisted they take me shopping for a new suit. It was a sensible call, as my clothes no longer fitted me. I wasn�t particularly domestic back then, and to get the washing machine going I simply turned the dial to the first setting, which turned out to be boil wash. I only learned of my mistake when Ana asked who was boil-washing their clothes, and I suddenly understood why they had become so tight. Under the sartorial gaze of Ana and Jane I bought a dapper suit, collared shirt and tie, and the next morning I headed off to my interview. I had to get from Sunningdale in Berkshire to north London in the rush hour, and this meant standing for most of the journey. As the train drew to a stop at Twickenham, the man standing next to me managed to spill most of his cup of coffee down my shirt and tie. Not a great start. Shortly afterwards I realized my journey planning was cutting things very tight, and a tube delay meant I had to run the mile from Camden Tube station to the Institute of Zoology on one of the hottest mornings of the year. I arrived drenched in sweat, coffee stained and smelling like a cross between Starbucks and a gym.

My interviewers, Steve Albon and Josephine Pemberton, gave me a few minutes to catch my breath before ushering me in. I was flustered and still sweating profusely, so much so that one of them offered me a towel to wipe my dripping forehead just after I�d answered the first couple of questions. Despite this, I must have impressed, for the following day I was offered the job. I had a three-year position at the Institute of Zoology to study the genetics and ecology of red deer living on the Isle of Rum in the Inner Hebrides and a breed of wild sheep living on Hirta in the St Kilda Archipelago, one of Britain�s remotest islands. Little did I know it back then, but I would continue to work on these two study systems for the next eighteen years. I became good friends with Steve and Josephine and many others who collaborated on the project, and I only stepped back from this study when a scientific disagreement over approaches threatened to jeopardize these friendships. I am pleased to say we all remained friends, and the move to work on other systems, including wolves in Yellowstone, guppies in the freshwater streams of Trinidad, and a small species of green bird called silvereyes that live on the islands off Australia, a study system my wife Sonya has worked on for over twenty-five years, proved to be a great decision.

In the years I spent working with red deer on Rum and Soay sheep on Hirta I became proficient in working with mark-recapture data, something I still do. To generate these data, individual animals living in the wild are humanely captured, often shortly after birth, and affixed with a unique mark. In the case of sheep, the mark is a numbered ear tag, for red deer it was an expandable patterned collar, in wolves it is a collar that sends out a bespoke radio signal that allows us to identify where the animal is, and in the guppies it is a tiny tattoo. After marking, animals are then released back into the wild at the same location they were captured, and each time they are subsequently seen or caught their location is recorded. Coupled with other data that are collected at the time of capture, including blood samples that allow family trees to be constructed for each individual, and phenotypic trait data on attributes such as body size, position in a dominance hierarchy, evidence of parasites, state of the animal�s dentition, and all sorts of other measurements, it is possible to study questions as diverse as the effects of climate change on wild animal populations, and why and how they evolved the phenotypic traits they have.

The reason I was so happy with my decision to work with guppies, wolves and silvereyes I quickly realized was that my knowledge of the natural world was very biased by what sheep and deer do. As I started to work with experts who had dedicated their lives to building incredibly valuable, individual-based studies of other species, I came to appreciate that each individual in each species really is unique, not just in its genetic code but also in how it lives its life. I would never have understood the way that nature works in as much detail as I do had I remained a sheep and deer expert. Those studies were invaluable for my training, and I will always be grateful for the opportunity to spend so long working on these systems. But it was the guppies, the wolves and the silvereyes that allowed me to flourish. Hours spent chatting with research collaborators in Yellowstone National Park, and with Sonya on walks with Woofler in and around Oxford or on visits to Australia, helped me break free from my narrow herbivore focus.

What I started to appreciate from working with each of these very different species is that individuals in each population experienced very different causes of death, and these causes mattered. Wolves tended to die in fights with other wolves or from disease; guppies that lived with predators would inevitably end up in the jaws of a predatory fish such as a goby, while those that lived away from predators would die from a lack of food; while Sonya�s silvereyes would often die during harsh winter storms, from disease outbreaks, occasionally from fights with other birds, or from unknown causes in old age. Just as important as the causes of death were events that prevented some individuals from breeding. In some cases, these events were a failure to lay down enough fat to produce offspring, or a failure to find a mate or breeding territory, or dominant individuals killing the young of subordinates, as happens in the wolves. Within each population, the major causes of death and failure to breed favoured phenotypic traits that reduced these threats, and by understanding what caused death and reproductive failure, and how different traits altered these risks, I could draw generalizations across very disparate study systems by linking their evolution and ecology. Natural selection is all about understanding how different phenotypic traits help individuals avoid things that can kill, while sexual selection focuses on how other phenotypic traits minimize the chances of failing to breed. Individuals with DNA sequences (genes) that code for the development of phenotypic traits that reduce the risks of death and reproductive failure leave more descendants. More descendants mean more copies of genes that code for the production of these beneficial phenotypic traits, and this means these beneficial genes increase in frequency within the population from one generation to the next. Evolution driven by this process has been continually ongoing since the first organism evolved.

Across the 3.9 billion years of the line of descent from LUCA to you and me, our ancestors have faced countless causes of death and things that could stop them reproducing and passing on their genes to the next generation. Yet each of our ancestors survived long enough to produce descendants, with evolution eventually producing us. The same is true for every other individual of every other species alive today. To achieve this astonishing feat, numerous challenges were faced and overcome, with evolution producing ingenious solutions to minimize the myriad risks our forebears faced. As the tree of life grew and life diversified, evolution moulded endless new phenotypic traits to help each species cope with the causes of death and reproductive failures that life threw at it. These phenotypic traits are coded in DNA, with the study of life�s most important acid having helped biologists piece together not just the tree of life but also much of its history.

For many genes, the two alleles are always identical in all individuals within a population, but for others they are not. For example, alleles at genes that control the development of our two arms and two legs are always the same within each of us, while alleles at genes that determine our hair colour can differ. It is genes that have more than one type of allele that make each of us different. The more closely related you are to someone, the more similar your genomes.

The further apart two species are on the tree of life, the more different their genomes are. Time has led to their genomes diverging since they shared a common ancestor. However, not all parts of DNA diverge at the same rate as we move backwards along our line of descent towards LUCA. Some genes have changed very little with time. For example, genes associated with translating the genetic code into proteins are said to be highly conserved, for they have hardly changed in the last 4 billion years. Other genes can differ substantially between even closely related species. Heterochromatin, for example, is an important gene that is involved in switching other genes off, and it differs in very closely related species of fruit fly.

New alleles, and new genes that produce new proteins, arise via mutations to the genetic code. Mutations occur because errors are made when strands of DNA are copied when cells divide. A mutation is a copying error, and it is how the Sars-Cov-2 virus mutated to create new strains.

Mutations appear to occur at random, with many impacting the way an organism develops. These impacts can be mild, resulting in outcomes such as my impaired vision due to a failure of my retina to develop properly. Other mutations can be much more debilitating, resulting in blindness, deafness, premature ageing, early death, chronic pain, Downs syndrome and some types of personality disorder. Deleterious mutations consequently prevent development proceeding as it should. However, a small proportion of mutations are advantageous. They result in phenotypic traits being expressed that allow individuals to better avoid death or reproductive failure. It is these advantageous mutations that have allowed life to thrive, to increase in complexity on some branches of the tree of life, and to conquer the world. Without these advantageous mutations, we could never have evolved from LUCA. However, the cost of life�s success is the deleterious mutations. The cost of our existence is countless premature deaths, suffering and failures to thrive of endless organisms that had flawed self-assembly manuals, which meant they left no descendants. Mutation and selection drive evolution, but because mutation can happen anywhere in the genome it means some individuals lose life�s developmental lottery while a few others win.

The simplest type of mutation occurs when a cytosine, thymine, adenine or guanine is not copied correctly. Such a mutation in one of my ancestors is responsible for my poor eyesight (more on that later). An example of a point mutation such as this might mean a cytosine nucleobase is accidentally copied as adenine at some point on chromosome 14. The new allele that is produced might change a single amino acid in the chain the gene codes for, and this could mean the protein it produces takes a different shape. The protein�s changed structure might slightly alter the way a phenotypic trait develops by slowing down a chemical reaction compared to the non-mutated allele. If the new phenotypic trait gives individuals that carry it a survival or reproductive advantage, the newly mutated allele should start to spread within the population, but most mutations are deleterious and are quite quickly lost from the population.

Single-point mutations, where one nucleobase is switched to another, is just one type of mutation. Other types involve more substantial changes. Perhaps the most intriguing are mutations that arise where �junk DNA� that didn�t produce a protein starts to. For a sequence of DNA to start producing a chain of amino acids, the molecules in your cells that read the DNA sequence and translate the code into a protein need to encounter a start codon that initiates the production of a string of amino acids. There are a number of nucleobase triplets that initiate the production of an amino acid chain, but ATG is a common start codon. There are also codons that tell the machinery to stop. Every now and then a point mutation will produce a new start codon in a stretch of junk DNA that was not used to produce proteins. When that happens, a completely new protein can be produced. In most cases, it will not serve a useful function, but on rare occasions the protein may find a role for itself. Mutations such as these are uncommon, but they can produce completely new phenotypic traits that have never been seen before by life.

We tend to think of viruses as being nothing but trouble, but they can also occasionally be the source of new, useful genes. Viruses, such as the coronaviruses that cause COVID or measles or rabies cannot reproduce on their own. They need to use the machinery of your cells, and they do this by infecting you. Once inside your cells, they hijack the cellular machinery to make copies of their genetic code and the proteins that coat their exterior, before self-assembling as new virus particles. Most cells that are infected by viruses die, but not always. The genetic code of some viruses can end up being permanently spliced into the DNA of an infected cell. If this happens in the stem cells that make sperm and eggs (the germline), then the new gene can be passed on to offspring, and can then be used to make proteins and new phenotypic traits. On most occasions these proteins may not have a useful role, but they sometimes do, and viral genes co-opted by hosts are known to have played a role in the evolution of the placenta in early mammals.

There are even more types of genetic mutation, including events known as deletions, insertions and inversions. Stretches of DNA can get copied from one part of the genome and inserted into another part, while DNA sequences can also get flipped around, appearing in reverse order. Sometimes two chromosomes can join to form a larger new one, while others may split in two. On occasion, the whole genome can be duplicated, doubling the number of genes by making a new copy of each one. Recent research has even revealed genes jumping from parasites into their hosts. DNA is dynamic and subject to change.

The history of mutations in our lines of descent means different species have different genes, and different alleles at some of the same genes, and they also have different genome sizes. The Japanese alpine plant Paris japonica has a genome consisting of 149 billion nucleobases. The human genome is fifty times smaller, consisting of a paltry 3 billion or so nucleobases. Compared to the bacteria Nasuia deltocephalinicola that lives inside insects, this is still vast. Its genome consists of only 112,000 nucleobases, nearly six orders of magnitude fewer than P. japonica. Some species appear to require more DNA than others to live their lives, although, perhaps surprisingly, the size of the genome doesn�t necessarily correspond to the number of genes. The plant Arabidopsis thaliana has 27,416 genes packed into a genome of 135 million nucleobases, while Norway spruce trees have about the same number of genes but a genome of 19 billion nucleobases.

The way that DNA is organized also differs between species. The atlas blue butterfly is the animal with the largest number of chromosomes, with 450 arranged into 225 chromosome pairs. In contrast, females of the jack jumper ant have only one chromosome pair. Males of the species, as in all ant species, have only one copy of each chromosome rather than two. Biologists describe them as haploid rather than diploid as their chromosomes do not come in pairs. Remarkably some species of plant have three or more copies of each chromosome. The adder�s tongue fern can have up to ten copies of each, while even the humble banana is triploid, with its chromosomes coming in three copies. Genome size, the number of chromosomes and the number of copies of each chromosome are phenotypic traits, and so can evolve much like any other.

By comparing genome similarity across species, biologists can piece together which species are closely related, and which are not. However, when an organism dies its DNA can quite quickly break down except for in very unusual circumstances. In the film Jurassic Park, dinosaur DNA is extracted from insects preserved in amber, but even when locked up in hardened tree sap, DNA cannot last anywhere near the 66 million years it would need to survive from the Cretaceous-ending meteor impact until now for us to genetically sequence a dinosaur. The oldest DNA yet found was beneath the Greenland icecap and is about 2 million years old. Because most DNA degrades quite quickly after death, we can only hypothesize about the size, structure and make-up of the genomes of T. rex, the first mammals and the common ancestor we shared with MK-D1 billions of years ago. We may be able to assert, reasonably confidently, that an extinct species had a particular gene if descendants from a shared ancestor with that species alive today all have the gene, but that only takes us so far.

DNA has allowed biologists to piece together the tree of life in astonishing detail, but it is of less use in working out what extinct species looked like. Even if we can assume an extinct species had a particular gene, that does not mean we know with certainty how the gene influenced the species� development or phenotypic traits. The same gene can often be co-opted for multiple different uses. For example, the protein GroEL is found in many bacteria species, and it plays a key role in ensuring that chains of amino acid fold into the correct protein shape. The same protein, produced by exactly the same gene, is also used as an insect toxin by antlions.

The larvae of a type of lacewing are known as antlions, or doodlebugs. These insect larvae dig cone-shaped pits in sand, partially burying themselves at the bottom. Unsuspecting insects, often ants but sometimes beetles, lose their footing on the edge of the sandpit and slide to its base, where the antlion injects them with a paralysing toxin before consuming their prey�s innards and discarding the inedible exoskeleton. The toxin is produced by bacteria that live in the antlion�s salivary gland, and it is the same protein used by bacteria to fold proteins. Many, and perhaps the majority of genes, produce proteins that are put to more than one use, and that makes it hard to know how an extinct species might have used a particular gene or which phenotypic traits it helped produce.

Different species also switch on genes for different lengths of time during development. Fish species that grow proportionally larger fins turn on some genes for longer as they are growing their fins than species that grow smaller ones. Many differences in body shape and organ sizes between mice, monkeys, humans and other mammals are due to different species switching genes on and off at different points in development, rather than because they are using different genes. A gene associated with brain development consequently may not tell you much about how big a brain an extinct organism might have had. We need to turn to fossils to understand the form of life past.

People have been aware of fossils for at least two and a half thousand years. The ancient Greeks realized that fossilized shells were once living shellfish, and when Xenophanes of Colophon uncovered such fossils, he correctly surmised that the land on which he had found them must once have been under the sea. Although many cultures from lands as far afield as China and the Middle East were aware of fossils and appreciated that they were records of long-dead organisms, it was not until the 1840s, when Richard Owen, the founder of London�s Natural History Museum, coined the name �dinosaur� that interest in fossils became mainstream. In retrospect, it became obvious that humans had stumbled across dinosaur fossils for millennia, but had attributed them to mythical monsters, including giants. Richard Owen�s breakthrough was to analyse bones from three different species of dinosaur and realize they all came from an extinct group of reptiles that could grow to enormous size. Dinosaur translates to �terrible lizard�, and once the public appreciated that monstrous beasts really did once walk the Earth, our fascination with dinosaurs began.

Dinosaur fossils are reasonably common, although the vast majority of dinosaur corpses rotted away leaving no trace. Their fossils are common because dinosaurs were incredibly successful, roaming the planet for 165 million years, and some species would have had large populations. Big dinosaurs also had large bones and teeth, and it is the hard parts of animals and plants that are most likely to fossilize. In comparison, individual cells or soft tissues such as muscles, brains and flowers rarely form fossils. Nonetheless, in some cases softer tissues have been preserved. Despite this, there has been a lot of life on our planet over the last 4 billion years that we know little about, even though palaeontologists have got very good at predicting where best to look for fossils from each past geological epoch.

It is not always straightforward to tell whether an animal is an adult of a small species or a juvenile of a larger one. As species develop, their body shape can change radically, and when only a few bones or teeth are fossilized, interpreting them can be hard. Juveniles and adults of a species can differ substantially in both form and size. Emu chicks are hatched at about 5 inches tall, about the size of the world�s smallest flightless bird, and are fluffy and remarkably cute. Yet they grow to a height of five feet seven, with the adults being monstrous and bearing little resemblance to the young.

A few years ago, while visiting my father-in-law�s farm in Queensland, we were brought a day-old emu chick that had become separated from its father. Emus, and other ratites such as ostriches, are unusual for birds in that it is the father that does all the parental care of the chicks. Young birds of most species are not as attractive as the adults, being born without feathers. Emus, chickens and ducks are different, in that their young do have feathers, and the emu chick we were given to care for was cute. And if we didn�t look after it, it would die.

I originally named the emu Emma, but on pondering whether it might in fact be male decided we needed a gender-neutral name. Emma-Steve imprinted on my father-in-law, and he on it, and he raised it on his farm. For months the bird was a beloved pet, and it turned out it would eat pretty much anything, including the keys to the tractor, but it was most partial to steak. Most emus eat a diet less protein-rich than Emma-Steve�s, but given the cattle on the farm, steak was not in short supply. By the time Emma-Steve became an irascible �teenager� at about a year of age, it was the biggest emu we had ever seen, growing to about six feet tall. If we hadn�t known what emu chicks looked like, we would have been in for a nasty surprise.

It turned out that Emma-Steve was a him, and he developed from an extraordinarily cute chick into a violent beast with a strong desire to top the dominance hierarchy. After Emma-Steve chased my daughter Sophie into the house, attacked my father-in-law and generally deterred all visitors, it was decided it was time to take action. He underwent what biologists refer to as a soft release, which meant we left the gate open before shutting it once he�d left. But we would regularly see him around. His large size did him no harm, and on one sighting we were excited to see him with a clutch of eight young.

My father-in-law has raised all sorts of orphaned animals on his farm, from wallabies to stray puppies. He has vowed to never again raise an emu. Emma-Steve�s initial cuteness deceived us, and even though we knew what sort of monster the chick would grow to be, we seemed to forget that in our desire to save the young chick�s life. Juveniles and adults of a species can be very different, but despite the challenge that development poses to palaeontologists, their careful study of the fossil record, coupled with the insights that geneticists studying DNA have produced, means we have a good history of life on Earth.

LUCA was a simple bacterium that used the same genetic code that we rely on but was otherwise very different from us. Attempts to work out what its genome looked like suggest that it probably ran its metabolism on hydrogen sulfide produced by volcanoes rather than by eating plants, fungi and animals. Oxygen would have killed LUCA, while we die without it. Its genome would have been tiny compared to ours, and sex was alien to it, while it is necessary for us to reproduce. There were very many mutations, some of which led to novel phenotypic traits, which helped our ancestors survive and reproduce, that were required for you and me to evolve from LUCA, and these played out over nearly 4 billion years. The construction of the tree of life through the study of DNA and fossils, coupled with key events in the history of our planet such as meteor impacts and periods of extreme volcanic activity, has allowed biologists to work out what had to happen for LUCA to evolve into humans. There is still much to learn, but we know a lot. The process of mutation impacting development with natural and sexual selection operating on the resultant phenotypic traits led to us. The molecular mechanisms involved are often very complicated and are beyond the scope of this book, but I will briefly consider a few of the most important evolutionary innovations on the journey from LUCA to us.

As life secured its footing on our planet it became adept at using a wider range of energy sources than the hydrogen sulfide that LUCA likely relied upon. By at least 2.7 billion years ago, and perhaps as long as 3.5 billion years past, life had found a way to run its metabolism using light. The first bacteria to do this were cyanobacteria, the type of bacteria that produce the stromatolites found at Shark Bay in Western Australia and in the Bahamas. They use photosynthesis, something that all plants, algae and many bacteria rely upon today.

Photosynthesis uses the energy from photons to break apart molecules of carbon dioxide and water before using the freed carbon and hydrogen atoms to build the amino acids needed to make proteins. In doing this, oxygen is produced as a waste product. Although each individual cyanobacterium produced very little oxygen, with billions of them releasing it into the environment over millions of years, all the elements in the Earth�s crust and atmosphere that could react with oxygen eventually became oxidized, at which point oxygen concentrations began to increase in the atmosphere. Unreacted oxygen first appeared in the Earth�s atmosphere 2.33 billion years ago, steadily increasing in abundance over the next few million years until it constituted 3�4 per cent of the atmosphere. This is only a fraction of 21 per cent of the atmosphere that oxygen constitutes today.

The cyanobacteria were the first global polluters, and although life went on to use the oxygen they produced to great effect it was poisonous to most forms of early life. Billions of bacteria were killed by cyanobacteria�s oxygen. But one species� poison is another�s meat, and life found a way of using oxygen. The endless puffs of oxygen from countless cyanobacteria billions of years ago paved the way for you and me to exist. The great oxygenation event in which oxygen increased to 3�4 per cent of the atmosphere was a necessity for our evolution, but it did not foretell it.

An increase in oxygen in the atmosphere allowed a new type of metabolism to evolve, with organisms adapting to use the reactive nature of oxygen to gain energy from glucose, a type of sugar that is used by a very wide variety of life forms. As evolution has shown time and again, when a new resource becomes sufficiently abundant, life finds a way to use it. Once life had mastered how to use oxygen to break down glucose it changed the world beyond recognition, allowing new ways of life and organisms built from more than one cell to evolve.

All animals alive today run their metabolisms off glucose sourced by breaking down large molecules called carbohydrates. Life uses carbohydrates to store energy that can be used at a later date, and for building other large molecules. Every cell contains carbohydrates, so the ability to run metabolism and DNA replication using glucose as a fuel opened up the opportunity for life to thrive by feeding off the living or recently dead. These lifestyles began with single-celled organisms feeding on one another, eventually via endocytosis, but led to predation, herbivory, parasitism and scavenging. Oxygen was consequently a double-edged sword. Not only was it a poison for many bacteria, it also heralded the arrival of a more dangerous world, as the world now contained a new way to die: death by other organisms.

As oxygen levels increased, cells evolved large sizes as more oxygen could diffuse deeper into them. These larger cells required more organizational structure than bacteria and archaea to run efficiently, and internal membranes evolved to help direct and shuttle chemicals around within the cell. These early eukaryote cells could be thousands of times larger than bacteria. Research into the evolution of eukaryotes is a highly active field, with many hypotheses proposed as to how the structures in eukaryote cells evolved. Currently we are not entirely sure how archaea evolved into eukaryotes, but there is one key event worth mentioning. At some point shortly after eukaryotes emerged approximately one and a half billion years ago, a failed predation event occurred when an early eukaryote cell attempted to consume a bacterium. The bacteria survived and it ended up setting up home inside the cell, where it divided to produce offspring. Over time, the early eukaryote and the bacteria evolved a highly effective symbiosis where the early eukaryote provided safety and glucose to the bacteria in return for it running the cell�s metabolism. The bacteria generated the adenine triphosphate, or ATP, that is the end-product of metabolism and is the fuel used to run the chemical machinery in all cells.

Descendants of these bacteria are found in all eukaryote cells, and they are called mitochondria: organelles with their own membranes and DNA. Mitochondria can no longer survive outside eukaryote cells, and eukaryote cells die without them. When our mitochondria cease to function, we die. Something else normally kills us first, but a key reason we age is because our mitochondria wear out. Sometimes the DNA in mitochondria experiences a mutation, meaning they do not work appropriately. When that happens it usually results in an early death. All eukaryotes, including plants and fungi, have mitochondria, with plant cells hosting the descendant of another symbiotic prokaryotic in their cells. These organelles are called chloroplasts, and this is where photosynthesis happens, with light being used to produce the glucose used to run plant metabolism. All complex life on Earth today owes its existence to mutualisms between bacteria and early eukaryote cells that were closely related to strain MK-D1.

Some eukaryote species, including humans, have sex to reproduce. Sex evolved quite early in eukaryote evolution in single-celled organisms similar to yeast. However, not all eukaryotes reproduce by sexual reproduction. In some species, females produce genetic clones of themselves, making males redundant, and this poses a paradox. Every time a clonal female divides, she produces an exact genetic copy of herself. Furthermore, every daughter can produce genetic clones herself. In contrast, every time a sexually reproducing female breeds, on average, only half of her offspring will be female and able to give birth to new descendants; the other half are male and they cannot give birth to new descendants. Each offspring born to a sexually reproducing female also has only 50 per cent of her genome, represented by one chromosome in each chromosome pair. All else being equal, genes in the asexually reproducing clonal strategy would spread through a population at twice the speed of the sexually reproducing strategy. Sexual reproduction would be outcompeted by asexual reproduction, and the sexually reproducing strategy would be driven extinct. Sex is consequently evolutionarily costly, so for it to evolve, there must be some advantages.

Biologists have identified a number of advantages of sex. For example, sexual reproduction allows for evolution to more rapidly rearrange the way a genome is organized, helping populations rapidly adapt to evolving viruses and bacteria. Sexual reproduction can also help populations rid themselves of deleterious mutations. Benefits such as these must have outweighed the costs of sex early in eukaryote evolution, and although sexual reproduction is not ubiquitous, it is the way that most animals and plants reproduce. Having evolved in single-celled eukaryotes, sex was adopted by more complex species once multicellularity evolved.

At some point between 600 million and 1.6 billion years ago, multicellular life appeared. Most life forms, including all bacteria and archaea, consist of a single cell that can survive and reproduce without relying on other cells. Multicellular life such as fungi, plants and animals is made up of lots of cells working together, with none of these cells able to thrive alone. In such organisms, each cell in the body has the same genome, but the cells that develop from it can be classified into different types, with each type performing different functions. For example, your skin cells are different from your nerve cells, which in turn are different from muscle cells, but they all contain the same DNA sequence wound up within a membrane called the cell nucleus.

Evolving multicellularity required two problems to be solved. An organism consisting of multiple cells had to develop from a single initial cell and, once the organism had matured, it needed to produce new single cells that could develop into new individuals. In sexual reproduction, initial new cells are the result of a fertilized egg. Most animals do this by making sperm and egg cells that have only one copy of each chromosome, making them haploid cells. Conception involves one egg and one sperm cell fusing to make a new diploid cell that then makes a copy of itself, before each of these two cells themselves divide to make four cells. The process is repeated until a ball of sixty-four or so cells called the blastula forms. The blastula starts to differentiate into different cell types, with the first differentiation into cells that become the placenta and those that become the embryo. Whether one of the blastula cells develops into a placenta or an embryo cell is determined by the concentration of chemicals produced by neighbouring cells. Those on the outside have fewer neighbours, so experience lower concentrations of some chemicals, and this lower concentration results in some genes being turned on while others remain off. In contrast, higher concentrations of these chemicals in cells deeper in the developing cell ball mean a different set of genes are turned on and off and the cell develops into another cell type.

Once the first differentiation has occurred, the two different types of cells produce chemicals that are often unique to that cell type. These molecules leave cells, and form chemical gradients, such that there are more molecules closer to the source cell type than there are further away. Concentration gradients of many different chemicals throughout the ball of developing cells provide the instructions to each cell as to which genes to switch on and which to turn off, and these determine the type of cell it becomes, and more and more different cell types are produced. Throughout development these chemical signals lead to muscles, kidneys, eyes, ears, heart, fingernails and all the different bits that you are up of arise from the same genome. In remarkable demonstrations of how important variations in chemical concentrations are for development, biologists have manipulated chemical gradients in fruit flies and other laboratory animals by turning specific genes on and off at various ages to produce monstrous flies.

You consist of about 30 trillion cells, classified into about 220 broad types. There is further variation within each type, and some scientists argue we should classify at a finer scale. During development these cell types organize themselves into the different organs and tissues that constitute you. The same genome produces these very different cell types by turning on and off different genes during development and by leaving some genes functioning for longer in some cells than in others. Turn one set of genes on to make proteins and you end up with a cone cell used for vision, turn on another set and you create a fat cell for storing energy. The self-assembly manual that is your genome contains instructions that determine when to turn particular genes on and off during development, creating concentration gradients of molecules that instruct other genes in other cells to be turned on or off.

Animals are the champions of cell differentiation, but plants, fungi and some types of algae such as seaweeds do it too. These other groups produce only a handful of different cell types from their DNA and have far fewer organs than animals. Construction of the tree of life reveals that multicellularity in eukaryotes evolved independently at least ten times. But how did it arise?

Over the billions of years that life was restricted to single-celled organisms, mutation and selection produced an ever-increasing number of useful genes. Bacteria evolved various ingenious ways of sharing DNA, and by this process some single-celled species became masters of multiple ways of life. Different genes would be activated depending upon the environment in which the cell found itself. For example, if oxygen was available to power metabolism, then glucose could be used as a source of energy. In contrast, if it was absent, try another source of carbon atoms. Flexibility to use different genes in different environments doubtless paved the way for the evolution of multicellularity, but it was only a single step of the many that were required.

A key challenge in making the first multicellular organism would have been to clump cells together. The most likely way for this to have happened was for dividing cells to fail to completely separate, such that a ball of genetically identical cells formed. However, that is not the only route to producing a ball of cells. Another hypothesis is that groups of cells from different genetic lineages started to cooperate and, via a mechanism that is yet to be identified, merged their DNA into a single genome. Genetically different cells of some simple species are observed to group together today, so there is some support for part of this hypothesis, but until evidence of genomes merging is documented, this will remain an unproven route. A third idea is that a single cell produced multiple nuclei, each containing a copy of the genome, before membranes separated the nuclei and nearby organelles, producing a group of cells. Such behaviour has been observed in some modern-day species, but it is relatively uncommon. The fossil record does not record processes such as those that resulted in multicellularity, so we may never know for sure how multicellularity arose, but it is possible that across the ten or more independent origins of it, evolution used each of these three methods.

The first multicellular animals likely consisted of a ball of cells with little differentiation between them. Nonetheless, natural selection favoured these simple balls. The first multicellular organisms may have been able to consume single-celled competitors. They may also have been protected from large single-celled predators, and they might have had a mobility advantage. In photosynthetic species, multicellular species could have been superior competitors for light, growing over the top of single-celled species. But there would have been costs too. Resources such as sugars and oxygen would need to be transferred to cells in the interior of the organism, for example, but the benefits outweighed the costs, and multicellularity was here to stay.

Multicellularity does carry several costs. For example, multicellular organisms delay reproduction while they develop, and this means that single-celled organisms can make copies of themselves at a faster rate than multicellular ones. All things being equal, because single-celled species can reproduce more quickly than complex animals, they should outcompete their larger competitors. Multicellular species also inevitably die. Although not an evolutionary cost, it is an undesirable feature of being a complicated organism whose bodies are divided into germ cells and the disposable soma. If the germline has done its job effectively it will be represented in offspring. All of your body, from your limbs to your brain, your eyes to your toes, is disposable soma, existing solely to maximize the probability you will produce offspring, each of which will carry half your genes.

In animals, plants and fungi the benefits of multicellularity must have outweighed the costs. However, in other eukaryotes, such as yeast and amoebae, it was not favoured. Most life on Earth is still unicellular. It is a very effective way of life that has persisted for billions of years longer than multicellular life has been around. We tend to think of multicellular life as evolutionarily superior, but that is not correct. It is just one way that life found to pass genes from one generation to the next. But how did a simple ball of cells evolve into complex animals like you and me?

Fossils reveal that in one multicellular lineage, the simple balls of cells evolved into tiny bowl-shaped animals. The bowl may have been a primitive trap for single-celled organisms that were then broken down by chemicals these primitive animals produced. Different forms evolved in the ancestors of plants that created branching structures, maximizing the surface area that could use light to photosynthesize. Multicellular life started to diversify, much as unicellular life had done, using any available resources it could find. A key step had been taken on the line of descent from LUCA to us.

Over the next half a billion years, more and more complex species evolved. There was a spectacular burst in the evolution of complex animal life lasting 13�25 million years just over 500 million years ago. The Cambrian explosion, named after the geological period that began 539 million years ago and lasted for over 53 million years, was when our ancestors, and indeed ancestors of all animals with a backbone, first appeared in the fossil record. High-tech imaging of sixteen fossils of a species called Pikaia gracilens revealed it had a nerve chord that, over the next few million years, developed into the backbone found in all vertebrates. Like many distant relatives, Pikaia gracilens would not have been an inspiring dinner-party guest, although it certainly would have stood out. It was about five centimetres long and looked like a primitive eel with tentacles on its head. There is a related group of animals alive today called lancelets, which resemble Pikaia gracilens. They live in shallow seas from the tropics to as far north as Scandinavia, and they feed by filtering bacteria, algae and small animals out of the water column. Pikaia gracilens pioneered this way of life that has survived for over half a billion years.

It is not just our ancestors that first appear in the fossil record during the Cambrian explosion. Some of the forms that evolved then have gone on to evolve into mussels, sponges, octopuses and lobsters. Evolution had mastered multicellularity, cellular differentiation and the use of chemical gradients, and during this period evolution produced not only the ancestors of all animals alive today, it also experimented with body forms that failed to survive evolution�s filter. The one species I wish had survived was Tamisiocaris borealis, which measured nearly a metre in length. It was closely related to an apex predator called Anomalocaris, an ancestor of modern-day shrimps and crabs, but Tamisiocaris borealis was altogether a much less fearsome beast. It would not have looked out of place in the seas of Pandora in James Cameron�s Avatar: The Way of Water. It fed on bacteria and algae using delicate bristles on its front arms, or legs, or whatever we should call them, to entrap them. If it was still alive today, it would be a marvellous addition to any large marine aquarium.

The Cambrian explosion was also the geological period when the modern-day animal head evolved. Evolution had produced cells and eventually organs that could sense light, electric fields, touch and sound waves, and likely taste and smell. Primitive brains evolved alongside these early sense organs, and during the Cambrian explosion eyes, ears and noses migrated to the same part of the organism that became a head and contained a brain. Although there were peculiar-looking beasts such as T. borealis, many animals that evolved during the Cambrian explosion would not look entirely out of place in today�s seas, as it was the period when many features of modern-day animals first appeared. But it was to be nearly another half a billion years before individuals of a descendant species of Pikaia gracilens gazed upon its fossils with wonder.

Towards the end of the Cambrian explosion, when animals rapidly diversified in the oceans, seeding the way for vertebrates, plants began to colonize the land, with the earliest evidence suggesting that the first species took root (literally) about half a billion years ago. Bacteria had been thriving in freshwater lakes and shallow seas for many hundreds of millions of years, and single-celled algae would have also colonized the same habitats once they evolved, but the first multicellular land plants came later. The earliest of these species would have grown roots in the sediment of lakes but started to evolve structures that allowed them to grow above the water to compete for light, in a similar way that mangrove species grow today. These early plants next adapted to thrive in seasonal lakes before colonizing areas that were flooded less frequently. As plants spread, organic soils followed, with earth and mud becoming commonplace about 400 million years ago.

Insects evolved a little under 500 million years ago, shortly after the first land plants established, and were the first land animals. The earliest insects looked a little like modern-day silverfish, and were yet to evolve flight. The first fossils of flying insects are 400 million years old, with flight evolving as a way of dispersing to new areas and of avoiding predation from the vertebrates that were by then well-established on land, having left the oceans 450 million years ago. The land plants and insects offered a resource that could not be resisted. A few species of fish had already evolved primitive lungs to gulp air as a way of supplementing the oxygen they extracted from the poorly oxygenated waters in which they lived. These fish may have also used land as a refuge from predators, or even moved short distances over land between temporary lakes or salty rock pools. Some modern-day fish still behave like this. Hart�s killifish jump out of water to escape predators, and they move between streams across the forest floor in tropical South America. Phenotypic traits such as these would have given our primitive fishy ancestors a head start in colonizing land. Over time, limbs evolved from fins, and gills were eventually lost altogether.

As plants thrived on land, they began to compete for light, and being tall was advantageous. To grow upright to large heights, land plants evolved a rigid protein called lignin. They could use this to defy gravity and grow away from the ground below. Lignin is quite a tough molecule. When plants die today, and trees fall in the forest, lignin and other parts of the plant are decomposed by bacteria and fungi, but it can take years. By 300 million years ago plants were using lignin to great effect, and vast forests covered the land. The climate was warm and damp, and oxygen levels were higher than today, with dragonflies and other insects up to a metre in length flying between the trees. The ground beneath the trees was often swampy, and when plants died they were not broken down by bacteria or fungi. Some scientists have argued that this was because fungi and bacteria were yet to evolve enzymes to break lignin down, but this hypothesis is contentious. However, even if bacteria and fungi had evolved these enzymes, they were unable to deploy them in the damp environment of the forest floor, for in those swamps dead plants did not get broken down but instead built up. Over time they became compacted by yet more dead plants and then by sand and soil, and over millions of years the dead organic material became coal. We run our power stations, cars and aeroplanes by burning the remains of ancient plants that past life had found no way to exploit. Animals are yet to evolve a way to digest lignin, but humans have found a way to exploit coal (and oil, which formed from ancient algae and bacteria in shallow seas) and in so doing we are changing our planet. Carbon dioxide levels in the atmosphere have increased to levels not seen on Earth for millions of years, and the energy from long-dead trees is used to power machines that cut down living ones.

Although our addiction to coal and oil is changing the world and is a contributing factor to the very large number of extinctions that have happened in the last 300 years, without the coal and oil that formed in the Carboniferous geological epoch, modern-day civilization would not exist. We would not have built up the vast amount of knowledge we have of the universe, and nor would we have invented computers, cars and electric ovens. Coal and oil are both a blessing and a curse.

The mass extinction humanity is causing is referred to as the sixth great extinction our planet has experienced, although there may have been more. The most famous happened 66 million years ago when a meteor killed off the dinosaurs, but other mass extinctions we know of occurred 440, 360, 250 and 210 million years ago. Scientists can read the devastating impact of these events from the fossil record, and each time a significant fraction of life on Earth died. The biggest mass extinction was the one that occurred 250 million years ago, marking the end of the Permian geological era. It is commonly referred to as �the great dying�. It wiped out over 80 per cent of species in the ocean, and 70 per cent of land-living vertebrates. It was caused by the huge amount of carbon dioxide released by the formation of the Siberian traps we encountered previously.

Mass extinctions often wipe out dominant species, opening the way for a new group to thrive, and the great dying was no exception. At the end of the Permian, our ancestors, a group called the synapsids, were the dominant species on the planet. The ancestors of dinosaurs, called sauropsids, were both rarer and smaller. The great dying killed off many though not all of the synapsids, including the two-ton herbivore Tapinocephalus, the largest animal of its time. It also killed several species of sauropsid, but those that survived got the upper hand in the new world, dominating it for the next 185 million years as dinosaurs, while relegating our synapsid ancestors to a life on the margins.

The synapsids had been the dominant group on Earth for over 100 million years, and, like all life forms, they evolved from an ancestor. The mass extinction prior to the great dying, which happened 110 million years earlier, had given a group of vertebrates called the amniotes a shot at being the dominant vertebrate fauna. Amniotes had evolved from fish and amphibian ancestors, with the first appearing 330 million years ago. They had the advantage of being less dependent on water than their ancestors, which helped them thrive in warm, dry climates. Their name comes from their ability to produce embryos surrounded by an amnion or amniotic sack, a membrane that prevents the developing foetus from drying out. Reptiles, birds and mammals are all amniotes, even though some lay eggs while others give birth to live young. A few million years after the first amniotes evolved, their branch of the tree of life split into the synapsids that were to evolve into mammals, some non-mammalian synapsids that subsequently died out, and the sauropsids that included the ancestors of dinosaurs, reptiles and birds.

Some of the extinct synapsids looked a little like modern-day carnivores, and they shared many traits with modern-day mammals. They were warm-blooded and had primitive mammary glands for provisioning their young, even though they laid eggs. Their jaws contained molars, canines and incisors, and some species appear to have had fur. The big synapsids including Tapinocephalus are not seen after the great dying, but some smaller species survived it, and by 160 million years ago had evolved into the first mammals. These mammals did not immediately go on to evolve into modern-day mammals like rhinos, hippos and humans because the surviving sauropsids, for reasons we do not understand, stole a march on the surviving synapsids following the great dying, growing to become the dominant class.

The dinosaurs that evolved from the sauropsids that survived the great dying roamed the Earth from about 240 million years ago. Like their famous descendant, T. rex, the first dinosaurs walked upright on their two back legs. They quickly rose to dominance and, once there, they enjoyed that position until 66 million years ago. Many dinosaurs were successful large species, and their success prevented mammals evolving to the sizes they achieve today. Mammals couldn�t effectively compete with the tyrannosaurs, sauropods, triceratops and other large species. Interestingly, there were no small dinosaurs. It is possible that the small mammals that survived prevented dinosaurs evolving small sizes, while the large dinosaurs prevented mammals evolving to large sizes.

There are relatively few ways of making a living if you are a vertebrate. You get your food by feeding on other forms of life. Species can feed on vegetation, on seeds or fruits, on other animals they have hunted, or those they find dead, and, in the case of vampire bats and occasionally the vampire ground finch of the Galapagos, on the blood of living animals. No vertebrates are able to survive by consuming decaying plant matter, a way of life termed detritivory, but many vertebrates are omnivores eating a varied diet. Grizzly bears, for example, eat fruit, vegetation, insects, fish and meat. Regardless of what their favoured food is, evolution has produced a plethora of ways for species to detect, acquire and then utilize food.

The first step in finding food is to detect it. Animals have senses to help them do this, and these range from vision, smell, and touch, which are very familiar to us, through to echolocation, used by some bats. Each sense requires a specific organ � eyes for vision, ears for sound, a nose for smell � plus an area of the brain to make sense of the information the sense organs collect. Some food types are easier to detect than others, with it being easier for a bison to find grass than it is for a woodpecker to find bugs in bark. When resources are hard to find, evolution hones sense organs and brain areas to be exquisitely sensitive. Elephants, for example, can smell water from up to 12 miles away, while cheetahs can spot potential prey up to 3 miles away.

Echolocation requires more than just hearing; it also requires the bats that use it to produce sound waves that bounce off objects before they detect them. Their brains then produce a form of map in reflected sound that the bats use to spot insect prey. Although producing sound to detect food is relatively unusual in the animal kingdom, it is not a trick unique to bats. The aye-aye is a lemur, a species of primate, that is found only on Madagascar. It survives on insect larvae it extracts from tree trunks. It finds them by tapping a specially adapted finger on the trunk and listening with its oversized ears until it locates a cavity underneath the bark where a tasty morsel may be hidden.

Once the food has been detected, it needs to be acquired. Some herbivores have it easy, with the bison needing to simply put its head down and graze. In contrast, wolves need to chase down their prey before killing it, while orang-utans need to track down fruiting trees, pluck fruit from them, and sometimes peel the fruit, before consuming it. Killing prey can be dangerous, with buffalo and giraffe sometimes killing lions, and bison mortally wounding wolves. All animals have adaptations to help them acquire their food. Having detected a cavity within a tree, the aye-aye uses its forward-pointing incisors to drill a hole into the trunk before using its significantly elongated middle finger to prise the insect prey from out of its hiding place. Both the angle of teeth and the extension of a single digit on each hand are unique to the aye-aye, being found nowhere else in the animal kingdom. These phenotypic traits have evolved to enable the aye-aye to thrive. Aye-aye ancestors with these traits succeeded in avoiding death and failure to reproduce by detecting, acquiring and using food, and the phenotypic traits, and the genes that determine them, spread through the aye-aye population.

Even if acquiring food can be relatively straightforward, it needs to be done without becoming food for something else. The antelope on the African plains needs to be vigilant for cheetah, lion and wild dog, while the mouse searching for seeds, fruits and insects in the leaf litter must do so while avoiding snakes. The impressive 100 km/h top speed of the pronghorn antelope, the colour-changing skin of the chameleon and the hedgehog�s resistance to snake venom are all adaptations to reduce the risk of death while foraging. Evolution has been inventive, with arms races between predators and prey resulting in remarkable camouflage, staggeringly fast top speeds and senses honed to detect the minutest smells.

Once food has been acquired, it needs to be utilized. Things can now become more challenging for herbivores like the bison, which need to ruminate. They initially swallow the vegetation they consume, before regurgitating it and further chewing it. As they chew their cud, they release an alkaline saliva that counters the acid in their first stomach, the rumen. They then swallow their twice-chewed meal, which can be more easily digested by the chemicals and bacteria in their guts. A bison can spend up to eight hours a day chewing its cud. Energy from grass is hard won. Koalas arguably have it even harder. They survive on a diet of eucalyptus leaves that contain many protective chemicals to keep insects at bay, and this makes them extremely hard to digest. The koala has a longer gut than other animals its size to aid with digestion. The longer it takes for the chewed leaves to move from one end of the gut to the other allows more time for leaves to be digested and goodness extracted. Still, despite this adaptation, the koala sleeps for up to twenty hours a day, during which it does nothing but digest.

Once the food is digested the energy needs to be allocated to all the different tasks an animal must do. Some energy will need to be dedicated to reproduction, some to fighting disease, some to detecting and acquiring the next meal, and some to just keeping the metabolism ticking over. Some species of animals use energy sparingly, eking it out over a long period, while others use it more quickly, often living a �live fast and die young� lifestyle.

The guppies in Trinidad where I work occur with predators in streams and rivers on the floodplains at the base of the Northern Range mountains. Guppies in these streams will inevitably be predated. Individuals do not live long, and the predators keep their prey population sizes small, meaning there is plenty of high-quality food for the guppies in the form of nutritious stream invertebrates. The guppies are well adapted to live in this environment. They reach sexual maturity at a young age, when they breed they produce large litters of small young, they have a very high metabolic rate and are able to swim quickly, and they are very vigilant for predators. Their mouths are adapted to suck prey out of the water column, and their shortish guts allow them to digest their nutritious food effectively.

If you follow one of the streams up into the mountains, you soon encounter a waterfall. Predatory fish are never found above these waterfalls, but guppies sometimes are, yet they are different from their relatives below the waterfalls. Because there are no predators, their population sizes increase until there is insufficient food to go round. Instead of dying by being predated, these guppies starve to death. Their high densities mean high-quality food is scarce, and the guppies switch their diet to include algae and bacteria. The predator-free, low-food environment selects for different phenotypic traits than the high-food, high-predation streams. Guppies evolve new jaw shapes to scrape algae and bacteria from rocks, they evolve slower metabolic rates so they use energy more efficiently, and their gut becomes longer to allow them to extract energy from their less nutritious diet. Because they do not need to sprint away from predators, their maximum swimming speed is slower. They grow more slowly than their high-predation cousins, reach sexual maturity at a greater size and age, they produce fewer, larger offspring, and their maximum longevity increases. In the absence of predators, males become more brightly coloured, and they compete more intensively for females. Numerous phenotypic traits change, and the fish, although clearly still guppies, differ considerably between predator-rich and predator-free streams.

These changes have occurred because different genes are switched on and off at different times during development. Natural and sexual selection has moulded the guppy to thrive in environments with very different causes of death and reproductive failure. It does this by favouring sets of phenotypic traits that help guppies avoid an early grave and a failure to breed in contrasting environments, and as these traits evolve, different alleles on different chromosomes become favoured and high and low guppy populations start to genetically diverge. In time, perhaps, a new branch of the tree of life will emerge. The guppies are a microcosm of how evolution has sculpted life over the past 4 billion years.

In this chapter I have explained how competition for resources is the driver of evolution, and how on our limb of the tree of life it has produced complex multicellular organisms that sexually reproduce. I have also given a high-level summary of how animal life has emerged and evolved until mammals inherited the Earth. Along the way, I have summarized how evolution is a powerful process that has created strain MK-D1, E. coli, aye-ayes, guppies, wolves and you and me. Each species is a success story, surviving in a world where death or reproductive failure is never far away. The ancestors of each organism alive today have survived periods of food shortage, landscapes rich in predators, cold climes, and warm ones. They have lived in water and on land and have survived volcanic eruptions and meteor impacts. They have found many ways to thrive, some as single-celled wonders, others as complex animals consisting of trillions of cells. In complex animals such as you and me, we have evolved keen senses to detect, acquire and efficiently utilize a wide range of food, and phenotypic traits to allow sexual reproduction.

For mammals to exist, multicellularity, sex and a whole host of other attributes I do not mention had to evolve. These characteristics all evolved in response to the environments that ancestral species on our branch of the tree of life experienced. Modern mammals are a product of billions of years of evolution of competition for resources in both cold and hot environments, as well as wet and dry ones, in places where competition was sometimes intense and sometimes a little more relaxed. At times the journey was driven by volcanoes, earthquakes and asteroid and meteor collisions, and at others by predators, disease and limited food. Sixty-six million years ago, mammals became the dominant animal species on land. They spread and diversified, and the next step on our journey is how they became human. But before I discuss that, there is an important phenotypic trait I am yet to mention: consciousness. And that is the topic of the next chapter.

Consciousness

If you were to encounter an intelligent alien, they might ask you what it is like to be human. I suspect they are much more likely to ask such a question than undertake the type of orifice examination reported by many who claim to have been abducted by little green men. Would the first thing you do on encountering a sentient species from another planet be to poke your finger into any openings it may have? Probably not. So how might you answer questions on what it is like to be human? You might talk about your body inhabited by your mind, where your mind is the part of you that reasons and thinks. You might say that your mind is what makes you you. That would be a sensible response given our minds feel special and unique, and for some people too good to be temporary as they believe their minds live on after death. Observation forces us to accept that when we die our bodies serve no function, and decay, but it can be harder to accept that our minds die too. We might fight the inevitable ravages of age on our bodies by eating healthily, exercising daily and avoiding guilty pleasures that doctors tell us are bad for us, but it can sometimes seem inconceivable that our minds will wear out and die. Alzheimer�s disease and dementia demonstrate that our minds can, and sometimes sadly do, fall apart like the rest of our disposable soma, but these illnesses do not stop many people believing their minds will live on post death. Our minds feel special and a little bit different from our bodies.

One reason that makes the mind seem special is we have been unable to locate it in the way we have the heart or spleen. Unlike a kidney, liver or lung, there is no organ called the mind that we can dissect out of the body and study. There is no mind transplant procedure, and even though science fiction writers describe futures where minds can be downloaded on to a USB stick and uploaded into a computer, this is not going to happen any time soon. Given we can�t locate the mind, how could it decay along with our brains, skin and muscles? However carefully we dissect the human corpse, no mind has been found, so perhaps it moves on after death. Personally, I don�t subscribe to this view. My mind resides in my brain, the organ (or more accurately the set of organs) found in my skull, and upon my death it will no longer exist. My brain, and therefore my mind, is part of my disposable soma and none of it has life after death. I die and the lights go out. Of course, this makes me sad, but it does raise another question: why do we have minds, why are we aware of our thoughts and existence? Is it simply the universe�s way of marvelling at its own magnificence, or just an accident of nature? Can we study the evolution of the mind in the same way we can study the evolution of any other phenotypic trait?

The hard problem of consciousness, a term coined by David Chalmers in 1995, is concerned with why we experience anything at all, and it has been central to consciousness research for the last few decades. The problem is wryly named, because all aspects of studying the brain and consciousness are hard; there are no easy bits. But the hard problem was seen as particularly difficult. Put another way, it asks, would it not be possible for a conscious-less zombie to behave just like us, doing everything we do but experiencing nothing? The hard problem is hard because it assumes that even if we get to a point where we understand everything about how the human brain works and are able to build an accurate computer model of it, the question of why we experience life would still remain.

What it feels like to be human is important, and perhaps the most important part of our existence. Given this, it is no surprise that researchers have been keen to understand why being human feels like it does. An obvious follow-up question is what does it feel like to be a bat, cat, or rat? We know the structure of the brain of each species holds the key to answering this question, and we also know that being conscious is the process that gives us the sense of self. Understanding how our brain works, and how our brain differs from those of other animals, has helped scientists begin to understand consciousness and why we have minds.

An influential scientific paper on consciousness by the philosopher Thomas Nagel has the great title, �What Is It Like to Be a Bat?� The paper starts by assuming that most animals have some degree of consciousness, takes the position that we can�t understand consciousness by breaking it down into physical processes such as interactions between cells in the brain or the workings of genes, and concludes that it is impossible to know what it is like to be a bat, but, because bats are conscious, it must be like something. The paper was published in 1974, and much has been discovered since. Psychologists are now confident that bats do have a degree of consciousness, and these researchers also have a reasonable understanding of how a bat�s brain works. But we still do not know what it feels like to be a bat. There are probably ups and downs, good times and bad, with bats experiencing a sense of pleasure and of pain. I doubt they feel anxiety about the fate of the insects they eat while pondering the pros and cons of vegetarianism (I�m assuming they�re not fruit-eating or vampire bats), they are probably unaware that death awaits them, but they take actions to avoid predators, and although they communicate with one another up to a point, they do not lecture one another on the wondrous achievements of bats past.

Humans

At this point in our journey, mammals have evolved, and these mammals have brains that have given them a degree of sentience. These brains consist of neurons wired to form enormous networks that allow at least one mammal species � us � to understand and remember things about the world. The stage is now set to consider how humans came to be, and how they conquered the planet. We are an important species, but we are still just a species. One of many hundreds of millions to inhabit the Earth.

Every now and again a species � or group of species � evolves that changes the planet. Humans are one of these, but by no means the first, and they are unlikely to be the last. Species of Cyanobacteria changed the planet billions of years ago via oxygenating the atmosphere. The first eukaryote species heralded an era of complex organisms. The first predator created a way of life never seen before, and the first land plants opened up the ability of life to colonize continents. Humans are just another species in a list of those that have changed the planet. We are an unusual species in that we have altered the planet very quickly, but we are a product of nature just like every other. The evolutionary biologist Olivia Judson has a nice way of describing epochs of life based on the way organisms acquire their energy: �geochemical energy, sunlight, oxygen, flesh and fire�. Humans are one of the first, but probably not the first, species to use fire as a form of energy. I focus on humans in this chapter because this is a book on why you and I exist. We are special because we have unusual attributes, but we are not so special we should think these traits mean we are in some way separate from, or above, nature. We are not.

The next step on our journey from the Big Bang to us involves understanding how modern humans, that have changed the world, came to be. You already know how evolution proceeds, so the next step is to consider how it moulded modern humans. Mammals needed to evolve from being small, solitary beasts into primates, and then into us. This required the evolution of several traits, and the ability to develop technology of increasing complexity, and that is the focus of this chapter. But before we start, let�s consider the end point � modern humans.

One thing that we do as a species that is unique is create ways to make our day-to-day lives frustratingly difficult by putting in place layers of bureaucracy. You might think that because universities are full of supposedly clever people they might run smoothly. You�d be wrong. The University of Oxford where I work is administratively complex, but I am sure that any organization that has been around for over 800 years would have generated enough red tape to drive even the career bureaucrat round the bend. In October last year, a colleague�s computer went missing during an office move. It was never found, and quite understandably she urgently wanted a replacement. She was told she could have one but would have to order it from a specific company. The company didn�t have any in stock and had no idea when they might be able to supply it, but it would likely take a few months. A local business could supply the desired model for the same price within twenty-four hours, but my colleague was not permitted to buy from that business as it was not an approved supplier. The process to approve a new supplier can take several months, the outcome is not guaranteed, and as far as I can tell appears to be entirely random. Almost equally astonishing is that six different people have to approve any expenses claim I put in. I have no idea why this degree of scrutiny is necessary, but wouldn�t be entirely surprised if it goes back to a ruling by Ralph of Maidstone, who was chancellor in 1231.

The head of the University of Oxford is the chancellor, which these days is a largely honorary and ceremonial role. His deputy is the vice-chancellor, and she is responsible for the day-to-day running of the university. She chairs a partially elected board of trustees that challenge or endorse programmes of reform. A group of pro-vice-chancellors report to the vice-chancellor, and those with portfolios are responsible for research, education, people, buildings or financials. Research and teaching are conducted within departments that group people by subjects such as History, Biology or Economics. Within these departments, various ranks of professors are responsible for delivering courses and conducting research, and they are managed by heads of department, such as me, who are ultimately responsible for hitting budgets, the successful delivery of taught courses, health and safety, and various other aspects of administration. Departments are grouped into four divisions called Humanities, Social Sciences, Medical Sciences and Mathematical, Physical and Life Sciences, and this level of organization is responsible for budget setting and strategy in these four broad areas. Thirty-nine independent colleges, some of which are extremely ancient and wealthy, admit students to courses, with many university-employed academics having second contracts with a college to deliver tutorials in support of the course the students they admit are studying. Staff employed by the university can join Congregation, which is the sovereign body of the university. Congregation can overturn decisions approved by the trustees, the vice-chancellor and the pro-vice-chancellors, but only occasionally does so. No other university is organized like Oxford, and although it has employed leading academics for over eight centuries, I am confident none of them would have advocated for the governance structure we have. A question that has long intrigued me is, how did evolution take us from a solitary mammal with a small brain to intelligent, highly sentient beings that work in complicated organizations and live in diverse societies that are so complex we do not really understand how they function?

Humans are the only species to have formed nation states and complex organizations with structures like the University of Oxford. We have been able to do this because of our abilities to think and communicate abstract ideas via language, coupled with most people�s general acceptance of the laws of the nation state in which they reside, however crazy they may be. In this chapter I explore how this astonishing social and organizational complexity arose.

The first hominins, species in the same evolutionary family as humans, walked the planet about 6.5 million years ago, while the first cities were formed about 9,500 years ago, and it is the period between these dates I focus on in this chapter. Once humans had developed cities, art and writing, the rest is history (literally). There are very many good books on this topic, and I do not attempt to summarize human history over the last 9,000 years or so, instead focusing on the journey from the first hominin to the first city. A key role of science in piecing together what happened between the first hominin and the first city is in ageing bones, teeth and artefacts. Palaeontologists and archaeologists have pieced together this history from bones, artefacts and genetics, and although a new find can lead to a revision of this history, we now have a reasonable account of the journey from a fruit-eating primate to a space-age ape.

You and Me

I have so far told the narrative of how the universe, the Milky Way, our Solar System and the Earth came into being. I have explored physics, chemistry and biology, the emergence and spread of life, the evolution of consciousness and the rise of humanity. I have covered 13.77 billion years of the past, and we are nearing the present day and the end of this extraordinary journey. Then I can answer the question I posed at its beginning: were we inevitable at the birth of the universe, or are you and I just incredibly lucky? As the history has unfolded, things have become ever more localized as we have moved from the history of the entire universe to the rise of the first civilizations in a small corner of our planet. The last step is more local still, and it involves exploring why you and I are as we are.

I face two challenges in navigating this part of the journey. First, I do not know you. One reason I have described a little about my history is so you have an idea about the sort of person I am. I focus on me because I cannot answer questions as to why you might be short or tall, optimistic or pessimistic, shy or outgoing, why you might be scared to snorkel but happy to jump out of a plane, or why you might have remained single or are on your fifth marriage. Second, it is challenging to identify the cause of particular characteristics within one person. At the level of the individual, science can struggle to explain why you are anxious or relaxed. Answering questions about individuals requires understanding the detailed genetic and developmental mechanisms that have made you an introvert or an extrovert, shy or bombastic, and currently we do not fully understand these mechanisms. Nonetheless, scientists have made progress. They know that our genes, our experiences and chance all contribute to our characters, and the focus of this chapter is on these three processes.

Because I do not know you, I have put myself and occasionally my wife Sonya under the spotlight throughout this book. Before I dive into the topic of this chapter, I will briefly summarize key aspects of my journey. My childhood was happy with supportive parents, but I didn�t enjoy school much. Despite this, there was much to read and satisfy my curiosity at home, which meant I did a lot of learning outside school. A spell in Africa as a young man changed the course of my life and inspired a desire to understand why I exist. Because I had little idea of what to do for a living, I did a Ph.D., and followed an academic career trajectory; I still study the ecology and evolution of animals. The job involves publishing academic papers, although between 2017 and 2024 I took on leadership roles with the University of Oxford, which means I have sacrificed my research to focus on building a legacy for current and future biologists at Oxford. Sonya, my children and Woofler are very dear to me and are a constant source of happiness. This is my first book, and I wrote it without much of a plan on what to do once the first draft was finished, but I found a wonderful agent and publisher, and that is why you are reading it now. The reason I have told this personal story will now hopefully start to become clear: I want you to know some of these details to illustrate how experience, genes and chance can influence us all.

Lots of measurements can be taken from individuals such as adult height, birth weight, how extrovert we are, anxiety levels, sprinting speed at age twenty and intelligence. Your birthweight might be six pounds five ounces, and your sprinting speed at age twenty might be 15.6 mph, and these are called phenotypic trait values.

Values of any phenotypic trait vary across individuals within a population. You were probably a faster runner than I was at age twenty, but you are likely to be shorter than I am. I can say this with confidence because I am below average in running ability, but I am taller than the global average height. I am five foot eleven, while the global average for men is somewhere between five foot seven and five foot nine.

If the values of phenotypic traits such as running speed at a particular age, the strength of the immune response you mount to fight an infection, or anxiety level are plotted against how frequently they occur within the population, a bell-shaped curve often describes the data well. A curve of this shape is sometimes referred to as a normal or Gaussian distribution. Carl Friedrich Gauss was the mathematician who first described this curve, and it is often named after him. The reason for the bell shape is that there are few individuals with either very small or very large values of a trait of interest, with most individuals falling in the middle of the distribution. If we take running speed as our x axis, with faster runners appearing to the left of the Gaussian distribution and slower ones to the right, Usain Bolt will be far to the left-hand tail of the bell shape because he was an exceptionally fast runner as a young man. I would be further to the right, beyond the highest point of the bell. The highest point of the curve is the average, or mean, value, while the width of the bell measured at a particular point on the graph is called the variance, and it quantifies the degree to which individuals in the population differ from one another. Scientists then identify factors that might explain some of these differences between individuals, and for phenotypic trait values these factors can be split into three classes: differences in our genes (nature), differences in the environment we experience (nurture), and something called developmental noise that is currently unpredictable and is thought to be due to chance.

The statistical approach of explaining variation in phenotypic trait values allows scientists to make statements such as �smoking shortens lives�. There is irrefutable evidence that this is true because on average within a population life-long smokers do not live as long as those who have never smoked. However, this does not mean that smoking will lead to an early death for everyone who consumes a packet of twenty cigarettes each day. The average smoker dies younger than the average non-smoker, but we cannot accurately predict at what age any individual will die, regardless if they smoke or not. Some smokers live to old ages, avoiding the smoking-related illnesses that can kill many other smokers, and some non-smokers die young because they develop lung cancer despite never having dragged on a cigarette, cigar or pipe. Scientists can make statements about what happens on average within a group of individuals, but they cannot say with certainty what will happen to an individual. In the same way, we can say that traumatic events early in life on average result in introspective adults, but we cannot state that a child who experiences such an event will become introspective. They are just more likely to do so. It is frequently impossible to say with complete confidence that the reason a person has a particular value of a phenotypic trait is down to nurture, nature or chance. It is consequently impossible to state definitively why each of us is the way we are, but we can identify probable causes and roles for genes and the environment.

I have already described genes and summarized how they work, but it is worth briefly recapping. A gene is made up of long strings of molecules called nucleobases of which there are four types, A, T, C and G. Reading your genetic code would rapidly get monotonous as it is a very long string of these four letters. Nucleobases are grouped into threes, with each group of three instructing the molecular machinery in your cells to grab a particular type of amino acid � a type of molecule made from carbon, oxygen and nitrogen atoms arranged in a particular configuration that enables them to form chains.

Imagine the first three nucleobases in a gene are CCC. The triplet tells the molecular machinery to grab an amino acid called proline from the molecules in your cells. The next triplet in the gene reads GCA, which codes for the amino acid alanine. The molecular machinery selects an alanine molecule and joins it to the proline molecule, with this process continuing until a stop sequence such as TGA, TAA or TAG is encountered which tells the molecular machinery that the protein�s production is complete. The molecular machinery that translates the genetic code to a protein contains lots of steps involving many types of molecules that I do not describe here.

The chain of amino acids then folds itself to create a protein. Proteins run our metabolism and are used to build our bodies. Something as simple as a change in a single amino acid in the chain can alter how effectively a protein does its job. For example, if the nucleotide sequence of a gene is changed even just a little, the sequence of amino acids in the chain may change, and the effectiveness of the protein produced may be impacted. If a nucleotide triplet CCC were to change to CAC, a proline in the chain would be replaced with a histidine and the chain might no longer fold as it should. When a mutation happens that changes the sequence of amino acids in the chain, the protein might stop working altogether, its function may be slowed, and in occasional cases it may even work more efficiently. When the order of amino acids in a chain is altered due to a change in the sequence of nucleobases, a genetic mutation has occurred. Each of us has different mutations � different variants of the human genetic code � and these differences can contribute to differences between us in phenotypic traits such as how outgoing we are, our birth weight, adult height, or our ability to run 100 metres quickly at age twenty.

I know of one genetic mutation I have because it prevents a protein functioning properly. I have a mutation in a gene called GPR143 that produces a protein with the snappy name of G-protein coupled receptor 143. The protein consists of 404 amino acids, and in my case at least one of these is not what it should be due to a mutation. I do not know how the mutation I have alters the order of amino acids in the protein the gene codes for, but the protein does not function as it should. In the database of human genes and what they do, GPR143 is described as encoding for �a protein that binds to heterotrimeric G proteins and is targeted to melanosomes in pigment cells. This protein is thought to be involved in intracellular signal transduction mechanisms. Mutations in this gene cause ocular albinism type 1, also referred to as Nettleship�Falls type ocular albinism, a severe visual disorder.� In more accessible language and drawing on descriptions of how the eye develops, this means that when the protein works correctly it acts to add colour to a small group of cells in the back of the eye called the fovea. My fovea lacks colouration, leading to a genetic condition known as ocular albinism. Such a small effect � a few uncoloured cells � might sound trivial, but it does mean my eyes wobble, and this is the cause of my poor vision. My eyes do this because the colour that most people have in the fovea helps stabilize their vision, and my vision is not stabilized as it should be. My mutation means I have about 25 per cent of normal vision, and this means I can only read down to the third row of the eye charts at the opticians. My eyes have always wobbled and always will. It hasn�t had too many detrimental impacts on my life other than that my eyesight is not good enough to allow me to drive safely. I must live within cycling or walking distance of work, and I am always the designated drinker if Sonya drives us to a social event.

The mutation I have means there is an error in my genetic code. Your genetic code is a self-assembly manual on how to build you, and the human genome is the generic self-assembly manual on how to build a person. My instruction manual to assemble a human eye has an error in it. Most of the instructions are correct, but the bit that says �now add colour to these cells� is missing. My copy has a typo in gene GPR143, and this means my eyes did not develop as they should. To understand what this means I turn to the science of developmental biology, the science of describing how we grow and how organs such as our heart, kidneys and eyes are formed.

Complex multicellular organisms such as you and I start out as a single cell that then divides. Such cell divisions keep occurring until a small ball of a few tens of identical cells exists. From that point onwards, different cells start to develop in different ways. Those on the outside of the ball will go on to develop into skin, while those in the centre will become our internal organs. The way that cells �know� how to develop is due to chemical signals. The concentrations of these chemicals determine when to turn particular genes on or off, and this determines how each cell develops and whether it will become a nerve cell, a muscle cell, a neuron in the brain or a blood cell in the veins.

As development continues, cells start to move around within the developing embryo, migrating to particular areas. In the brain, for example, the end of a neuron is guided into its final position, where it joins with other cells by means of things called neuronal growth cones, that are a little bit like molecular tractors that move along concentrations of particular chemicals in the developing brain. These chemicals act like signposts, telling the molecular tractor to keep going, head right or left a little, or stop.

By the time we are fully grown, our body consists of over 30 trillion cells. Many of these cells migrated into their final positions within the developing body following gradients of chemicals. These chemicals are produced by genes, or by reactions involving the proteins that the genes code for. Your genome does not contain a map of where each cell in your body will be and how they link to one another. Instead, it consists of a set of instructions for when each protein should be produced in each cell type, and when the rate of production of that protein should be turned up or down or turned off. These on and off instructions result in gradients of chemicals in your developing body, and these chemicals allow your body to construct itself. Your genome codes for instructions that state �turn on if such-and-such a chemical is below a certain concentration, and turn off otherwise�.

Development is complex, and this means that most phenotypic traits are not determined by a single gene. Your adult height is determined by how long and how quickly your bones grew. Tall individuals have produced longer bones than those that are smaller because their bone cells continued to divide for longer during development or they divided more quickly. Growing bones is a complex task, with genes controlling the rate at which they develop, the directions in which they grow, the shapes they develop into, and the length of time they grow for. Your height is consequently not determined by one gene but by very many that control all these aspects of bone growth. There are many genetic ways to be tall or short, thin or fat, funny or serious.

When scientists discovered genes they did not have this understanding of how they control development. Early in the genetic revolution, when scientists discovered they could read DNA sequences, some biologists thought they would find single genes for nearly all our phenotypic traits. A gene for intelligence, another for life expectancy, and a third for the ability to play cricket. Some researchers dreamed of using genetic engineering to make people smarter, longer-lived, or better batters at cricket or baseball. Many benefits could be seen if genes were to work like this, with all sorts of diseases potentially becoming treatable. Perhaps a murderous psychopath could be turned into a thoroughly decent chap following some genetic engineering. But it was not to be. As we have learned more about the way our genes work and our bodies develop, we now see our genomes as self-assembly manuals, explaining how to build a human from a single, fertilized cell. There is no genetic homunculus determining your exact developmental trajectory from conception. The values of most phenotypic traits are controlled by lots of genes, and my genetic condition, ocular albinism, is unusual in that the extreme phenotypic trait value I have for my eyesight can be attributed to a single gene. Because only one gene causes ocular albinism, it could be a candidate for gene therapy. Despite this, my mutation does not operate completely independently, and although all males who have the mutation develop poor eyesight, we are not all equally partially sighted.

There are many members of my extended family who carry the mutation in the GPR143 gene and several men who have poor eyesight because of it. Although we all have worse than average eyesight, some of us can see better than others. The mutation in the GPR143 we have means we all lack pigmentation in our fovea and that impacts our sight, but other genes involved in the development of the eye mean that the impacts of the mutation on our eyesight differ between us. These other genes contain instructions on which types of cells to produce and where they should be positioned. No gene works independently when building a body or a phenotypic trait such as adult height, and this means that identifying how a particular gene influences the value of a particular trait is usually very hard. The reason that biologists know GPR143 influences vision is that all males who carry my mutation have eyesight that is a long way from the average of the bell curve, and geneticists have worked out what the protein the gene produces does. Biologists have also worked out a lot about how the eye works and have shown how important colouration in the fovea is for normal eyesight.

The self-assembly instruction manual for a human body is so complex, and involves so many steps and chemical gradients, it is perhaps not surprising that if you run the identical code multiple times it does not produce exactly the same result. Variations in chemical concentrations that we cannot predict caused by molecules bouncing around off one another within and between your cells as you develop mean that if multiple bodies are produced by the same genome, they are not completely identical but can have different phenotypic trait values. Biologists refer to these unpredictable impacts on development as developmental noise, and they have produced a number of clever methods to explore the impact of chance on determining our phenotypic traits. For organs that come in pairs like the kidney or feet, biologists can compare how these differ within individuals. Many of us have better eyesight in one eye than the other, with my left eye being slightly less bad than my right. Each eye develops in the same way, using the same genetic code and developmental steps, so any differences are due to events that we cannot predict and that consequently appear to be random. Biologists can also use differences in phenotypic traits between genetically identical twins to investigate the role of developmental noise.

Identical twins have identical genomes and they often look very similar, but there are differences between them that their parents are usually very quick to spot. These differences are due to developmental noise caused by small differences in the concentration gradients that developed within each twin as they grew. Such developmental variation means that if you were to create a hundred clones of yourself, they would not be perfectly identical. They will each have developed in slightly different ways, with each having a slightly different height, or face shape, or brain structure. Some traits seem to be more susceptible to developmental noise than others, with some psychologists claiming that some bits of personality may be strongly impacted by developmental noise. A hundred identical clones of you might vary in their degree of extroversion, with their individual scores producing a bell curve, with one or two being extrovert, one or two being shy and the rest being somewhere in between.

I have described how two processes, genetic differences between us and chance, can influence who we are, but there is a third process to consider, the environment. The term �environment� captures a lot. It includes, among other things, the country where you were born, the month and year of your birth, the culture you experienced while growing up, the altitude at which you spent your youth, your socioeconomic class, the amount and types of food you ate as a child, the people you grew up surrounded by, and events you have experienced. Identifying which aspects of the environment to measure and to focus on in scientific research is hard.

In most scientific studies the environment is characterized quite coarsely, using variables like class, the religion of the household you grew up in, the number of siblings you have, whether you are the oldest or youngest sibling or somewhere in the middle, nutrition during development, or the cohort of individuals you spent your formative years with. A typical study will correlate values of a phenotypic trait with aspects of the environment such as these. As an example, many studies have found birth order correlates with aspects of personality. In this case, some measure of personality such as anxiety level is the phenotypic trait, and birth order captures an aspect of your environment as a child as it correlates with the amount of attention your parents were able to give you. The firstborn child tends to be conscientious and cautious, while the youngest is more fun-loving, attention-seeking and manipulative. Those in the middle are more rebellious and have a large circle of friends. These differences are thought to arise because the oldest, being the first, gets lots of parental attention. Each middle child receives a little less, with the youngest having to compete the most for attention against its older siblings that still require parental care. Although these effects are undoubtedly real, you shouldn�t read too much into them. Some firstborn children are rebellious and manipulative, and some last-born offspring are conscientious and cautious. Your birth order means you are slightly more likely to have a particular personality trait rather than that you will. Birth order explains only a small part of why people differ, with scientists saying it explains only a small proportion of variation in personality between people.

Although a single characteristic of the environment such as amount of parental attention explains only a fraction of variation in phenotypic trait values such as anxiety level, this does not mean the environment is unimportant. Instead, what such results reveal is that no one aspect of the environment, such as birth order, explains many of the differences between us. When scientists include many measures of the environment into analyses, such as birth order, the type of school you went to, how many overseas holidays you went on with your parents, or the ages of your parents, the total impact of the environment on phenotypic trait values increases. The environment is devilishly complicated, it is hard to quantify, but for many phenotypic traits it plays a key role in how they develop.

The genetic code is long and complex, development is complicated and involves many steps, and the environment is a multifaceted web of lots of different things. As if these drivers of phenotypic differences are not complicated enough, things become even worse when the influence of some aspect of the environment on phenotypic trait values depends upon your genes. Biologists refer to these effects as gene-by-environment interactions.

Smoking is something that is bad for you, and exposing your lungs to cigarette smoke is part of the environment you experience. On average, smokers do not live as long as non-smokers, in part because they are more likely to get various types of cancer, including bladder cancer, as well as the more obvious lung cancer. A smoker is at least three times more likely to develop bladder cancer than a non-smoker. However, not all smokers face the same risk of developing bladder cancer, with the chance of a smoker getting it being dependent upon the alleles they have at a gene called NAT2. Smokers with some genetic mutations at NAT2 are about three times more likely to develop bladder cancer than those with a normal copy of the gene. Your genetic code at gene NAT2 influences the likelihood of poisons in cigarette smoke causing a mutation in a cell in your bladder that makes it cancerous. Your genetic code is said to interact with an aspect of the environment, influencing the chances of developing a nasty disease.

Much of my focus in this chapter so far has been on physical traits such as eyesight or running speed at a particular age, but we also have personality traits. When we describe ourselves, we often mention things such as whether we tend to get stressed, are laid back, or have a fear of spiders. These aspects of our personality are also phenotypic traits, and they too can be influenced by genes, the environment and chance. We are often intrigued by why one sibling is shy and another outgoing, or why we are anxious but our partners are not. The reason is down to nature, nurture and chance, with nurture a function of the environments you have experienced. Despite the complexity of personality traits, psychologists have made progress in exploring the role of genes, chance and the environment on our personalities.

Personality traits come from the structure of our brains and the way brain cells link to one another via synapses. I discussed synapses and how they are involved in the ways our brains work when I explored consciousness. Brains are extremely complex, consisting of between 80 and 100 billion neurons linked by upwards of 600 trillion synapses. It is these linkages that determine how we experience the world, what we remember, how we think and why we have the personalities we do. Psychologists who study the brain do not have a mechanistic understanding of why the networks created by synapses between brain cells make some of us extrovert and others introvert, make some people scared of spiders and others indifferent towards them, or indeed any aspect of our characters. Despite this, psychologists have been able to work out which parts of the brain are associated with particular personality traits, and that genes, chance and the environment all impact our personalities much in the same way that they determine our physical phenotypic traits.

Some of the first breakthroughs in understanding brain structure were achieved by studying how unfortunate accidents have damaged parts of the brain in some patients and how these injuries have altered their personalities. Scientific papers on these cases typically refer to patients by numbers to protect their true identities. In one paper, scientists report how a thirty-year-old man referred to as Patient 2410 suffered a brain bleed towards the front of his brain during surgery. Before the injury his wife described him as short-tempered, often angry, and mopey. The surgery changed him. Afterwards he was much more laid back and spent more time laughing and joking. Another example of a positive personality change was seen in Patient 3534. At age seventy parts of the front of her brain were damaged when a tumour was removed. Her husband, who had been married to her for decades, described her as being stern, irritable and grumpy pre-surgery. Post-surgery she was happier, outgoing and chatty. Sadly, brain injuries do not always result in increases in happiness. They often turn once happy people into more troubled and aggressive beings.

Brain-imaging studies, where patients lie motionless in large machines which create 3D models of their brain, have allowed psychologists to identify parts of the brain that are active when a person thinks about a particular topic or event. Such work has revealed that groups of neurons in different parts of the brain are linked and are associated with positive and negative feelings, anxiety, despair and happiness. These scans have also revealed there are many differences between each of our brains, with no two brains being the same. We are each wired a little differently, with this wiring due to difference in our genes, our experiences and developmental noise.

Our brains, and to some extent our personalities, change with age, and this is because the way our brains work is not constant, as networks of neurons can be repurposed. Many of us will know children who were shy and reserved but who developed into outgoing adults. We may also know outgoing individuals who have become more introverted following a traumatic event. These changes occur because the role of neuronal networks in your brain is a little different from year to year. Some synapses linking neurons are reinforced by experience, while others fade if they are rarely used, with others being completely repurposed. Although we cannot predict how a particular network of neurons will result in a personality trait, we do know our brains are flexible, and that who we are changes from one month to the next as connections in our brains change as we experience the world around us.

These experiences are a function of the environments we encounter. As previously mentioned, for other physical phenotypic traits, environment captures a lot, including nutrition and socio-economic class, but also things that made us nervous, scared, happy or excited in the past. And because we learn, many of the environments that impact the wiring of neurons within our brains are not random. As we develop, we learn what makes us ecstatic, and what makes us fearful or uncomfortable, and we tend to select environments we feel comfortable in. If you are scared of heights, you won�t become a mountaineer. I don�t enjoy dance clubs and so actively avoid them. My experience of dance clubs is consequently limited, and they have contributed little to who I am.

Personality traits, like any other phenotypic trait, are determined by genes, chance and the environment, but they are harder to study because we do not understand the mechanistic cause of traits like introversion or extroversion. Unlike my poor eyesight, where scientists understand the genetic error and how it manifests, we do not have a good grasp of which brain structures equate with outgoingness or humour. Without a mechanistic understanding of how brain structure maps to personality we must rely on statistical correlations, but this too is not straightforward, as my next example demonstrates.

Sonya has a fear of sharks and is never comfortable going into the ocean. She tells me that her fear started after seeing two of her friends swimming in the shallows and spotting a large shark swimming towards them. She shouted and waved a warning and they safely swam ashore, with the shark turning back to the depths. There is no doubt that the experience was frightening, and for a few seconds she was not sure how it would play out. Nonetheless, if that event hadn�t happened would she be unfazed by the idea of swimming in the ocean? Was she already a little nervous about entering the sea before this event? We can never know with certainty because a version of Sonya who did not witness the shark close encounter does not exist, and so we cannot make such a comparison. Because each of us is unique, we cannot do the experiment where we compare versions of ourselves that did and did not experience a particular event.

Even if my wife had an identical twin sister who had not experienced the shark close encounter and who was happy to swim in the ocean, I still could not say for certain that the day on the beach when her friends could have been injured or killed was the cause of Sonya�s fear. Some other differences in the experiences of identical twins, or even apparent randomness during development in the womb or early childhood could be the cause of their different views of the safety of the sea.

Although scientists cannot attribute cause to effect within an individual in terms of why they have the personality they do, they can draw insights by comparing populations of people. If I were to identify 1,000 individuals who had observed their friends experience a close encounter with a shark, and 1,000 who had not, I could measure their levels of anxiety when I take them to the ocean and ask them to swim. I might do this by measuring changes in levels of adrenaline or cortisol, hormones associated with stress, in their blood. If the two groups of individuals did not differ in other ways, I might be able to conclude that seeing a shark swim towards your friends was sufficient to induce a fear of entering the ocean. I would reach this conclusion if individuals in the close-encounter group were, on average, more reluctant to enter the water than those in the non-close-encounter group. To draw this conclusion, the two groups must be as well matched as possible for all aspects of experience, biology and background, other than having seen their friends encounter a shark while swimming. If everyone in the close-encounter group was female, for example, while the non-close-encounter group was all male, I could not distinguish between sex and shark-close-encounter experience in determining fear of the ocean. Perhaps men are just more likely to be scared of the ocean than women and this could explain the difference between the two groups.

Even if my two groups were indistinguishable from one another in all other aspects of their lives except for the shark-close-encounter experience, I could not say with complete confidence that any single individual fears the ocean because she experienced a shark encounter unless all those who experienced a near shark attack were scared of the ocean, and all those who did not were happy to dive into the sea. In most comparisons like this, you do not find that all of one group exhibit one view, while all the second state another. It would be highly likely that some of the thousand individuals who had not experienced a shark close encounter were still scared of the ocean, and vice versa. The best I can say is that, out of a balanced and random sample of 2,000 people, those who experienced a shark close encounter were more likely to fear the ocean than those who had not. I can�t say they will, or will not, fear the ocean, just that they are more likely to do so. Sonya did experience a shark close encounter so it is possible that this has caused, or contributed to, her fear of the ocean, but neither she nor I can ever really know despite her vociferous insistence that this was the cause.

Despite scientists� inability to link particular experiences to personality traits, all of us have experiences that we believe have moulded our personalities. Some people recount events to explain why they are shy, anxious, funny, talkative, or scared of almost any object you can imagine. For example, a woman from the West of England cannot walk down the frozen food aisle of a supermarket because of a fear of frozen peas. She explains, �They tend to just look at me � ganging up on me.� She wasn�t involved in a terrifying pea prank as a youngster, or a collision with a lorry carrying a shipment of peas, but maybe something triggered it when she was younger. Regardless, the fear is very real and it is part of her personality. With something as complicated as a personality, how can psychologists make progress in working out why we have the characters we do?

The first step is to simplify the problem, and psychologists frequently do this by classifying people on five dimensions of personality: extroversion, neuroticism (or emotional stability), agreeableness, conscientiousness and openness to experience. These are called the big five personality traits, and are part of a framework developed in the 1990s to assign individuals into categories by having them complete a questionnaire. Psychologists like these traits because they capture some of the differences between individuals, and scores for any one individual tend to be very similar if the same individual does the test on multiple occasions spanning months or even years. If acquaintances are asked to complete the survey for someone, there also tends to be agreement between the classifications provided by the subject and her colleagues.

By analysing brain scans, aspects of the environments in which people have lived and genetic sequences of study participants, psychologists have been able to show that, as with many other phenotypic traits, genes, the environment and developmental noise contribute to determining the big five personality traits. Environmental drivers such as nutrition as a child, socio-economic background or the country you are born in tend to explain little of the differences between us. You might think this means that the environment is not important in determining our personality, but that would be wrong. It would be a strange world if all people born rich were shy and all those born poor were not, or if the French were much more anxious than the Germans. The influence of the environment is important, but proving it is hard because individual experiences mould our personalities. Extremely traumatic events, such as abuse as a child, have been shown to impact openness to experience. Fortunately, such traumatic abuse is unusual and most of us do not experience such extreme environments, but that is not to say less traumatic events do not shape who we are. Sonya is convinced her fear of the ocean stems from one experience that lasted less than a minute, and certainly before it she had a job that required her to spend a lot of time in the ocean studying turtles. It is possible she had subconscious anxiety about the ocean that only manifested itself consciously following her close-encounter experience. But scientists cannot prove that.

Big Five Personality Traits

Despite the enormous difficulties of studying personality traits, we are confident they are like any other phenotypic trait in that genes, the environment and chance determine their values. They are harder to understand because classifying the environment in a way suited to studying personality traits is hard and because a single experience might change your personality but it wouldn�t change your eye colour or height. Although I know I can�t prove it, I believe that my genes and a few key events really have given me my character. Telling the story of these events is useful as it allows me to turn to the role of personal narratives for making sense of the world, and of who we are.

I enjoy life, and most of the time I do not find it too stressful. When I conducted an online survey to score myself on the big five personality traits, I came out as more extrovert than most of the population, emotionally reasonably stable, more agreeable than four out of five folk, at least averagely conscientious, and open to new experiences. I have been told I am intelligent, but those who have said this have never seen me attempt to put together flat-pack furniture.

I have not always had these personality traits. As a child I rarely felt confident, was not particularly popular with my peers, and often found social situations stressful. I was not conscientious with respect to schoolwork and would rather read books and magazines or watch TV, but for as long as I can remember I was fascinated by science. How did I get from the geeky, lazy and shy youth to who I am now? It is due to my genes, the environment I have experienced and apparent chance. I also believe, irrationally, there are a few experiences that changed my personality and my outlook on life.

The mutation at gene GPR143 that impacts my eyesight changed the course of my life when I was a child. My father was a successful pharmacist before he retired at sixty, and this meant that my parents were reasonably financially secure when I was growing up. Dad had gone to the Perse School for Boys in Cambridge, the premier private school in the city, and he thought it would be a good option for me. As I neared my eleventh birthday and the time to move from primary to secondary school approached, my parents arranged to meet with the headmaster of the Perse school. The meeting initially went well, and the head of the school was encouraging that they apply for me to attend and that I would be likely to be offered a place. Towards the end of the meeting my father mentioned my genetic condition and poor eyesight, and explained I would not be opening the batting for the school�s cricket team. On reflection, the headmaster stated, it turned out I probably wasn�t a great fit to the school. He apparently wanted his students to be competent cricketers. Instead, I went to Comberton Village College, a state school in a local village. I suspect that this school was a better choice of school for me, and that the headmaster of the Perse school was correct, but for the wrong reason. My mutation caused a change in my educational environment, the friends I would go on to make and the teachers who struggled to educate me.

I was lazy at secondary school as I didn�t enjoy the drudgery of learning. I liked finding out facts for myself, via experimentation, but I didn�t enjoy learning lists of facts. My dislike of rote learning meant I was hopeless at French, German and Latin, because I refused to learn vocabulary. Because most of the schooling involved rote learning, I did not engage with it. The subjects I liked were maths and science. I liked science because I could do experiments. I devised a school project with my dad where I grew broad bean plants in different types and amounts of soil. Each plant got the same amount of light and water, and I would measure them daily to monitor their progress. It was fun, as I was finding out stuff for myself, and this revealed there were rules to the world. I enjoyed discovering these rules, rather than lists of facts such as the principal cities and towns of all the counties of the British Isles.

Maths was a little like science. There were rules on how numbers could combine or equations could be manipulated. If I understood the rules, I wouldn�t need to learn my times tables but could instead easily work out the answers from first principles. From an early age, I wanted to know why the world worked as it did, rather than learning enough facts to help me navigate it. But it wasn�t all plain sailing. I was taught calculus at secondary school by being given a set of recipes to integrate and differentiate various mathematical functions. I didn�t understand where these recipes came from, and that frustrated me. Up until this point I had been good at maths and was in a class slated to do an examination certificate halfway through the two-year course. I achieved 6 per cent in the mock exams, possibly the lowest mark in the class. On the night before the exam, the penny eventually dropped as I thought things through, and as I stared at the recipes I suddenly understood where they came from and what differentiation and integration were doing, and I could see why they were such incredibly useful tools. The next day I did the exam. I hadn�t learned the recipes but I understood how the bits of calculus I needed worked, and I came fourth highest in the class, being one of only a handful of students who passed. I wished we had been taught why calculus worked, rather than simply how to apply it.

At the age of seventeen, with my A-level exams looming, I was supposed to decide what I wanted to do with my life. Training in mathematics would surely be a safe bet for a reasonable job and so I applied to university to study it. I subsequently received offers, subject to securing the right A-level grades. At around the same time I was applying for university we had a presentation from an ex-pupil who had taken a gap year. He had spent the year overseas teaching in a school in Honduras. I enjoyed travel, this sounded fun, so I decided to apply to Project Trust, the organization that had placed him.

After an interview and a set of selection tests on a remote Scottish island I was told that I would be posted to a school called Chemanza in central rural Zimbabwe, along with three other school-leavers. Just before departing for Africa, I received my A-level results, and I had the grades to study maths at university on my return from my year away. There is absolutely no doubt that my year overseas was formative. It was a remarkable experience, and I will be eternally grateful to Project Trust. It was the first time in my life I had worked hard at anything, and I strove to do a good job. Students that I taught performed at least as well as those in other classes. I grew up a lot in that year, I became much more outgoing and confident, and I hope I inspired some of my students in their educational journey.

During my year of teaching, I fell in love with Zimbabwe and with the people I met. It was also when I decided I didn�t want to be a mathematician, and reapplied to university to study biology, which I eventually did at the University of York. As soon as I was back in the UK, I yearned to return to Africa, and the opportunity arose during my second year at university, when I had to choose an undergraduate research project that would be conducted at the end of my second year. It turned out it was possible to design your own project if you could persuade a faculty member to supervise it. I was fortunate enough to persuade a lecturer in the department to supervise me.

A cousin arranged for me to go to Kenya, to the bush camp of George Adamson. My project involved comparing the behaviour of wild lion cubs with three hand-reared cubs whose mother had been shot, and which George was raising for their eventual release to the wild. I spent about a month at the camp, studying lion behaviour.

There were various important experiences during this period that likely contributed to who I am, including the murder of George Adamson and two of his assistants by Somali bandits two days after I left camp. He was the first person I knew who had met a violent death. It was also during this trip that I contracted malaria, which eventually led me to write this book, but I had another formative experience that resulted from this trip that played out on my return to the UK.

Although I had a great time in Africa, I hadn�t collected much data for my undergraduate project on lion behaviour. The problem I faced was this. Once I had entered all my data on to the mainframe computer I had access to at university and had conducted analyses, I found that when I drew graphs there did appear to be some differences between the behaviour of the three hand-reared lion cubs and the observations I had collected from wild cubs from a nearby pride. However, when I conducted formal analyses, these differences were not statistically significant. What this means is that the differences I observed probably arose by chance. I started to ask myself how many litters of lion cubs I would have needed to collect data from for these results to be significant.

I explored this question by writing some computer code to simulate data. In particular, I used the computer to create a larger data set than the one I had collected, but which had the same statistical properties as the real data. Each column of the made-up data had the same average value as the real data, and the same degree of spread. Some of the columns in the real data were correlated, and the simulated data were correlated in the same way. I would then analyse the made-up data sets and see how big they needed to be to achieve statistical significance. I continued to add rows of data to my simulated data set until an analysis of it reached the threshold of statistical significance.

Much later in my career I learned that there was a much simpler way of solving this problem � an approach that statisticians called power analysis. Although I could have saved a lot of time if I�d known this before, I learned a huge amount about statistics from my simulation exercise. By doing the simulations I taught myself how to code in a computer language called Fortran. I also learned a lot about distributions of data, and about statistical tests. And just as at school I loved learning new things by trial and error, this made me determined to pursue a scientific career. I still get an adrenaline rush when I solve a problem or discover something new. I like the idea that no one else has ever known what I have just found out, however trivial it may be. These breakthroughs are very minor in the grand scheme of things and nothing I have done has had a major impact on the world, but that doesn�t lessen the sense of excitement.

Simulating the data taught me more about statistics than any of the lectures I attended, but it also made me realize that my undergraduate project was deeply flawed. Much in the same way that I cannot prove that a single event radically altered my life trajectory, it is not possible to study three hand-reared lion cubs and a similar number of wild ones and draw any meaningful conclusions. In the same way that no two people are the same, no two lion cubs are equivalent. For my project to have worked I would have had to record the behaviour of many more hand-reared and wild lions, but that was not possible. I described the limitations in the discussion section of my study, submitted my project, and this was enough for me to pass my degree.

Despite being well aware of the limitations of science in being able to assign particular events that happened early in life to personality traits or even the course of one�s later life, I still, irrationally, believe the narrative I describe above. I know I shouldn�t, but I do think that malaria made me decide to try to understand existence, and that a poorly designed undergraduate research project made me a more competent scientist. Can these narratives really be true? Would I have become a scientist and author if I did not have the mutation in GPR143 and had gone to the Perse school instead of Comberton Village College? Would I have become a mathematician rather than a biologist if I had not gone to Africa? Would I have decided to write this book if I had not caught malaria and nearly died? And did doing a dodgy undergraduate project really set me on a course to become an academic? I can never know because I can�t do the experiments required to test these narratives (I�d need many clones of myself), but I do know that all aspects of me are determined by genotype, environment or apparent chance, so perhaps I have constructed this narrative because it includes a little of each.

I have likely decided to believe in the narrative I tell to justify my approach to the science I do or because I wanted a reason for writing a book on what science can tell us about how we got here. As humans we have a strong tendency to look for causes that explain why something happened, and we do this because we want to live in a world we can understand. Most of us don�t enjoy change or the unexpected, and constructing narratives helps bring a sense of order to the unpredictable.

The narratives we tell serve two functions. They are used to justify our behaviours and events we have experienced, and we also use them to make sense of the world around us. Our narratives about our life trajectories can contain egregious acts committed by people that have prevented us from achieving our true potential. For example, if Fred Bloggs had behaved more professionally towards my friend Elmer, he would be a better person, would have won multiple Nobel Prizes, and would have wealth and power beyond his wildest dreams. Sure. Sometimes these slights may be real, other times they are imagined. There are very many well-documented historical cases where individuals have acted vindictively to prevent someone else from achieving success or being recognized for it, including in the sciences, and perhaps that is why we sometimes blame others for failures in our lives.

Isaac Newton is arguably the greatest scientist who ever lived. But he wasn�t a very nice person. He was paranoid and held grudges, particularly when it came to scientific precedence. He fell out with many other scientists who worked on gravity and calculus, including another great scientist of the era, Robert Hooke. Before Hooke died in 1703, he had been lauded by the Royal Society and a portrait had been painted of him that would help ensure his scientific legacy. Newton became head of the Royal Society in the same year as Hooke�s death and he oversaw the relocation of the Royal Society to new premises a few years later, and among the small number of things that went missing were Hooke�s portrait and many of his papers. Scientific historians have repeatedly suggested that these losses were not accidental but were overseen by Newton. He was angry, incensed that Hooke had succeeded while alive and accused him of stealing his ideas, and so Newton did what he could to ensure that Hooke was forgotten to history.

Fortunately, not everyone behaves so meanly. Some narratives can contain acts of kindness that are perceived to have altered the trajectory of a life. People who are generally happy with their lot and at home in their skin appear to be more likely to have narratives based on acts of kindness or on their ability to overcome adversity. In contrast, individuals who feel they should be performing better than they are hold narratives where the bad behaviour of others has prevented them from achieving their potential.

I have held the narratives I described earlier for nearly my entire adult life, and they have not changed. But other aspects of the narratives I tell have changed throughout my career. I am currently happy with my lot and am largely content with who I am. The reason for this is that I believe I have achieved more than I expected to in my career. I am in a senior position at one of the best universities in the world, and many of my peers have some respect for me. I am in love with and very happily married to a woman who accepts me as I am and who treats me well. And my children are currently all happy.

During the early parts of my life when I was unsuccessful in applications for fellowships and university jobs, when I was struggling to make progress on the scientific questions that interested me, or when I was in a marriage that had run its course, aspects of my narrative were different. I did not take as much responsibility for the failings that troubled me as I should have done but instead constructed stories where particular individuals whom I felt had slighted me were out to deliberately hinder my progress. In retrospect, they would have had no reason, nor the power and influence, to do so. My perceived lack of progress towards my career goals was because I was not writing strong enough grant and job applications, and the papers I was submitting for publication were rejected because they were not as convincing to the reviewers as they were to me. Science is a competitive enterprise, and I was not as good at it as I thought I was or as others whom I was competing against. Because some of the narratives I told in the past were false it casts doubt on the ones I still believe and tell. Perhaps catching malaria and simulating data as part of my undergraduate project had no impact on who I am, or my trajectory throughout life.

We know that personal narratives can be wrong. When some people are down on their luck or unhappy with their lot and there is no obvious person known to them that they can blame, they can construct narratives that are easily disproved. These narratives posit the existence of illuminati, shady characters that run global affairs from the shadows that actively hold back and suppress the enlightened who know the truth. One of the more surreal illuminati conspiracy theories posits that the British royal family is descended from humans who mated with a powerful race of malign extraterrestrial reptiles, and this is how they gained their power. A second theory asserts that Prince Harry and Meghan Markle married as part of a devious plan by the British to regain control of America. Specifically, a child of the marriage could become heir to the British throne and President of the United States, providing a route to the British recolonization of America.

Narratives are central to being human and they help us survive in an uncertain world. Science has developed ways of testing many aspects of the stories we tell. The scientific method does this by assigning confidence on whether a cause is linked to a particular event. When the mechanism that links a cause and an event are well understood, confidence can be high. I know that I have a genetic mutation in a particular gene that impairs my vision, and scientists understand that this means that pigment is not produced in cells in the fovea, and that means my eyes wobble. Mutation to this gene repeatedly has the same effect in different people. When science has been unable to determine mechanistic links between cause and effect it does less well and must rely on statistical correlations, and these cannot explain why specific individual events, such as writing this book, occurred. We do not yet have a good mechanistic understanding of how experience influences our personalities or makes us as we are and we must rely on statistical correlations. Experience clearly plays a role in influencing our personalities, but exactly how a particular experience in the past has influenced your, or my, personality is currently impossible to prove.

Because science has some power in deconstructing narratives that are often firmly held, many people are wary of it. Conspiracy theories are easily debunked, yet many people believe them. As an example, a 2019 survey revealed that one in six Britons believe the moon landings were faked, despite having clearly happened. Yet given the ease with which we construct and believe false narratives, it is perhaps unsurprising that scientific knowledge is often disregarded or treated with scepticism. Although science does not have all the answers, it provides narratives that are supported by data and are often testable with experimentation.

Some parts of the scientific narrative I have told in this book are backed up by more data and experimental tests than others, and some scientific disciplines are more mature than others. A mechanistic understanding of the workings of the fundamental forces of nature is quite mature, and this means chemists and physicists rely less on statistical correlations than do those working in biology and psychology, where a mechanistic understanding is frequently lacking. A consequence of this is biologists tend to think of the natural world as unpredictable and rather random, while some physical scientists speculate that the universe may be deterministic and, with enough understanding and computing power, perfectly predictable. Whether we can achieve a mechanistic understanding of the complex chemistry of living beings such as us is an open question, but currently there is a long way to go. Science can�t tell me why I chose to write this book, but statistics could tell you that a majority of people want to understand why they exist.

Having told the narrative of the universe and of me, I can now address the question that motivated this book: were you and I inevitable at the birth of the universe, or are we just incredibly lucky? It is not a straightforward question to answer. In researching this book, I have been astonished at the breadth and diversity of human knowledge. I have also realized how flexible the scientific method is, and how researchers in different disciplines use different aspects of it in different ways. I have also learned that although the development of the scientific method is humanity�s greatest achievement, it is not perfect. For example, although science can identify that genetic variation, developmental noise and the environment will all have played a role in determining who you are, and that each process plays different roles for different phenotypic traits, science cannot yet tell me why you might be outgoing, empathetic or conscientious. While answering the question that motivated this book, I now also consider how much science has achieved, and how much more there is to learn.

Why We Exist

When I was in hospital in 1989 recovering from my bout of malaria and deciding that by the time I lay on my eventual deathbed my hope was to understand why I had come into existence, I hadn�t really thought it through. Scientific knowledge is vast, filling endless libraries, and it grows on a daily basis as more and more papers reporting research are published. Between 2020 and 2023 there were at least four and a quarter million new scientific papers published in English each year. I haven�t read them all.

After failing to make any progress towards my life�s goal for the first couple of years post-malaria, I finally came up with a plan on how to tackle the Sisyphean task I had bitten off. I started by writing down a list of key things, or entities: energy, force fields, quarks and electrons, neutrons and protons, atoms, molecules, stars and planets, simple life, more complex multicellular life, sentient life, and you and me. I then made a list of the processes that linked each thing to the next on the list. For example, the formation of stars from molecules of hydrogen in nebulae is known as star formation, while the process that resulted in life becoming more complex on our branch of the tree of life is evolution. By focusing on the processes, I realized I could describe what science can tell us of how you and I came to exist in manageable chapters. I could start with how energy became fundamental particles and work through the formation of atoms, stars, molecules, Earth, life, consciousness, humanity and you and me and explain how each thing led to the next. I also came to understand how important interactions were in our existence. I learned how quarks interact to form neutrons and protons, how these interact to form atomic nuclei, and how these, in turn, interact with electrons to form atoms. The atoms then interact to produce molecules, that in turn interact to produce planets, and then � on Earth at least � life. Force fields enabled these interactions. Interactions between competing individuals drove the evolution of complexity, eventually resulting in humans. I realized how the outcome of each interaction was something a little more complicated than the constituent parts. For example, molecules are more complicated than their constituent atoms and protons are more complex than quarks. By the time I had children, I had a plan on how to structure my book. I knew the structure would come at the cost of not describing a lot of scientific knowledge and, in particular, I would skip over a lot of mechanistic understanding.

By focusing on processes, I realized I could address the question of why we exist by describing what had to happen at each stage of our universe�s history as one entity on my list transformed into the next. The mechanisms, in contrast to the process, describe how each transition happened rather than why. One reason for skipping over much of the how is that mechanisms can become fiendishly complicated and difficult to write about in a non-technical way. For example, a mechanistic description of developmental biology, a topic I discussed in the last chapter, would require covering how DNA is unzipped, copied and translated into chains of amino acids, how proteins fold, and how different proteins run chemical reactions that play key roles in the construction of our bodies. The subject is fascinating but complex, and a mechanistic description of how you developed from a single cell would run to hundreds of thousands of words, many of which would be technical with very specific meanings. If this book has piqued your interest in any aspects of the science I have summarized, I list some books in the appendix that provide descriptions of the mechanisms that I do not describe.

As I started to plan this book, I delighted in researching and reading about the processes, and the challenge I had set myself became more manageable. I started to gain some understanding of how a simple, minuscule pinprick of extraordinarily hot energy could transform itself over billions of years into you and me, extremely complex organisms built from trillions of cells working together. Yet there were still choices on what to, and what not to, describe. One early challenge was whether to deal with the three laws of thermodynamics. The first states that energy can neither be created nor destroyed but it can be transformed between different forms, the second that systems tend to become more disorganized and less ordered over time, and the third that the level of disorganization eventually settles down to a specific value (I�m not going to explain the value) as the temperature gets close to absolute zero, the coldest temperature that matter can obtain, or −273.15 degrees Celsius. The second law is the most bewildering, given that clearly you and I are highly ordered beings and we have arisen from a point of intense energy over the course of 13.77 billion years. We are not just a random set of molecules that happen to be in the same place at the same time. The history of Earth and of you and me appears to contradict this second law, yet all reputable scientists accept it as correct.

The apparent paradox between the second law of thermodynamics and our existence is that the law does hold if we consider the entire universe. On average, the universe is becoming less ordered, but there are corners of it, such as Earth, that have become more organized over time. Averages are useful, but they do not always explain what is happening in a particular location. While Earth has become more ordered, with atoms assembling themselves into you and me, other parts of the universe have become much more disordered than average. These bits of space will have become colder, contain very little matter, are much less structured, and simpler. If we could examine the history of every cubic inch of the universe, we would find that on average they have become less ordered, but there is a distribution of change in how ordered things are, with each bit of the universe having a different history.

The second law of thermodynamics holds because, on average across the universe, the degree of disorganization, or entropy to give it its scientific name, is increasing. I have written this book focusing on Earth. If I had chosen a point in deep space, somewhere beyond our solar system but before another star system starts, the history would be rather less exciting. I would have described a history of space cooling, the density of energy and matter thinning, and not a lot happening. The book would have been easier to write, shorter, but (I hope) not as interesting to read.

I chose not to introduce the laws of thermodynamics early in the book as I wanted to tell the history of our neck of the cosmic woods, and I wanted to avoid getting into discussions of some of the scientific conundrums scientists have grappled with, or those they still work on. But as I near the end of the book and turn to the questions of why we exist, and whether our existence was inevitable or not, I need to consider some of the hardest questions that scientists, and philosophers, are trying to solve. One of the questions that motivated the book is whether our universe�s history was predetermined from its birth, or whether randomness has played a role.

If our universe is deterministic it would develop in exactly the same way each time my repeat universe experiment described at the start of the book was run, as long as the starting conditions were identical. Each universe would end up with identical histories. If the initial starting conditions were to differ between two runs, even by just a tiny amount, the resulting universes would be different. We would consequently be inevitable in a deterministic universe with the same starting conditions as ours. In such a universe, scientists would, hypothetically at least, be able to exactly predict the history of every single particle in our universe from its inception until its demise. Such a feat is technologically impossible with today�s computers and is likely beyond the wit of any advanced civilization.

Some scientists argue that the universe is deterministic because they can accurately predict the behaviour of many things, such as the orbits of the planets, and have become better and better at doing so as various scientific breakthroughs have been made. Science progresses and as it does so it increases our understanding. There is only one direction of travel: ever better predictions. For example, 500 years ago we were poor at predicting many celestial events. But today, using equations discovered by Isaac Newton and Albert Einstein, astronomers can very accurately predict the return of comets to our night skies decades into the future, the trajectories of spacecraft, and the orbits of planets around our sun. Similarly, physicists can predict how magnetism will move objects, and chemists can predict what will happen when some types of chemicals are mixed in beakers, in ways that were unimaginable even a century or two ago. Many bits of the natural world are now predictable, and that gives some scientists hope that the future of our universe might be perfectly predictable.

There are many things that we still cannot predict. I can�t tell you when my teenage son will get out of bed at the weekend with any accuracy, we can only predict the weather in my hometown of Oxford a day or two ahead with any confidence, and we cannot predict when volcanoes will erupt or earthquakes will shake the ground on which we stand. Given our inability to predict many things, why do some scientists think the universe might be deterministic? Their argument is that we are yet to identify the mathematical equations that would allow us to perfectly predict the currently unpredictable, but that these equations do exist, and it is only a matter of time before we discover them. Scientists have improved our ability to predict all sorts of things, from the way that proteins fold to the existence of new fundamental particles such as the Higgs boson. Scientists will continue to improve predictions, but that does not mean the universe is deterministic.

When scientists are unable to accurately predict things like the weather, they sometimes state that the systems they are failing to predict are stochastic. Those championing determinism would counter that they are not strictly stochastic but that the apparent randomness is due to our failure to identify the equations that accurately describe the system. We might think the weather is stochastic, but this apparent randomness may simply be because we are not very good at predicting the weather more than a day or two into the future. When we discover the right equations, we will be able to accurately predict that 4.2 millimetres of rain will fall on the Champs-�lys�es between 9 a.m. and 10 a.m. on 7 January 2081 and we would classify the weather as being deterministic.

One consequence of a deterministic universe, where everything is theoretically perfectly predictable, is that all our actions would be predetermined and we would have no free will. The ways that the atoms from which our brains are built interact with one another would be inevitable, and so too would be each of our actions. Free will would be an illusion in a deterministic universe. I would have been destined to write this book 13.77 billion years ago when the universe began, and I never really made the decision to do so. I could never have resisted the urge to write it, even if I had wanted to. You, too, were destined to buy it.

The idea of free will being an illusion will sit badly with most people, and personally I don�t like it. The choices I make feel very real, and I believe we are responsible for our actions. I do not like the idea that our actions might be completely predictable. However, this is just a feeling, and is not scientific. Just because I don�t like a hypothesis does not mean it isn�t true.

Quantum mechanics very accurately describes the behaviour of the very small, and all the evidence says that aspects of the behaviour of particles such as electrons are random. In my opinion, Occam�s razor, the argument that explanations should be as simple as possible yet no simpler, points to a stochastic universe. Once again, such an argument is based on opinion rather than science. These feelings are, of course, important, but they are not scientific. You and I are made from particles, and particles obey the law of physics, so if their behaviour is predictable, then does that mean our behaviour must be deterministic? To disprove the idea of determinism I need to offer an alternative, and the alternative must explain where randomness comes from.

The only true source of randomness so far identified in the universe is described by the theory of quantum mechanics, something I described when discussing the two-slit experiment. Quantum mechanics accurately, yet probabilistically, describes the behaviour of electrons, protons, neutrons and atoms. It is one of the greatest achievements of physics, and explains one of the most peculiar aspects of our universe: that particles are also waves. The theory describes properties of particles as probability distributions. For example, a particle�s position is described with a wave function, which describes where it is more likely to be, and where we�d be unlikely to find it. Although we cannot directly observe the wave function of any particle, because measuring it would force the particle to entangle with our measurement device causing the particle�s wave function to collapse, we can imagine what it would look like if we could. If we were to scale the quantum properties of particles like electrons to the scale of our solar system, the moon (and sun and all the objects in the night sky) would appear as a distant mist, with some parts of the mist being thicker than others. The thicker parts would be where the moon would be most likely to appear if forced to interact with something.

Quantum mechanics is the most successful theory in science. It very accurately describes how fundamental particles behave, yet despite this accuracy, some physicists argue about the interpretation of the theory. Some claim that the apparently unpredictable and random behaviour of particles will eventually prove to be deterministic, and when that happens free will will be banished to the trash can of discredited ideas. One example of a hypothesis that makes the random behaviour of particles predictable is the Many Worlds hypothesis. Put very simply, and skating over many nuances, this states that we experience one outcome when a wave function collapses, but that all possible outcomes occur in parallel realities that we do not experience. It is a clever way of making the stochastic deterministic by saying all possible things that could happen do, but that we only experience one of the outcomes � our reality.

I didn�t mention the Many Worlds hypothesis, or other interpretations of quantum mechanics, earlier in the book when I described the two-slit experiment, because they are currently untestable. There is no evidence for realities beyond our own, and at present there is no way of testing hypotheses that predict them. The Many Worlds hypothesis provides a way to make our universe deterministic and in that respect it is cute. But many creation myths are quite cute too and that doesn�t make them right.

Untestable thought experiments such as the Many Worlds theory play an important role in science. They allow us to think through a range of possible interpretations. What I don�t understand is why there is a desire among some physicists to prove the universe is deterministic. The available data we have suggests the behaviour of fundamental particles is stochastic, and that opens up the possibility of a stochastic universe and of free will. But it creates a new challenge � how does randomness at the level of the tiny particle translate into non-deterministic behaviour of much larger objects such as you and me?

When the wave function of a particle collapses, such as when a single atom collides with the detector screen in the two-slit experiment, the particle becomes entangled with the detector screen � an object whose wavelength is so small it can be ignored and which behaves like those we are familiar with in everyday life. The screen is designed to detect the particle by interacting with it, and the screen and the particle become a single object. The screen and entangled particle are a large object consisting of millions of atoms, and it behaves like all large inanimate objects in predictable ways. Knock it off the bench and it will fall to the floor, or throw a ball at it and the object and the ball will bounce off it. It is such reassuring, familiar behaviours that make the idea of a deterministic universe attractive to some scientists. But not all solid objects behave quite like detector screens, and perhaps life has found a way to exploit the random behaviours of particles to enable free will.

The field of quantum biology is in its infancy, but exciting insights are beginning to emerge. For example, photosynthesis, the process by which algae, some bacteria and all plants create sugars from sunlight, water and carbon dioxide, relies upon quantum mechanics to transform photons of sunlight into energy that can be used to produce sugars. Photosynthesis evolved early in the history of life, perhaps as long as 3.5 billion years ago, suggesting that life discovered how to exploit quantum behaviour early on.

Evidence is also beginning to emerge that quantum mechanics plays a role in the workings of some enzymes, in the way neurotransmitters work in the brain, and recently biophysicists at the University of Surrey in the UK have data and models that point to some genetic mutations being due to quantum randomness. Genetic mutations influence the development of embryos, and are the fuel for evolution and the spread of life, providing some evidence that we owe our existence to stochastic processes, and that we were not inevitable at the birth of the universe. You and I would not occur in my universe rerun experiment if the universe is stochastic and quantum processes influence evolution.

Some claims of quantum biology are still controversial, and more evidence will be required to convince sceptical scientists. Many biological molecules are too large to behave like single atoms, and some researchers have argued that the warm, wet world inside our cells would prevent life from exploiting the quantum properties of the very small. Nonetheless, given our biological understanding that interactions between pairs of molecules impact many cellular processes, and that this is the scale at which random quantum interactions happen, there is potentially a route via which randomness could make our existence down to chance. A number of scientists now argue that the warm, wet environment inside cells is ideal for quantum processes to scale up to influence the dynamics of whole organisms. However, this does not answer how quantum mechanics might result in consciousness and free will. At this point, the evidence becomes sparse and I am straying into conjecture. The Many World proponents may now see an opportunity to accuse me of generating an untestable hypothesis. But I would argue that my theory is more testable than theirs.

In the chapter on the evolution of consciousness I described how the neocortex processes information. The neocortex, a highly folded part of the brain, is very large in humans, and is the part of the brain that makes us intelligent. Inputs to the neocortex from our sense organs � the eyes, ears, nose, skin and tongue � lead to a propagation of signals through networks of cells until neurons fire that allow us to recognize particular objects: remember the Jennifer Aniston neuron. Neurons in different clusters across the neocortex may fire, with signals from multiple neurons processed before a consensus is reached about what we are experiencing. Other neurons then propose how we should respond and a decision is made.

Although data are largely lacking, a number of psychologists, mathematicians, philosophers and physicists have proposed a role for quantum mechanics in consciousness, and given that sentience appears to arise from electrical activity of neurons interacting via synapses, there is certainly potential for stochastic quantum processes at the level of interacting molecules to contribute to consciousness, and ultimately to free will. Exactly how is currently a matter of speculation. Nonetheless, we cannot rule out a role for quantum randomness across the hundreds of thousands of synapses that fire every second that contribute to the way we make decisions. I have strayed from what science can currently tell us into guesswork, so let me bring it back to a line of evidence that is a little more concrete.

Many scientists and philosophers have drawn comparisons between computers and the human brain. They usually point out how aspects of the way computers operate mimic the workings of bits of the human brain. Such comparisons are made more and more frequently as artificial intelligence becomes more embedded in our everyday lives. Many of the algorithms that computers use to make choices when playing a game such as chess, or to find optimal, or near-optimal, solutions to currently analytically intractable problems, rely on random numbers. For example, ChatGPT relies on random numbers when you ask it to complete a task and computer scientists use randomness to help solve all sorts of computational problems that, today at least, cannot be solved without recourse to chance. Randomness can be used to identify solutions to problems, and if genetic mutations are due to chance, as seems probable, then we owe our existence to random events. It might seem counterintuitive that randomness can help evolution find solutions to make organisms more competitive, or computer scientists can use randomness to find optimal solutions to problems, so I will briefly give an example of how I have used random numbers in my research to solve problems.

My Ph.D. research investigated the impact of seed predators and herbivores on tree reproduction in forests in the US and the UK. In hindsight, it didn�t move the field forward much, but I don�t think it set it back at all either. I did learn a lot, and one thing I learned was how I could use a random number generator on the computer to construct a statistical model of seed and seedling predation by squirrels. I had collected quite a lot of data from experiments where I placed acorns in various densities across the forest floor, some in deer- or chipmunk- or squirrel-proof enclosures, others not. I would then return to see how many had been eaten. I then wrote down a mathematical model to describe the impact of vertebrate seed predators on the likelihood of seeds in different places germinating. The next job was to combine the mathematical model and the experimental data to identify the values of parameters that gave me the model that did best in explaining variation in the data I had collected.

I wrote a program in a computer language called Turbo Pascal to find the most likely values of each parameter in the model. The program implemented something called the Metropolis�Hastings algorithm that uses random numbers to hone in on the most likely solution. The way the algorithm works is akin to finding the highest point in a mountain range in thick fog at night using a teleportation device and random numbers.

At each location in the finding-the-highest-point analogy you can measure the altitude. Having done so, you randomly select a direction to move in and then teleport a fixed, pre-determined distance before measuring the altitude at this new location. You then compare the old and the new altitude. If the new location is higher, you move to it 70 per cent of the time, but if it is lower, you stay put in your original position, also 70 per cent of the time. You do this again, and again, exploring the mountain range. In the Metropolis�Hastings algorithm you are exploring a likelihood surface. Instead of latitude and longitude coordinates of the surface of our planet, and an altitude, the lats and longs are replaced by parameter names � perhaps alpha and beta � and altitude is replaced with a statistical quantity called the likelihood. The likelihood describes how well the model predicts the data you have collected given the values of alpha and beta. The highest value of the likelihood is called the maximum likelihood and it is analogous to the peak of the highest mountain in a range. The Metropolis�Hastings algorithm is one way to explore the likelihood surface and find the maximum likelihood.

The mountain-range analogy breaks down a little when a model has more than two parameters. If a model has, say, four parameters called alpha, beta, gamma and delta, instead of the latitude and the longitude coordinates on our mountain range analogy we now have four dimensions to explore instead of two � one for each parameter. With altitude, the latitude and longitude coordinate produce a total of three dimensions, while with four parameters and the likelihood my example model has five. I cannot visualize a surface that has more than three dimensions, but mathematically it does exist. I find it much easier to imagine a mountain range, and that is why I use the analogy. The Metropolis�Hastings algorithm works in any number of dimensions, allowing scientists to find the values of all the parameters in a model that maximize its likelihood.

There are two tricks in getting the Metropolis�Hastings algorithm to work. First, you need to select the distance to move such that you can explore the entire likelihood surface. If the distance between where you are now and the next place to sample is small, all that happens is you end up on the top of the mountain you start out on. You might be high, but there may be taller peaks in the range, which means you won�t be at the highest point. In maths-speak, you would be at a local maximum rather than the global one you are looking for. Second, how often should you not move to a higher location? The 70 per cent value works well, but other acceptance rates may on some occasions be slightly better (it depends on the specifics of the problem being addressed).

Assuming the distance to move and the acceptance rate are appropriate, the Metropolis�Hastings algorithm is a faster way of finding the best solution to a problem than deterministically evaluating the likelihood at each point on a grid laid across the likelihood surface and mapping the entire mountain range. The algorithm is also only one of very many that computers use to solve all sorts of problems in statistics and artificial intelligence. Computers are often described as mimicking the human brain, and perhaps we too use randomness to make decisions on what to do and how to behave. The only known source of randomness we could use is quantum randomness at the level of the fundamental particles, atoms and molecules, but perhaps science will one day uncover other sources of randomness in the universe.

Evolutionary biologists use a similar analogy when thinking about how evolution can produce more competitive individuals, lineages and species. Instead of being parameters, latitude and longitude are now values of phenotypic traits, and altitude is replaced with how good individuals with particular trait values are at surviving and reproducing. Random mutation means some individuals are less good at surviving and reproducing than their parents because they have less fit phenotypic trait values. However, every now and then a mutation generates phenotypic trait values that increase survival and reproductive rates. Evolution slowly climbs towards the survival and reproductive peak, but in doing so produces individuals that struggle to survive or are unable to reproduce.

Randomness is potentially pervasive through biology, yet research into its role is still in its infancy. Quantum biology and quantum consciousness are very new fields of scientific endeavour. If I were starting out in science today, I think I would seriously consider immersing myself in one of these fields. I reached this conclusion, because in researching this book, I have formed the opinion that until physicists are able to produce data to demonstrate that quantum mechanics is attributable to some underlying deterministic process, then I will take the quantum strangeness at face value, and conclude that the universe is stochastic. If I was to rerun my replicate universe experiment, each outcome would be different. Perhaps I would find the physical universe is deterministic, and our solar system will exist in each universe rerun, while life is stochastic. Maybe sometimes when we visit Earth in each universe, humans will exist, while at other times perhaps a species of malevolent intelligent lizard is the dominant species on our planet. Or perhaps I am just trying to find a way to bring the physicists who argue for a deterministic universe and the biologists who are convinced the universe is random together. Regardless, this discussion does not answer the question of why we exist.

I started my personal journey to try to understand why I existed when I lay on my deathbed. Writing this book does not, I hope, signal my imminent demise, as there is still much more I want to understand. Nonetheless, I have some idea of why it is we exist.

One downside of the universe being stochastic, and the Earth being the only planet where we have been able to study life, is that it is impossible to confidently assign probabilities to events that we only have evidence of happening once here on Earth. In particular, Earth is the only planet on which life is known to exist, so assigning an accurate probability of life evolving on other planets, even if they are similar to Earth, is currently impossible. We need more data before we can state if life arises frequently or not.

Despite this challenge, scientists have made remarkable progress in working out what had to happen for us to exist, why it happened, and even how it happened, but there are a few key events in our history we can still only speculate about.

The first key event we cannot assign a probability to is the birth of our universe. We have never observed another universe, and we have never observed anything beyond our universe. We don�t even know where the edge of our universe might be, or indeed if it even has one. We cannot look beyond our universe in an attempt to observe others coming into existence (or not). We consequently do not know whether universes are common or if there is only one, and if they are common, whether ours is typical or unusual. Put another way, we do not know why there is something rather than nothing.

What we do know with reasonable confidence is that when our universe formed, it consisted of a singularity of extremely hot energy. The four fundamental forces quickly emerged as the singularity expanded and cooled, and the fundamental particles from which all matter is made came into existence. Much like the formation of the universe, the four forces emerged only once, so we do not know whether it was inevitable or fortuitous that gravity, electromagnetism and the weak and strong nuclear forces emerged with the strengths that they have. Perhaps a different set of forces would emerge if we were to conduct the rerun experiment, and there may be fewer, or more, than the four we observe in our universe. Another unknown is consequently why we have the forces we do.

What we do know is that life as we know it requires all four forces, and computer simulations reveal that even small differences in each of their strengths could result in universes without atoms, molecules, stars or planets. Ultimately the reason we exist is that the fundamental forces took the values they did, and this allowed protons, neutrons, atoms, stars, molecules, galaxies and planets to emerge. We are here because the fundamental forces are just right. Given they have these values, science has a very good, but not yet complete, understanding of why these things formed and why they behave as they do.

Some scientists and philosophers have marvelled that our universe has forces of the right strength for life to exist. Some even interpret this coincidence as evidence that our universe could not have arisen by chance. However, if our universe did not have forces that were just right for life, then life could not evolve and no one could observe it. We can only observe the universe because it is just right for life to evolve. There may be many other universes that weren�t right for life, and they have never been observed. The fact we can observe our universe because things were just right for life to evolve is called the anthropic principle. We exist because the early universe developed in a way that would become favourable for life (at least in one location). We don�t know why this happened, but we should not be surprised, because if our universe hadn�t developed like this we could not exist or wonder about our existence. I think there are more exciting questions to ponder than the anthropic principle, which I find a circular argument.

Unifying the standard model of electromagnetism and the strong and weak nuclear forces with Einstein�s theory of the way gravity works would provide a more complete understanding of our universe, and is an exciting research area in theoretical physics. Theories such as string theory and quantum loop gravity provide approaches to unify the four forces, but testing them, and showing which theory is right, requires so much energy it is currently beyond our technical expertise. Nonetheless, Einstein�s theory of general relativity and the standard model provide good explanations for why matter behaves as it does. In principle, large particle accelerators might one day be able to re-create conditions a little more similar to those found in the early universe than we can achieve today, and experiments in these machines might shed light on why and how forces emerge with particular properties, but currently our technology is not sufficiently advanced to build such a facility.

If I was to create universes, each with our gravity, electromagnetism and strong and weak nuclear forces, where quantum behaviour is truly random, the universes would look similar, but not identical, to ours. In any universe with identical forces to ours, physical and chemical processes will be the same as those uncovered by scientists in our reality. The first generation of stars would consist of hydrogen and helium, and their deaths would create heavier elements in the ratios we observe. The molecules we are familiar with would also form. Water, carbon dioxide, nitrogen, methane, gold, uranium and other elements and molecules would be as common as they are in our universe. The fundamental forces determine the dynamics of all elements and molecules, so as long as these forces emerge as they are in our universe, and quantum behaviour is random, the processes we observe in our universe will be identical to those in our experimental reruns, yet each universe would be unique. We could exist, but we wouldn�t necessarily do so.

It would be likely that our solar system would not exist in other universes in my rerun experiment if our universe is stochastic. The early universe was not completely uniform. There were small variations in the intensity of energy, and in the density of the fundamental particles that formed. Some parts of the very young universe consequently had more quarks and electrons than others, and this variation translated into the distribution of galaxies across the night sky. Mathematical models suggest this early variation in the distribution of matter might have been caused by quantum randomness. In a stochastic universe, such randomness would result in a different layout of galaxies each time a universe like ours formed. There would be the same distribution of small and large galaxies, spiral and elliptical ones, but each galaxy in each universe would be different. Only our universe would have the Milky Way, the sun, the Earth and the other planets in our solar system.

Although unique, the Milky Way is not unusual. It is a galaxy of average size. Similarly, our solar system is one of a kind, but it too is not odd. Our sun is only a little larger than the average star, and observations of other stars through telescopes reveal that the planets that orbit our star are not particularly unusual. NASA reports that at least one in six stars has an Earth-sized planet, but some of these are orbiting stars that are very different from our sun. Another study has revealed that about one in five sun-sized stars have Earth-sized planets in their habitable zones. In contrast, only about one in twenty stars that are the size of our sun are orbited by gas giants like Jupiter and Saturn, planets that played such a key role in determining Earth�s current orbit. We can conclude that systems like ours, with planets like Earth that might be able to support life, are not particularly unusual in our galaxy, and we would expect them to occur at the same frequency in other galaxies in our universe.

Assuming four forces of the right strength and a large universe, Earth-like planets orbiting the right distance from sun-like stars for life to form appear to be quite likely. However, the suitability of Earth for life is dependent not just on its position in the habitable zone but also on its orbit and speed of spin. We currently do not have the data to study these attributes of other Earth-like planets, although scientists have proposed ways of doing these studies, so the data may be available in the not-too-distant future. But as I write we do not know whether our twenty-four-hour days are typical or unusual. Mars, our closest planet, has a day length very similar to that of Earth, but the next closest rocky planet, Venus, has a day length of 5,832 hours. Much more work is required, but the number of Earth-like planets suggests that some must have orbits and rates of spin appropriate for life.

The moon is also important for life on Earth, helping stabilize the climate by reducing our planet�s orbital wobble. We do not have the technological refinement to count moons around Earth-like planets in other star systems, but we do know they are common around the planets in our solar system, with over 200 moons counted. They can form at the same time as the planet, be captured by planets having started their lives as meteors and comets or even moons of other planets that escaped their orbits, or via a collision, as happened to produce our moon. We are the closest planet to the sun to have a moon, and our moon is the fifth largest in the solar system. Our moon may yet prove to be a little unusual, but it seems likely that some Earth-sized planets around other stars could well have one or more moons.

Part of the reason we exist is that the Earth has the right orbit, the right day length, the right axial tilt and a moon. Are these properties unusual? More observations of the heavens will help answer this question. Even if we conclude that close twins of Earth are rare, the cosmos is so large there may still be hundreds of thousands, or millions, of them in our galaxy alone. Yet life may not need an Earth-like planet to get going. Life could potentially arise on planets very different from Earth, or even on moons that harbour liquid water. Enceladus, a moon of Saturn, and Europa, a moon of Jupiter, both have salty oceans of liquid water below a thick layer of ice. Mars had surface oceans similar to those found on Earth today, although by 3 billion years ago they had been lost to space. Another four or five moons of the outer planets potentially have oceans trapped below a thick layer of ice. Could life have evolved on an ancient Mars or on these far-flung moons? We have no evidence that it did, but we have also been unable to look in any detail, so what can we say about life? Is the universe teeming with life, or is restricted to Earth?

The building blocks of life � nucleobases, amino acids and other organic compounds � readily form in space as revealed through analyses of meteors, meteorites and space dust. These molecules also form on Earth. The key molecular building blocks needed to get life started are consequently common, but that does not mean life routinely springs into existence � an energy input is needed. Volcanic energy sources likely powered the emergence of life on Earth. Volcanoes are not unique to our planet. Volcanoes, some dead, have been observed on Mercury, Venus, the Moon, Mars, and Jupiter�s moon, Io. Quite how volcanic energy and organic compounds combine to produce life is unclear, although scientists have plausible hypotheses of how life got started. However, they have not been able to identify the requisite conditions for it to get going in the lab. Nonetheless, membranes readily form, autocatalytic chemical reactions are not uncommon, and simple metabolism seems inevitable. What we don�t know is how easy it is for these key components of life to combine.

Although we do not know the conditions required for life to get started, there is no reason to suspect that the conditions on early Earth were particularly unusual. We have no reason to suspect that the emergence of life is a rare phenomenon. Of course, we cannot rule out that something extraordinarily unusual is required for life to get started and that only happened on Earth, but in the absence of any evidence of this, it seems reasonable to conclude that primitive life is a common feature of the universe. Whether it always uses RNA and DNA is unknown. Perhaps self-assembly manuals can be written in other molecular codes, but given amino acids are found on comets and meteors, my suspicion is that life will usually use RNA and DNA for coding instructions on how to build a cell. But much as French, English and Spanish use very similar alphabets to form a different language, the same base-pair triplets will not necessarily code for the same amino acids in independently evolved DNA-based life. Cytosine�adenine�guanine codes for glutamine on Earth, but it might code for valine or serine elsewhere. Or perhaps a completely different set of amino acids will be used.

Once life begins, competition between individuals, either of the same or of different species, is inevitable. The competition will always be for resources, and it leads to the evolution of life forms that can thrive on a wider range of resource types or exploit new resources. Evolution will always select for individuals that are more efficient at detecting, acquiring and utilizing these resources. Competition between life forms (where they exist) will be pervasive throughout the universe as the fittest replicator will always displace those that are less fit. On Earth, this competition has led to an increase in diversity and complexity in the eukaryote branch of the tree of life. As life diversifies, some of it may inevitably increase in complexity, as competition from a more diverse array of competitors will select for strategies to thrive in ever-more complex environments. Once life gets started, I suspect it will routinely proliferate and increase in complexity.

The next question on the journey from the Big Bang to you and me concerns whether multicellular life will inevitably evolve. First, multicellularity on Earth requires an oxygenated atmosphere. Oxygen will be a waste product of most forms of early life that use chemosynthesis and photosynthesis to power metabolism. It is also extremely reactive, and free oxygen cannot accumulate in the atmosphere until elements or molecules it easily reacts with have been oxidized. Planets are large, and the oxidization of surface molecules takes time. On Earth it took about 1.5 billion years before atmospheric oxygen began to increase.

Multicellular life required atmospheric oxygen to evolve, but once oxygen levels started to rise it evolved at least twenty-five times. Eukaryotes needed mitochondria to use the oxygen to run their metabolism. Mitochondria evolved from bacteria that were able to survive inside other cells. Predation or parasitism had to evolve to allow these bacteria to enter other cells before they were co-opted into a mutualism by the host. Over time, these bacteria evolved into organelles such as mitochondria, that are used by all multicellular organisms to run their metabolism. Evolution is very good at producing organisms that can exploit abundant, under-utilized resources, so it is possible that predation and parasitism would evolve given enough time on other planets. It also seems probable that once predation and parasitism evolved, mutualisms with bacteria living inside host cells could also arise. All eukaryote cells contain mitochondria, derived from a mutualism with aerobic bacteria, while plant cells also contain chloroplasts that evolved from photosynthetic bacteria such as the cyanobacteria. Given such mutualisms evolved independently at least twice on Earth, it seems plausible that they could evolve elsewhere too. My hunch is that multicellular life will almost inevitably evolve from simpler life forms given sufficient time.

On Earth, a lead time of over 2 billion years was required before multicellular life evolved. Our planet had to remain habitable while avoiding cataclysmic extinction events such as local supernovae, meteor impacts, or the loss of atmosphere from solar winds that could wipe out all life. Earth did this, but we currently have no idea whether this is typical or not. It seems likely that once life evolves, it inevitably becomes more complicated, but life is fragile, and is permanently at risk of being killed off by violent cosmic events. We exist because Earth remained habitable for 4 billion years, allowing evolution to work wonders, but we do not know whether such durations within habitable zones are commonplace or rare.

Once complex multicellular life had evolved on Earth, there was no guarantee that humans would appear a little over half a billion years later. Random, unpredictable events driven by mutations mean that evolution is unpredictable. In an experiment with the bacterium E. coli, Rich Lenski allowed twelve colonies of bacteria to adapt to the same environmental conditions. At the time of writing, the experiment had been running for 75,000 generations. Initially it looked as if each population was evolving in a phenotypically similar manner, but examination of the genomes of the bacteria revealed that different colonies accrued different mutations. In other words, although phenotypic adaptations initially looked similar, they were not achieved via the same genetic mutations. Such a pattern is not unusual � there are often multiple genetic routes to the same phenotypic solution.

The most remarkable adaptation occurred in one of the twelve colonies shortly after generation 31,000. In the presence of oxygen, E. coli use glucose as a primary source of energy to run their metabolism, but they can also run their metabolism off a compound called citrate in the absence of oxygen. E. coli in the laboratory are typically fed a diet of glucose and citrate, but will not utilize the citrate in the presence of oxygen, and this was the diet that was fed during Lenski�s long-term evolution experiment. One colony evolved to utilize citrate in the presence of oxygen. It is difficult for E. coli to do this because it involves a series of mutations and not just a single one. These mutations occurred only in one colony. The bacteria in this colony can now use a resource unavailable to the other E. coli colonies. Over longer periods, phenotypic evolution is not repeatable, and there is no guarantee a particular life form will evolve.

Such rare mutational events, perhaps driven by quantum fluctuations, will have played occasional but important roles in the evolution of life on Earth. It is highly improbable that life on other planets would experience the same set and order of random mutation events as occurred on Earth, particularly if it uses different triplets to code for amino acids. Scientists do not know enough about the genetic architecture of most phenotypic traits, or about which phenotypic traits are most likely to arise, and which are less likely to evolve. Life on Earth today is a combination of what has gone before in terms of mutation and selection. Although natural selection can frequently be repeatable, genetic mutation is less repeatable, and this can lead to quite different outcomes over millions of years. Deer and kangaroos have similar ecological roles respectively in Europe and Australia, and they are both capable of thriving on vegetation, but they look very different. The evolution of multicellular life on other planets may result in some outcomes that look familiar to us, but others that might look very odd. Aliens really could be little green men.

Once multicellular life arose, it seems inevitable that evolution would find ways to detect light, sound waves, aromatic molecules and touch, and this would require a central nervous system and some form of brain. Consciousness would then naturally emerge, and in time intelligence would appear. Intelligent life arose on Earth in the form of humans, and their close cousins, the Neanderthals. Other, less closely related species, including dolphins, elephants, crows and octopuses, stand out as also being able to solve quite complex tasks. Although complex language and technological development have evolved only in primates, this does not preclude it from evolving in other species.

Several writers and scientists have concluded that life, and intelligent life in particular, are likely to be rare. I don�t believe we can make that call yet, as we just do not have enough evidence. Most things in our universe, be they galaxies, stars or star systems, have distributions. For example, galaxies may contain a thousand stars or as many as 100 trillion. Their size varies enormously, with the Milky Way lying somewhere in the middle of the distribution. In the absence of evidence, the most sensible thing to conclude is that what you are observing is more likely to be average than not. Our galaxy is average, the sun is average, and rocky planets in the habitable zone are also not at all unusual. It may turn out that Earth is quite typical and that life is abundant. We will not find it on every planet, or even around every star, but if I were forced to guess, I would say that life evolves frequently on rocky planets with liquid water and volcanic activity, and that we are far from the only planet where intelligent life has emerged. Life is chemistry, and chemistry follows sets of rules, so as long as conditions are right, life will emerge.

As a species, humans are very probably unique in the universe. I cannot see how species with the same genetic code as ours evolved to live on planets around other suns. Genetic mutations occur at random. We are a product of genetic mutations raining down on our ancestors� genomes for 4 billion years. An identical sequence of random events will not have happened on another planet, anywhere, so as a species we will be unique.

The development of technologically advanced civilizations will require easy access to energy. Without it, we could not make accessible the large amounts of information necessary to develop and manufacture telescopes, computers and particle accelerators. We built our civilization on coal and, later, oil, and I doubt we could have developed it on tide, volcanic or wind power alone. How common will easy access to energy such as coal be? It is possible that advanced alien civilizations built on fossil fuels are rare, although once again we lack evidence to state this with any confidence. Coal formed on Earth because bacteria and fungi were unable to access the dead plant material from which it formed as a resource, and it consequently formed the hydrocarbons we have built our civilization on. Evolution is a potent force, but it requires genetic mutations that appear at random. Coal formed because the genetic mutations did not happen that would have allowed organisms to use the dead trees of the warm Carboniferous as a resource. If the development of advanced civilizations is dependent on chance genetic mutations, they may be rare. We know how evolution works and why it produces new life forms, and we know we are an accident of evolution. So too may be our civilization, and this means that perhaps very few species achieve an understanding of the universe that is as well developed as ours.

My personality, like yours, is also an accident of history, even if I cannot tell you with confidence which historical events made me the way I am. I have experienced a unique set of events and experiences, and these have helped make me. My personality, my desires and my obsessiveness to understand why I exist are due to a mix of my genes and the environment I have experienced throughout life. Had I been born into Tudor England, Napoleonic France or even ancient Athens, I would likely have developed other desires and motivations. I believe my brush with malaria was key to me being who I am, but I cannot prove that. Despite being an accident of history, scientists have some idea why I have the personality I do: chance, nature and nurture will all have played a role.

I exist because the fundamental forces have the right values, because these forces produce galaxies, stars and planets, and because chemistry is such that life can evolve when conditions are right, and evolution can act to make it complicated and intelligent. I am persuaded that our universe is stochastic, and this stochasticity manifests at the level of fundamental particles, and complex living organisms have free will because life has found a way to exploit this quantum randomness at the level of living beings. Despite this there is still much I want to know before I lie on my eventual deathbed. I want to know why there is something rather than nothing, and what the conditions were that led to life beginning. I doubt scientists will solve the first question in my lifetime, but I hope we will be able to make simple life in the laboratory by the time I die. It may be a long shot, but we are making progress. I know many of the reasons why we exist, but not all. Science has brought us a very long way, but the history of why we exist is not yet complete.

Acknowledgements

I have loved researching and writing this book. I have had a huge amount of support and help, both knowingly and unknowingly, over the years from an army of friends and colleagues, and I owe them all a huge debt of thanks. I cannot list them all, but below are those who have been particularly important. First, thanks to my family. In particular, thanks to my wife, Sonya Clegg. Without her support, encouragement and endless conversations about science and existence, this book could not have been written. Thanks, too, to my children, Sophie, Georgia and Luke. Perhaps one day they will read the book in its entirety. Or perhaps not. They have had to listen to me wittering on about science and existence all their lives.

When we got Woofler as a family pet, Sonya, Sophie, Georgia and Luke agreed to regularly walk him. They did for the first week or two. Thanks for not walking him much after that, and thanks to Woofler for needing walks. It was during these walks around the parks of Oxford and Brisbane that I composed much of the prose I wrote down on my return home. I could not have completed this book without being forced away from the keyboard to think.

The first full draft of this book was written during a sabbatical year in Australia, much of it spent at my father-in-law�s farm. Thanks to farmer Col for putting up with the Oxford don sat writing on the veranda, and his friends Eve-Ann Springate and Robert Hudson, who I spent many evenings chatting with. Thanks for the beer and good humour you provided while we sat around the best stove on the planet. Thanks too to Angela, Alex, Ava and Stella for making my year in Australia so enjoyable.

Rebecca Carter, my agent, has been amazing from the day I met her. Thanks for believing in me and in the idea of the book, and trusting that I could deliver it. Thank you also for answering all my questions about publishing, and for finding such a great publisher. Without Rebecca this book would not have seen the light of day.

Penguin Michael Joseph has been fantastic throughout the whole process. Alan Samson has been a wonderful editor. He has provided insightful and thoughtful comments that have improved the text no end, making incomprehensible first drafts of chapters accessible. Alan�s patience, good humour and fabulous editing have made writing this book enjoyable. Dan Bunyard has kept the project on track, moving it seamlessly through each stage of the publishing process. Sarah Day has been a keen-eyed and expert copy-editor and Sukhmani Bhakar has organized the illustrations with supreme efficiency.

Claiborne Hancock and Jessica Case at Pegasus Books bought the rights to publish in the US. Thank you for believing in the book, and for being so wonderfully efficient in designing the US release. Margaret Halton, Terry Wong and Rebecca Sandell at Pew Literary have made securing overseas publication rights very smooth, and I am extremely grateful for all their hard work.

Bryony Blades, Sonya Clegg, Richard Dawkins, Jane Hodgson, John Park, Quentin Painter, Ana Rivero, Rick Stockwell, Tabby Taberer, Tianqi Wang and Andrew Wood all provided insightful comments on drafts of the complete manuscript, while Phil Burrows, Sarah Hilton, Chris Summerfield, Anna Vinton and Wai-Shun Lau have commented on the chapter in areas they research. Their advice and correction of errors are greatly appreciated. Any inaccuracies or misrepresentations that remain are entirely my fault.

I am grateful to numerous friends and colleagues who have helped me become a scientist and thrive. In particular Steve Albon, Charlie Canham, Tim Clutton-Brock, Mick Crawley, John Lawton, Steve Pacala and Josephine Pemberton were inspirational mentors during my graduate and postdoctoral studies. More recently, I have had wonderful and illuminating discussions in Trinidad and Yellowstone with Ron Bassar, Dan MacNulty, David Reznick, Doug Smith, Dan Stahler and Joe Travis. I have also collaborated a lot with my close friends Jean-Michel Gaillard, Pete Hudson and Shripad Tuljapurkar over many years, and I have spent countless enjoyable hours discussing numerous topics with them. Their views on science and the way it is done have helped mould my take on the universe and how it works.

Conversations with Anne Carlson, and then Amanda Niehaus, gave me the belief I could write this book, and I am extremely grateful to both of you for your encouragement. My friends Greg Devine, Djami Djeddour, Dominique Eza, Mike Furlong, Sally Gibbons, Nigel and Gen Griffiths, Sunetra Gupta, Richard Hobbs, Loeske Kruuk, Bob Montgomery, Ian Owens, Rick Paul and Sue Scull are yet to read the book, but have all helped keep me sane and happy at various points over the last thirty-five years. I have had conversations with each on the meaning of existence at some point. Some conversations were more memorable than others, but primarily due to the good food and wine rather than the content.

Lastly, thanks to my parents, Anne and Patrick, and my sister, Fiona. Without their encouragement and support over the last fifty-five years I could not have pursued a career as a scientist nor written this book. Sometimes my parents have not been entirely certain what it is I do for a living, so perhaps this book provides some answers. I have been doing quite a bit of thinking. I know they won�t agree with all I have written, but I do hope this book makes them proud, and I look forward to discussing life, the universe and everything with them for many years to come.

About the Author

TIM COULSON is Professor of Zoology at the University of Oxford. He has worked as an academic teaching biology for three decades and has published over two hundred peer-reviewed articles. Tim chose to study biology because he was drawn to its inherent complexity, as his desire was to become a specialist in complex systems. His teaching and research have been rewarded with international awards from the Royal Society, the British Ecological Society, the Swedish Oikos Society, the Zoological Society of London, Imperial College London, and the University of Oxford. Tim regularly gives interviews on the BBC, Sky, and Channel 4 as a talking head. Tim lives in Oxford, England.

Further Reading

Each chapter in the book describes what had to happen for us to exist. There is a lot that is not discussed. If any part of the book piques your interest and you wish to learn more, I provide details of a few of the very many texts I have read while researching this book. Each book goes into more detail than I was able to. Because popular science books tend to be on a specific science such as chemistry or physics, I do not group them by the chapter titles in this book.

Image Credits

p. 23: The Scientific Method � Dima/Alamy Stock Photo

p. 112: The Two-Slit Experiment � Samya Jakane/Alamy Stock Vector

p. 115: Image of a Hydrogen Atom, reprinted figure with permission from A. S. Stodolna, A. Rouz�e, F. L�pine, S. Cohen, F. Robicheaux, A. Gijsbertsen, J. H. Jungmann, C. Bordas, and M. J. J. Vrakking, Physical Review Letters, 110, 213001, Published 20 May 2013 � 2013 by the American Physical Society: http://dx.doi.org/10.1103/PhysRevLett.110.213001

p. 138: The Sun�s Position in the Milky Way, courtesy NASA/JPL-Caltec

p. 156: Anatomy of the Earth � Yuri Arcurs/Alamy Stock Photo

p. 177: DNA Structure � Jawad/Alamy Stock Vector

p. 275: Structure of a Neuron Cell � KKT Madhusanka/Alamy Stock Photo

p. 279: A Network of Neurons � Siarhei Yurchanka/Alamy Stock Photo

p. 306: Timeline of Human History, illustration of the first primate � Friedrich Saurer/Alamy Stock Photo, and hominin illustration � Puwadol Jaturawutthichai/Alamy Stock Photo

p. 360: Big Five Personality Traits � Chavapong Prateep Na Thalang/Alamy Stock Vector