The Explosion and the Wipeouts

For three billion years the fossil record is a record of the microscopic and the simple. Then, in rocks a little more than five hundred million years old, it changes so abruptly that the early geologists who found it could hardly believe it belonged to the same planet. There is a hint of what is coming just before: the Ediacaran, a strange lost world of soft, flat, quilted organisms, some of them meters wide, unlike anything alive today, so alien that we still argue about which of them, if any, were animals at all. And then, in a window of perhaps twenty million years starting around five hundred and forty million years ago, an eye-blink by the standards of this book, the seas fill with animals. Not the ancestors of a few modern groups, but the sudden first appearance of nearly all of them at once: creatures with shells and segments and jointed legs and guts and gills, and, for the first time in the history of the world, eyes. Biologists call it the Cambrian Explosion, and it is the moment complex life announced itself, drafting in one furious burst almost every basic body plan that has existed since.

We know this burst in extraordinary detail because of a few rare places where soft bodies, not just hard shells, were preserved. In the Burgess Shale, high in the Canadian Rockies, and in an even older and richer site at Chengjiang in southern China, the mud captured whole animals in fine detail: Anomalocaris, a meter-long apex predator that cruised those seas with grasping appendages and a circular mouth; a menagerie of creatures so bizarre that one was formally named Hallucigenia; and, easy to overlook among the monsters, a small, soft, finger-length swimmer called Pikaia, which had a stiffening rod running down its back. That rod is the first whisper of the spine. Pikaia, or something very like it, is our own distant ancestor, the thread from which all fish, all reptiles, all mammals, and every human being would eventually be drawn. You can stand in a museum and look at the flattened smear of one of your earliest relatives in the rock.

Why the explosion happened then, and so fast, is still genuinely argued, and the honest answer is that several causes probably conspired. Oxygen, after the long buildup of the last chapter, had at last risen high enough to fuel the expensive, demanding business of being a large and active animal. The deep genetic toolkit for building complex bodies, the master control genes that tell a growing embryo where to put a limb or an eye, had been quietly assembled over the preceding hundred million years and could now be used to draft new body plans at speed. There is even a serious argument, sometimes called the "light switch" hypothesis, that the sudden evolution of the eye was itself a trigger: the instant one animal could see another, the whole ocean became a theatre of hunt and flight, and an arms race began that has never once let up. Eyes to find prey; shells and spines and armor to foil the eyes; speed to flee; jaws and claws to overcome the speed. Charles Darwin himself fretted about the Cambrian, because the abruptness of it looked, at first, like a problem for a theory built on gradual change. We can now see it was not a refutation but a detonation with a very long fuse: the genetic groundwork was laid slowly and invisibly across tens of millions of years, and then a threshold was crossed and it all expressed itself at once. In a geological instant, life went from drifting to striving, and, astonishingly, the basic blueprints it sketched in that burst are still the ones in use. You are built on a Cambrian design.

From that explosion the story becomes the one we vaguely picture when we think of prehistory: the long parade. Animals hauled themselves out of the sea and learned to breathe air. Plants greened the bare brown continents and then grew into the first forests. Backbones and jaws and four limbs took shape and spread. Flight was invented, and here is a detail worth pausing on, because it tells you something deep about how evolution works: flight was invented not once but at least four separate times, independently, by the insects, then by the great flying reptiles, then by the birds, then by the bats, each lineage arriving at the wing by its own completely different road. When the same excellent solution keeps getting discovered over and over by unrelated creatures, it is a sign that evolution is not wandering at random but is drawn, again and again, toward the handful of answers that actually work. Hold that thought; it will matter when we ask, much later, whether minds like ours are a fluke or another one of those answers the universe keeps finding.

For all its endless invention, though, this rising complexity was never a smooth ascent. It was repeatedly, savagely interrupted. Five times in the last half-billion years the fossil record shows life collapsing on a global scale, the great mass extinctions, and they are the dark twin of the Cambrian's exuberance. In each, in a geological blink, most of the species on Earth simply ceased to exist. It is worth naming them, because their rhythm is part of the story. Around four hundred and forty million years ago, an ice age at the end of the Ordovician killed something like eighty-five percent of species, when all life was still in the sea. Around three hundred and seventy million years ago, the Late Devonian crisis, drawn out over millions of years, took perhaps three-quarters of species, though our own ancestors, the first creatures venturing onto land, scraped through. And then came the worst thing that has ever happened to life on this planet.

About two hundred and fifty-two million years ago, at the end of the Permian period, life very nearly ended altogether. We call it, without any melodrama, the Great Dying. In the immense volcanic province of Siberia, the Earth's crust split and poured out lava on a scale almost impossible to picture, millions of cubic kilometers of it, burning through coal beds and belching carbon dioxide for hundreds of thousands of years. The planet overheated by perhaps eight to ten degrees. The oceans turned acid, then ran out of oxygen, then filled with poisonous hydrogen sulfide belched up from the depths. When it was over, something like ninety-six percent of all marine species were gone. The trilobites, that great and ancient lineage of armored sea-creatures that had crawled the ocean floors since the Cambrian and survived every prior catastrophe, finally vanished forever. Life took something like ten million years to recover, the slowest healing in the whole record. It is the closest the living world has come to total erasure since life began, and had it tipped just a little further, this book would have no author and no reader, because our own thread ran through the eye of that needle.

Life did recover, as it always has, and the recovery set up the fourth great extinction and one of its strangest consequences. Around two hundred and one million years ago, another vast volcanic episode, this one tied to the breakup of the supercontinent Pangaea, triggered the end-Triassic extinction. Among the groups it cleared away were many of the competitors of a then-minor set of reptiles, and in the emptied world that followed, those reptiles rose to dominance and held it for the next hundred and thirty-five million years. They were the dinosaurs. And so we arrive at the fifth and most famous wipeout, which is not the largest but is the one that made us, and which shows better than any other how much of this entire story turns on sheer chance.

Sixty-six million years ago the dinosaurs were the unquestioned masters of the Earth, as they had been for an almost unimaginable stretch, and the mammals were small, timid, mostly nocturnal things scurrying in the undergrowth, waiting out an age that showed no sign of ever ending. Then, on what began as an ordinary day, an asteroid roughly the width of a city struck the shallow sea off what is now the coast of Mexico, near a town whose name, Chicxulub, is now written into the rocks of the entire planet as a thin layer of impact debris. The blow threw so much rock and soot into the sky that the world went dark and cold for years. Photosynthesis faltered, the food chains collapsed from the bottom up, and roughly three-quarters of all species died, including every last dinosaur that could not fly. And in the ruined, darkened, emptied world that crept out the other side, the small furry survivors found a planet suddenly swept clean of its ancient rulers, and they inherited it. Every mammal alive today, every whale and bat and elephant and mouse, every primate, every human being who has ever drawn breath, is here because a mountain of rock happened to cross the Earth's orbit at exactly the wrong moment for the dinosaurs and exactly the right one for us. Calculations suggest that had the asteroid arrived only minutes earlier or later, it would have struck deep ocean instead of that shallow, sulfur-rich shelf, thrown up far less world-shrouding debris, and perhaps let the dinosaurs live on, in which case the Earth might still be theirs, with no one here to wonder about any of it.

That is the lesson these wipeouts press on us, and it is worth carrying forward into everything that follows. The history of life is not a ladder climbing steadily and inevitably toward us. It is a story of long, patient building repeatedly knocked flat and begun again, in which survival often owes as much to luck and timing as to any virtue or fitness, and in which catastrophe, terrible as it is for those living through it, keeps clearing the stage for whatever comes next. Recall the shape we first met with the Moon-forming impact in Part I, catastrophe as a strange kind of stabilizer, disaster as a doorway. Here it returns, larger and bloodier: again and again, a world-ending calamity for the old order becomes the opening scene for the new. The mammals did not out-compete the dinosaurs in some fair contest of merit. They out-survived a catastrophic afternoon. And we will meet this exact pattern a third time, near the end of this book, when we turn to ask what kind of afternoon our own civilization may be walking toward, whether we are living in another Permian buildup without knowing it, and whether we, unlike the trilobites and the dinosaurs, might be the first form of life on Earth with the power to see the catastrophe coming and step aside. It is worth noting, plainly and without comfort, that many scientists judge we have already begun a sixth mass extinction, this one with no asteroid and no volcano to blame, only us.

But that reckoning is far ahead. For now, hold the image at the close of Part II: a world, sixty-six million years ago, wiped raw and then slowly flushing green again, its great reptiles gone, its forests and skies and shorelines suddenly, shockingly open. And among the survivors creeping out into that emptied morning was a modest, unpromising group of warm-blooded animals with grasping hands, forward-facing eyes, and brains a little larger, for their size, than they strictly needed. Nothing about them looked like destiny. But the arms race that began in the Cambrian, the endless spiral of eyes and speed and cunning, was about to enter a new and far stranger phase, one that would, for the first time in nearly four billion years of life on this planet, produce a creature that could turn around and ask where it had come from.

The story was about to grow a mind.