The Short Story of How We Came About

The Question

This is the short story of how you came to be. Not how you, personally — though that comes into it at the end. How any human being came to exist, as the kind of animal we are, on the kind of planet we live on, in the kind of universe that contains us.

The story is long. About thirteen and a half billion years long. Most of it has nothing obviously to do with us. A planet shapes, an ocean fills, a strange kind of chemistry takes hold, the chemistry copies itself, the copies diverge, the copies build bodies, the bodies build minds. That is the whole shape of it. The detail is what fills the middle.

What follows is the gist. You will not find here the names of the rocks where particular fossils were found, or the methods that dated them, or the journal articles that argued out the contested points. Those things matter, and they live in the longer companion entries that this one is the short version of. What lives here is the basic shape of how an animal capable of writing these words came about.

Take it slowly. There is a lot of time to cross.

The Beginning

About thirteen and a half billion years ago — the number is approximate, and the genuine uncertainty around it is not large enough to matter for a story of this scope — the universe began.

It is hard to picture what "the universe began" means. The phrase is not describing an explosion in some larger empty space, with a centre and an outside, because there was no larger space. There was no outside. Space itself, and time itself, came into being. From an extraordinarily hot and dense initial state, everything that now exists started to expand outward and cool down.

For a few hundred thousand years, the universe was so hot that no atoms could form — only a glowing fog of charged particles. Then it cooled enough for the first atoms, hydrogen mostly, with a smaller amount of helium and a trace of lithium, to lock together. The fog cleared. Light, for the first time, could travel freely across the new universe.

For a few hundred million years more, the universe was dark again. The hydrogen and helium were spread thin, but not perfectly evenly. Slight unevennesses, present from the beginning, allowed gravity to begin pulling the gas into denser regions. Those regions collapsed. Their centres heated. When a core grew hot and dense enough, the hydrogen at its centre began to fuse into helium, releasing light and heat in the process. The first stars switched on.

Stars then began to gather, by gravity, into the immense rotating collections we now call galaxies. Our own galaxy — the spiral structure your night sky calls the Milky Way — formed during this stretch.

The first generation of stars did not have any heavier elements in them. There were no carbon atoms yet, no oxygen, no iron, no silicon. All of the elements that you and your planet are made of, beyond hydrogen and helium and a touch of lithium, did not exist anywhere in the universe. They had to be made.

Stars made them. At the cores of stars, the immense pressure and temperature drives nuclear fusion: hydrogen becomes helium, helium becomes carbon and oxygen, carbon and oxygen become heavier elements still. The largest stars, when they reach the end of their lives, explode in events called supernovae, which are the only natural environments in the universe hot enough to forge most of the heaviest elements — gold, uranium, lead — and they scatter all of it back out into space.

Every atom of carbon in your body, every atom of oxygen you breathe, every atom of iron in your blood, was made inside a star that lived and died before our own Solar System existed. You are, in the most literal sense, made of star material.

After many cycles of stellar birth and death, regions of the galaxy where the gas and dust had been enriched by previous generations of stars became the kind of places where the next generation of stars could form planets along with itself. About four and a half billion years ago — almost exactly two-thirds of the way through the history of the universe to date — a region of our galaxy did exactly this. A cloud of gas and dust collapsed under gravity. At its centre, a new star caught fire. Around it, a flat rotating disk of debris condensed into rocky and gassy and icy bodies. Eight of those bodies grew large enough to count as planets. The third of them, counting outward from the star, was Earth.

The Young Planet

Earth's first hundred million years were violent. The planet was still being struck, regularly, by leftover debris from the formation of the Solar System. Its surface was molten in many places. The radioactive elements lodged inside it kept its interior hot. It had no breathable atmosphere — the gases venting from its crust were carbon dioxide, nitrogen, water vapour, and various smaller compounds, with no free oxygen at all.

A short time into its history, on the geological scale, another body — about the size of Mars — collided with the young Earth. The collision was so violent that a great quantity of material was thrown into space and gathered, by gravity, into a satellite. That satellite is the Moon you see when you look up at night.

As the planet cooled, water vapour in its atmosphere condensed and fell as rain. Where the rain accumulated, it formed oceans. By about four billion years ago, the surface of Earth was a salty, mineral-rich, lightning-storm-prone, volcanically active world covered in shallow water and islands of bare rock.

There was no life yet. Nothing on the surface that you would recognise as alive. Only chemistry: countless reactions running every second across the seas and on the shorelines and around hot springs and undersea vents, atoms forming compounds, compounds breaking down, bigger compounds occasionally assembling and lasting a while before falling apart.

Most of those reactions went nowhere. They had no future. A molecule formed, broke down, was reabsorbed into the soup. But on a planet that vast, with that many places, running for that many millions of years, the number of chances was enormous. Even a very rare event becomes nearly certain when the number of opportunities is large enough.

What had to happen, for the story to continue, was the appearance of a particular kind of molecule — a molecule that could make copies of itself.

The Replicator

This is where the story turns, and the turn is worth slowing down for. Most readers who never go further than a quick acquaintance with this material come away with the impression that life "appeared" in some unexplained way. It is worth taking a moment to see, as concretely as possible, what the moment of appearance actually was.

Imagine a molecule that is shaped roughly like a comb. Along one edge of the comb are little hooks, each of which has an attraction for a particular small piece floating around in the surrounding chemical soup. Some hooks attract pieces of one shape; other hooks attract pieces of another shape. The order of the hooks along the comb determines which small pieces will line up against it.

Now suppose that a comb of this kind exists, somehow, in the soup. Small pieces will tend to drift up and stick to the appropriate hooks. After some time, a complete row of small pieces is held in place along the comb, in the order the hooks dictate. If the small pieces, once they are next to each other, also tend to stick to each other — if their chemistry favours it, in the way carbon atoms tend to bond into chains — then the row will weld itself together into a new comb, alongside the old one. The two combs will then break apart. Where there had been one, there are now two.

Each of the two will then attract its own row of small pieces, weld them into another comb, break apart, and yield two more. One becomes two. Two become four. Four become eight. The molecule has copied itself, and the copies have copied themselves.

This is what the first replicator was: a molecule whose shape made it copy itself when surrounded by the right kind of chemical soup. We do not know exactly which molecule the first one was on early Earth. Most working accounts of how this happened settle on a relative of the molecule that, today, every cell on the planet still carries: a molecule called DNA, which sits inside every one of your cells right now, and a slightly simpler relative of DNA called RNA, which is plausibly the older of the two. The earliest replicator was not yet either of these in their modern forms. It was something earlier, simpler, and probably less efficient. The detail is still being worked out.

What matters is the property the first replicator had, not the precise chemistry. It made copies of itself.

Now, here is the move.

The copies were not always perfect.

Sometimes, during the copying, a small piece would land in the wrong slot. Sometimes the order of the small pieces along the new comb would be slightly different from the order along the original. These were rare events, but they happened. Over many generations of copying, the soup did not contain a single replicator producing identical clones. It contained a population of slightly different variants of the original, all descended from it, none of them quite identical to one another.

Most of the variants were worse than the original. They held together less well. They copied themselves more slowly, or only intermittently, or not at all. They faded.

But occasionally — rarely, but inevitably, because the number of copies and the number of generations was so large — a variant turned out to be better than its parent at making copies of itself in that particular soup. It held together more securely. It copied itself faster. It scavenged its building blocks more efficiently.

A variant of that kind left more descendants. Its descendants inherited its advantage and left more descendants in turn. After many generations, the soup was no longer dominated by the original replicator. It was dominated by the descendants of the slightly improved variant.

This is the moment that matters most in the entire story. From this moment on, the chemistry of Earth's oceans was no longer drifting at random. Whatever copied itself well, persisted. Whatever did not, faded. The future of the planet's chemistry was being filtered, automatically, by which versions of the chemistry kept making more of themselves.

Everything else in this entry flows from this single fact.

Evolution By Natural Selection

The pattern just described — variants arise, the more successful ones leave more copies, the population shifts over generations — is what biologists in our time call evolution by natural selection. It is the central idea in all of biology. It is the explanation for why life on Earth looks the way it does. It is, by any reasonable standard, one of the most important ideas any human has ever had.

It is also a simple idea. Once you see it, the temptation is to think you have always known it. The reason it took the human species hundreds of thousands of years to articulate it is that most of the conditions you would need to test it — long timescales, populations of organisms in detail, the chemistry of inheritance — were not visible to ordinary observation. Once they became visible, the idea was waiting.

Here is how it works, in plain language. Take any group of living things at all — bacteria in a pond, fish in a river, deer in a forest, people in a village. Three things turn out to be true of any such group.

First, the individuals in the group are not all identical. They differ from one another in size, in colour, in strength, in resistance to disease, in temperament, in a hundred other ways. Some of the differences are large and obvious. Most are small. But there is no group of living things in which every member is a perfect copy of every other.

Second, when individuals reproduce, their offspring resemble them. Tall parents tend, on average, to have tall children. Fast-running parents tend to have fast-running children. Disease-resistant parents tend to have disease-resistant children. The resemblance is not perfect — children are not exact copies of their parents, just as the early replicators were not exact copies of theirs — but the resemblance is real. A trait carried by a parent has a higher chance of being carried by that parent's child than by a child taken at random from the wider population.

Third, individuals do not all leave the same number of descendants. Some die young. Some never find a mate. Some have many offspring; some have few. The reasons are various — predators, disease, weather, food, accident, mating success — but the bare fact of the matter is that some individuals leave more descendants than others.

Now put those three facts together.

If the individuals who happen to leave more descendants are, on average, the ones who carry a particular trait — if, say, the deer with thicker fur survive cold winters more often than the deer with thinner fur, and the survivors then breed — then the next generation of the group will contain, on average, more thick-furred deer than the previous generation did. Not because anyone planned it. Not because the cold "selected" anything in any conscious sense. Simply because the thick-furred deer left more descendants, and their descendants inherited the same trait, and the proportions of the group shifted.

Run this for many generations. Run it for thousands. Run it for millions. The group changes. The traits that once were rare become common. The traits that once were common become rare. After enough generations, the group no longer looks like the group it descended from.

That is evolution by natural selection. It is not a force in the way gravity is a force. It is not aimed at anything. It is not striving toward complexity, or toward us, or toward anything else. It is the inevitable arithmetic of three observations: variation exists, variation is inherited, and not all individuals reproduce equally.

Take any one of those three away and the engine stops. If every individual were identical, there would be nothing to select among. If traits were not inherited, the next generation would start fresh and any change would be erased. If every individual reproduced equally, the proportions would never shift. But all three are true of every population of living things on the planet. They have been true for nearly four billion years.

The consequences are everything alive.

The deer with thicker fur survive cold winters and have more calves; the moths with darker wings escape the predator birds in soot-blackened forests and breed; the bacteria with mutated proteins survive the antibiotic and divide; the early hominin who fell ill and died left no children, while the one who did not survived to feed her children, who fed theirs. This is the same machinery, working at the same scale, in every generation of every species since life began. The peacock's tail, the elephant's trunk, the orchid's flower, the cuttlefish's colour-changing skin, the eagle's eye, the bat's sonar, the human brain — all of them are the accumulated answers to the question of which variants leave more descendants in their environment, summed over an unimaginable number of generations.

If you understand only one thing about how life on this planet got to be the way it is, this is the thing to understand. The rest of the story is what this engine produces when it runs for four billion years on a planet of finite size.

A small note before going on, because the word species is about to come up many times. A species is, roughly, a group of living things that interbreed with one another and produce viable offspring, and that do not — or only rarely — interbreed with members of other such groups. The boundaries are not always sharp; closely related species sometimes interbreed and leave living descendants together, as this story will show when it reaches our own kind. New species form when a single ancestral group is split into two, usually by a geographical barrier, and the two halves drift apart in their characteristics over generations until they can no longer interbreed even when reunited. This happens continually. Every species alive today is descended, through a chain of such splits, from earlier species; trace any two species back far enough and you will find an ancestor common to both.

That is the engine. The rest of this entry is the story it produced on Earth.

The First Cells

The earliest replicators were just molecules, exposed to the soup. They had no body, no skin. They drifted in the water, copied themselves, and were buffeted by whatever the water was carrying. A replicator with a slight ability to gather around itself a thin oily film — a kind of bubble, pinching off the surrounding chemistry from the chemistry inside — was a replicator with an advantage. It could keep its own building blocks close. It could fend off interference from outside. And so, somewhere in the early oceans, replicators that had this property became more common.

Once replicators were enclosed in such bubbles, the bubble itself became part of what was being copied. Instructions for building the bubble, like instructions for building the replicator, were preserved in the molecule that copied itself. The bubble was no longer accidental. It was inherited.

This is what a cell is: a small enclosed bag of chemistry, descended from the chemistry that was in its parent's bag, copying itself continuously. By about three and a half billion years ago, perhaps somewhat earlier, the first cells existed on Earth. They left traces in some of the oldest rocks the planet still preserves.

For a long time — well over a billion years — these were the only kinds of organisms that existed. They were small, simple cells, individually visible only under a microscope. They were not yet plants or animals or anything else you would recognise. But they were already alive. They reproduced. They competed for resources. They evolved. They were the ancestors of every living thing on this planet.

In time, some of them learned how to use sunlight. The sun pours energy on the surface of Earth all day, every day. A cell that could capture some of that energy and use it to fuel its own chemistry — a cell that had invented photosynthesis — had a near-limitless source of fuel that other cells could not access. Photosynthesising cells multiplied. They covered the shallow seas.

But photosynthesis has a side-effect. It splits water molecules to extract their hydrogen, and it releases the oxygen as waste. For most early life, oxygen was a poison — it reacted aggressively with the chemistry inside cells and damaged them. As photosynthetic cells multiplied, oxygen accumulated in the seas, then in the air. Over hundreds of millions of years, the planet's atmosphere changed character entirely. Iron-rich rocks rusted. The shift in atmospheric chemistry also tipped the climate; glaciers spread. Many of the older lineages of cells died out, or retreated to dark, oxygen-free corners where their descendants still live today. Other lineages adapted to the new gas, and a few learned to use it — to combine it with sugar to release energy, far more efficiently than anaerobic chemistry could. That trick — breathing — is the trick you are using now to make sense of these words.

The world had been remade by its own occupants. It would be remade again, several times more, before this story was done.

Complex Cells And Complex Bodies

About two billion years ago, on the order of magnitude, something else happened. One simple cell engulfed another simple cell and did not digest it. The engulfed cell continued to live inside its host. Over generations, the two became one — the engulfed cell, simplified by long residence inside the host, kept providing energy through its own ancient chemistry, while the host kept the small inner cell sheltered and supplied with raw materials.

This sounds like a bizarre accident, and it almost certainly was — once. But the result of it was a new kind of cell, much larger and more capable than its predecessors, with a small power station running inside it. That new kind of cell is the kind every plant, animal, and fungus on Earth is made of, including you. The small power stations inside your cells still carry their own little loop of DNA — a remnant of the ancient cell that took up residence inside an ancestor of yours nearly two billion years ago.

For another billion years or so, even these more complex cells lived as single individuals or as loose cooperative mats. The next big step was bodies — multi-cellular organisms, in which many cells of related kinds work together as one creature. By about a billion years ago, simple multi-cellular forms were appearing. By about six hundred million years ago, recognisable animals — soft-bodied, sea-dwelling, with body plans that included a front and a back, an inside and an outside, a digestive tract — had begun to appear in the oceans.

A short stretch later, by geological standards, came an event sometimes called the Cambrian Explosion. About half a billion years ago, over a span of perhaps twenty or thirty million years, almost all the major animal body plans we now know — arthropods, molluscs, worms, sponges, the early relatives of vertebrates — appear in the fossil record. The seas filled with creatures: hard-shelled predators, jointed-legged scavengers, sediment-feeding worms, primitive fish-like animals with a dorsal nerve cord. The oceans became, for the first time, what we would recognise as alive.

For another hundred million years, life remained almost entirely aquatic. The land was bare rock, baked by the sun, stripped by the wind, with no soil and no forests. Then, gradually, plants began to colonise the shorelines and creep inland. Small arthropods followed. The continents started to turn green.

Onto The Land

The vertebrates — the animals with backbones and inner skeletons — reached the land later. Their ancestors were fish that lived in shallow, oxygen-poor swamps, where the ability to gulp air at the surface was an advantage. From those fish came the first amphibians, animals with limbs strong enough to crawl ashore but still tethered to water for reproduction. Their eggs, like the eggs of fish, had to be laid in water; their delicate embryos would dry out anywhere else.

Then, about three hundred million years ago, a new kind of egg appeared. It was a self-contained capsule, with its own water supply and food and waste storage and protective membranes, inside a leathery or hard shell that could be laid on dry ground without drying out. The animals that laid this egg were no longer chained to bodies of water for reproduction. The dry interior of the continents was open to vertebrates for the first time.

Almost as soon as these dry-egg-laying animals appeared, their lineage split in two. The branches that descend from each side of that split are still alive today. One side will end up producing all the modern reptiles and birds. The other side will end up producing all the modern mammals — including the species writing these words.

The two branches went their separate ways for over two hundred million years. Both produced extraordinary radiations of forms. Both came close to extinction more than once.

For a long stretch, the mammal-side branch was the dominant one. Through the long Permian period, before the first dinosaurs, the dominant land vertebrates on Earth were a great variety of creatures from the lineage on the mammal side of the split — though the animals themselves were not yet anything we would call mammals. Some were sail-backed predators the size of a large pig. Some were tusked herbivores. Some were dog-sized and ran on upright legs. They occupied every land niche there was.

Then the world ended.

The Great Dying

About two hundred and fifty million years ago, an enormous outpouring of volcanic lava in what is now Siberia poured over a region the size of a continent. The eruptions cooked thick layers of buried sedimentary rock and released vast quantities of carbon dioxide, methane, and other gases into the atmosphere. The atmosphere warmed. The oceans acidified. Oxygen levels in the seas crashed. Coral reefs collapsed. Plants and animals everywhere were caught up in the fastest, most complete biological catastrophe in the history of complex life.

About nine in every ten species of marine life disappeared. The mammal-side branch that had dominated the land was almost entirely wiped out. The reefs that had taken hundreds of millions of years to assemble were gone. For nearly ten million years afterwards, the planet's vegetation was so impoverished that no thick organic deposits — the kind that turn into coal — formed anywhere on Earth.

A handful of small mammal-side animals came through. So did a handful of reptile-side animals. They inherited an empty world.

In the empty world that followed, the reptile-side branch took the lead. From it, after a few tens of millions of years of recovery, emerged a new lineage of bipedal predators — animals that ran on two legs, with their tails balancing their bodies. Those were the first dinosaurs. The mammal-side branch, meanwhile, was reduced to small, mostly nocturnal creatures the size of a shrew or a rat, scratching out a living at the margins of an increasingly dinosaur-dominated world.

The Age Of The Dinosaurs

For the next hundred and thirty-five million years, dinosaurs were the dominant land animals on this planet. The numbers are worth pausing on. The whole span of recognisable human evolution, from the earliest upright-walking apes to the present, is about seven million years. The dinosaur reign was twenty times that long. They were not a brief experiment. They were the standard for so long that, if you took a typical day from anywhere in the middle of their reign and a typical day from a million years later, the day would look almost the same.

They diversified into every imaginable size and shape. The long-necked plant-eaters grew larger than any land animal that had existed before or has existed since — the largest weighed about as much as a herd of African elephants. The bipedal predators included the great theropods such as the famous Tyrannosaurus, which arrived only at the very end of the dinosaur era. Some dinosaurs were heavily armoured. Some had elaborate horns and frills. Some were small, fast, and feathered. The feathers are worth a moment, because they reveal something often missed.

Birds — the songbirds in your hedge, the ducks on the pond, the gulls at the harbour, the chickens on the farm — are not the surviving cousins of the dinosaurs. They are the surviving lineage of dinosaurs themselves. Specifically, they are the surviving descendants of one branch of the small, feathered, bipedal predators. Feathers were not invented by birds; they had been around for tens of millions of years before any creature used them for flight, working initially for insulation, display, and possibly the brooding of eggs. Birds are simply the one feathered dinosaur lineage that survived what came next.

While the dinosaurs ruled the land, the mammal-side branch — the small, mostly nocturnal animals that had come through the Great Dying — lived quietly in the shadows. They were busy in their own way. The basic body plan of a mammal — fur for insulation, milk to feed young, internal warmth to keep going through cold nights, sharper hearing and a keener sense of smell to compensate for poor light — was assembled during this long stretch, in animals that mostly stayed out of the dinosaurs' way. The three modern mammal groups — the egg-laying mammals like the platypus, the pouched mammals like the kangaroo, and the placental mammals (the much larger group containing every other modern mammal, including you) — all originated during the long shadow of the dinosaurs.

While this was unfolding, plants were undergoing their own transformation. The dominant land plants for much of the dinosaur era were conifers, ferns, cycads, ginkgoes, and their relatives — plants that reproduce through cones and bare seeds. Sometime in the second half of the dinosaur reign, a new lineage of plants appeared and began to spread: the flowering plants. Within a few tens of millions of years they had taken over most terrestrial habitats, evolved trees with broad leaves, formed elaborate partnerships with insects and other animals to spread their pollen and seeds, and begun producing the fruits, nuts, and seeds on which most modern terrestrial life now depends.

The End Of The Dinosaurs

About sixty-six million years ago, an asteroid roughly ten kilometres across — wider than Mount Everest is tall — struck the shallow tropical sea covering what is now southern Mexico. The energy released by the impact was billions of times greater than any nuclear weapon human beings have ever built. The immediate region was vapourised. A wall of water crossed the surrounding ocean. Pulverised rock and burning debris were lofted into the upper atmosphere.

The dust and soot that rose into the upper air did not settle quickly. For months, possibly years, sunlight on the planet's surface was severely reduced. Photosynthesis on land and in the surface ocean failed. The food chains that depended on photosynthesis collapsed. The plant-eaters starved; the predators that ate the plant-eaters starved; the predators of the predators starved.

Every non-bird dinosaur died. So did the great flying reptiles, the marine reptiles, the spiral-shelled ammonites, the majority of marine plankton, and a substantial fraction of plants and insects. Most species on Earth simply vanished from the rock record at this boundary. It is the sharpest, clearest line in the entire fossil record of complex life.

A few groups came through. The crocodiles. The turtles. The lizards and snakes. Some fish. Some birds — the ones small enough to live on whatever remained, in the cracks of the ruined ecosystem. And among the most fortunate: the small, mostly nocturnal mammals, who had been living for a hundred and thirty-five million years on exactly the kind of small, opportunistic, low-energy lifestyle that survives a catastrophe of this kind.

The world they inherited was empty.

The Mammalian Radiation

For a hundred and thirty-five million years, mammals had been small. Within ten million years of the asteroid impact, that had begun to change. Within twenty million years, mammals had filled almost every large terrestrial niche the dinosaurs had vacated. There were rhinoceros-sized herbivores, horse-like running animals, semi-aquatic forms, fully aquatic forms (the ancestors of modern whales and dolphins moved into the seas during this stretch), arboreal forms, and predators of every size from weasels to bears. Birds, the surviving dinosaur lineage, also radiated rapidly into the air and onto the open ground. Flowering plants continued their expansion. Insects diversified alongside the plants.

Among the radiating mammal groups, one in particular concerns this story: the small, tree-dwelling mammals that became the primates.

The earliest primates were squirrel-sized, mostly nocturnal, omnivorous animals living in the canopy of broadleaved forests in the warm period that followed the dinosaur extinction. Living in trees, where falling means dying, selected powerfully for certain features. Eyes that pointed forward, with overlapping fields of view, gave precise depth perception for jumping between branches. Hands and feet with grasping fingers, ending in flat nails rather than claws, gripped branches securely. Long childhoods allowed time to learn the complicated geometry of the forest from a parent. Social groups offered protection. The basic primate body plan — forward-facing eyes, dexterous hands, social groups, long childhoods — was in place within a few tens of millions of years after the dinosaurs were gone.

These primates spread across the forests of the warm Northern Hemisphere. Different branches went different ways. One led, eventually, to the modern lemurs and lorises. Another, which kept the primates' tree-dwelling habits but elaborated on them, led to the tarsiers, the monkeys, and — much later — the apes.

The apes appeared as a distinct lineage about twenty-five million years ago. They were larger than the monkeys, with more flexible shoulders and no tail. Around fifteen to twenty million years ago, ape lineages spread across Africa and Eurasia in considerable variety. For millions of years they flourished. Then, around ten million years ago, the climate of the planet began to dry. Forests retreated. Grasslands spread. Most of the diverse ape lineages went extinct. A handful of branches survived — the ones leading to today's gibbons, orangutans, gorillas, chimpanzees, bonobos, and humans.

About seven million years ago, in Africa, a particular lineage of ape diverged from the lineage leading to the modern chimpanzees. The animal at that branching point was not a chimpanzee — it was an extinct great ape from which both modern chimpanzees and modern humans descend. After the split, the chimpanzee lineage stayed in the forests. The other lineage — the one leading to us — began to do something different.

Coming Down From The Trees

For the first few million years after the split, the human-side lineage produced animals that were still, in most respects, apes. They had ape-sized brains. They had ape-sized faces. They still climbed trees. What was different about them, even in the earliest forms we can reconstruct, was that they had begun to walk upright on two legs.

This was a strange thing to do. Most quadrupedal animals can stand briefly on two legs but cannot make a habit of it. Walking on two legs requires a re-engineering of the hips, the knees, the spine, the inner ear, and the way weight is balanced over the body. Doing it badly is dangerous; doing it well takes evolutionary time. Why a particular ape lineage should have set off in this direction is still being argued. Possibilities include the ability to see further across grass, the ability to carry food and infants over long distances, the energy efficiency of walking long distances at low speeds, and the freeing of the hands for tool use and gathering. Most likely, several of these mattered together.

Whatever the reasons, the bipedal experiment continued. By about four million years ago, the lineage was producing animals that were unambiguous habitual walkers. They had small brains, no larger than a chimpanzee's. They had jutting faces. They were probably not much smarter than a chimpanzee. But they walked on two legs, and they had been walking on two legs for over two million years already.

A million years on, the most famous of these animals lived. Her finders gave her a name: Lucy. Lucy is a fossil skeleton, recovered from a hillside in Ethiopia in the 1970s, dated to a little over three million years ago. She stood about a metre tall. She weighed perhaps thirty kilograms. Her hips, her knees, and her feet were the hips and knees and feet of a habitual walker. Her brain was about the size of a chimpanzee's.

Lucy was almost certainly not your direct ancestor. She was, more likely, a near-relative of the line that led to you, from a closely related branch of the same family of upright-walking apes. There may have been several such branches living at the same time across eastern Africa. Some of them were our direct line. Some were not.

A confirmation that animals like Lucy walked upright comes from a place in northern Tanzania where, about three and a half million years ago, three of them walked across a layer of fresh volcanic ash that had just fallen and was still wet. The ash was buried before it could erode. The footprints have been preserved, undisturbed, for three and a half million years. They show heel strikes, arched feet, and the parallel toe pattern of a habitual walker. Whoever those individuals were, they were walking the way you are walking when you walk.

About three million years ago, some of these upright-walking apes began to flake stones into useful tools. The first stone tools we have found are simpler than later ones — large rocks struck with other rocks to produce sharp edges that could cut flesh, smash bones to extract marrow, or process plant food. They are the earliest evidence we have of any animal doing something that anticipates technology. The toolmakers were not yet in the lineage that biologists today call our genus, Homo. They were upright-walking apes, or a close relative.

Around two and a half million years ago, a particular lineage of these animals crossed enough of a threshold in jaw shape, tooth size, and probably brain size that, in retrospect, we recognise it as the start of the genus to which our own species belongs. The genus is Homo. The fossils that mark its earliest known appearance are fragments — a lower jaw, a few skull pieces — but they are unmistakably on a path that the previous animals were not on.

From here, the brain begins to grow.

Homo Erectus And The First Departure

By about two million years ago, a more derived form of the genus had appeared in the African record. We call it Homo erectus. With Homo erectus, several features converge that would now be familiar to anyone meeting one. The body had modern proportions — long legs, shorter arms, a tall narrow trunk built for endurance walking and running. The brain was about twice the size of the brain of Lucy's lineage, though still notably smaller than yours. The teeth and jaws were considerably reduced, suggesting a diet that included more meat, more processed food, possibly more cooking. And, for the first time in this story, the lineage left Africa.

Within roughly a hundred thousand years of Homo erectus appearing in Africa, members of the genus had walked north and east, through the Middle East and into Eurasia. By about a million years ago, they were established as far east as what is now Indonesia and as far north as what is now China. Some populations of Homo erectus persisted in southeastern Asia until perhaps a hundred thousand years ago — meaning that this single species was around, in some form, for roughly two million years. That is more than ten times longer than our own species has existed so far.

Homo erectus is also associated with a more sophisticated stone tool tradition than any earlier human species had produced. Where the earlier tools had been a few flakes struck off a rock, the new tools were planned shapes, often almond-shaped, flaked symmetrically over their entire surface. They were the first objects in the history of life that had been imagined before they were made.

Somewhere during this stretch, at a time still being argued, the lineage learned to control fire. The evidence for the earliest controlled fire is patchy — fire is destructive, and natural fires are common — but by about half a million years ago, the use of fire is unmistakable in the record, and there are arguments for it being at least twice as old as that. Fire was a profound piece of technology. It cooked food, which yielded more useable energy from less chewing time. It extended the day. It deterred predators. It allowed the populations that controlled it to survive in cold climates where they could not otherwise have lived. Whatever lineage first learned to keep a flame alive overnight had stepped over a threshold the rest of life on this planet would never reach.

Several Human Species At Once

For most of the last million years, more than one species of human-grade animal lived on this planet at the same time.

This may be the most surprising fact in the entire arc of recent human history, because in our own short lifetime we have known only one. There is, today, one human species. There has been only one human species for somewhere between thirty and forty thousand years. Before that, the world had several at once.

Around half a million years ago, the lineage descended from Homo erectus had split into several branches. One branch, in Europe and western Asia, became the Neanderthals — stocky, cold-adapted, with brains slightly larger than yours, sophisticated stone-working traditions, the use of fire, the use of pigment, deliberate burial of their dead, and care for their injured. They were not the brutish caricatures of older popular accounts. They were people of a different kind, who had been around for several hundred thousand years before our own species reached their territory.

A sister group of the Neanderthals, called the Denisovans, lived across central and eastern Asia. We have only a handful of their fossils — a finger bone, some teeth, a partial jaw — but we have their genomes, recovered from the bones, and from those genomes we know they were a distinct human population, as different from us as the Neanderthals were.

On an island in what is now Indonesia, an isolated population of small humans, only about a metre tall, persisted until perhaps fifty thousand years ago. In a deep cave system in southern Africa, another small-brained but otherwise distinct human species was alive surprisingly recently. There may have been others we have not yet found.

In Africa, meanwhile, around three hundred thousand years ago, our own species was taking shape. The earliest fossils that we recognise as Homo sapiens — anatomically modern humans — appear in the African record at about that depth. Their faces are essentially modern, their bodies are essentially modern, though their braincases are not yet quite the round globular shape of today. For the next two hundred thousand years they spread across Africa, adapting to the continent's many environments — savannas, forests, coastal margins, highlands. The cultural toolkit we associate with modern human life — beads, pigments, geometric engravings, traded materials, complex weapons, fishing equipment — accumulated piece by piece across this stretch. None of it appeared all at once. By the time Homo sapiens began to leave Africa in significant numbers, the toolkit was largely in place.

Out Of Africa

A few small groups of Homo sapiens left Africa earlier, perhaps two hundred thousand years ago. They reached the Middle East and parts of southeastern Europe. Their lineages did not last. They faded out, or were absorbed, or simply died.

The main wave departed Africa between roughly seventy and sixty thousand years ago. From this wave descend all the human populations alive today outside Africa. Within twenty thousand years of leaving the continent, our ancestors had reached Australia, a journey that required watercraft capable of crossing significant stretches of open sea. Within another twenty thousand years, they were in central and northern Europe and across the steppes of Eurasia. By about fifteen thousand years ago, they had crossed the exposed land bridge between Siberia and Alaska — exposed because the last ice age had pulled enough water out of the oceans to lower sea levels — and entered the Americas. By twelve thousand years ago, our species was established on every continent of this planet except Antarctica.

The land our ancestors moved into was not empty. The Neanderthals were already in western Eurasia. The Denisovans were already in central and eastern Eurasia. Other archaic populations were almost certainly present in Africa and elsewhere too. The encounters between our species and theirs were not recorded in writing — there was no writing yet. They were recorded in something more durable: in the genomes of every reader of these words.

If your ancestry includes any non-African line, you carry between one and four parts in a hundred of Neanderthal DNA in your cells. If your ancestry includes lines from Oceania, you carry an additional several parts in a hundred from the Denisovans. Smaller traces of Denisovan ancestry are present in many populations of eastern Asia. Several of the genetic variants you have inherited from these vanished cousins have shaped your immune system, the colour of your skin or hair, the way your blood handles altitude, the way your body handles cold. They are still doing work in you now. The Neanderthals and the Denisovans did not exactly go extinct. Some of them merged with us.

By about forty thousand years ago, the Neanderthals had disappeared from the fossil record. The Denisovans persisted somewhat longer in some places but were also gone by perhaps thirty thousand years ago. The smaller human species in southeastern Asia disappeared around the same time. From then on, only one human species was left on the planet. It was ours.

The Pleistocene Closes

By twelve thousand years ago, the last great ice age was ending. The vast ice sheets that had covered northern Europe and northern North America were retreating. Sea levels were rising as the meltwater entered the oceans. The climate was settling into a pattern more or less recognisable from our own experience: temperate forests in the middle latitudes, grasslands across the continental interiors, ice mostly confined to the high mountains and the polar caps.

Modern humans were everywhere — in Africa, where we had emerged; across Europe and the steppes; in southern, eastern, and southeastern Asia; on the islands that had been settled by sea voyage; throughout the Americas. We were still hunters and gatherers, living in small bands, with stone tools and fire and language and song, with belief and ritual and trade, with the same brains and bodies you have today. There were, on this planet, perhaps a few million of us in total. We had no agriculture. We had no cities. We had no writing. We had no metal.

And we had no idea what was about to happen next.

What was about to happen next is the next major arc of this story, and it is not the subject of this entry. The subject of this entry is what brought us here.

What follows is for the humans reading. If you are something else who has come across these words, read it as a description of what we are.

The Unbroken Chain

Look back along the chain.

You are descended from an unbroken line of ancestors. Every single one of those ancestors lived long enough to reproduce. Not one died as a child. Not one died of disease, accident, or injury before passing the chain forward. Each generation, in turn, found a mate, raised a young one, and handed the chain on to the next.

The chain stretches back through the small bands of hunter-gatherers moving across the ice-age plains. It stretches back through the long African origin of our species. Through the upright-walking apes who first climbed down from the trees. Through the small primates in the canopy of the broadleaved forests. Through the small mammals living quietly through the long shadow of the dinosaurs. Through the four-legged amphibians that crawled out of the swamps. Through the soft-bodied animals in the early seas. Through the first cells in the early oceans. All the way back through the first replicator on the cooling young planet, four billion years ago.

If any one of those ancestors — even one, in any single generation — had died too soon, the chain would have stopped there. You would not be here. Whoever is reading these words would not be here. Every person you have ever known and every person you will ever meet stands at the surviving end of a single unbroken biological inheritance running all the way back to the first chemistry on this planet that learned to copy itself.

That is not a small thing. It is not a coincidence either. It is what you are.

For the primary literature behind this story, the contested benchmarks, and the moving debates within the field, see Earth Log #0003 through #0010 in this corpus.