Sapling probably would never have come up with this insight by himself. But once she had gotten through to him he had grasped the concept quickly; after all his mind wasn’t so much different from hers. As Sapling hauled the throwing stick forward, the great force it applied to the spear made it bend: the spear, flexing, actually seemed to leap away, like a gazelle escaping a trap. Mother’s mind spun with satisfaction and speculation.

“Sick.” The flat, ugly word cut through her euphoria. Sour, her aunt, was standing outside the shelter they shared. She pointed inside.

Mother ran across the trampled dirt to the shelter. As soon as she stepped inside she could smell the harsh stink of vomit. Silent was doubled over, clutching his distended belly. He was shivering, his face sleek with sweat, and his skin was pale. Vomit and shit lay smeared around him.

Standing in the bright light outside the shelter, Sour was grinning, her face hard.

It took Silent a month to die.

It nearly destroyed Mother.

Her instinctive understanding of causality betrayed her. In this ultimate emergency, nothing worked. There were some illnesses you could treat. If you took a broken limb, pulled it back into shape and bound it up, very often it would set as good as before. If you rubbed dock leaves on insect bites, the poison could be drawn. But there was nothing she could do for this strange wasting away for which there wasn’t even a word.

She brought him things he had loved — a tangled chunk of wood, bright bits of pyrite, even a strange spiral stone. In fact it was a fossilized ammonite, three hundred million years old. But he would just finger the toys, his eyes sliding, or he would ignore them completely.

There came a day when he didn’t stir from his pallet. She cradled him and crooned wordlessly, as she had when he was an infant. But his head lolled. She tried to cram food into his mouth, but his lips were blue, his mouth cold. She even pressed those cold lips to her breast, but she had no milk.

At last the others came.

She fought them, convinced that if she only tried a little longer, wanted it a little more, then he would grin, reach for his bits of fool’s gold, and get up and run into the light. But she had let herself grow weak during his illness, and they took him away easily.

The men dug a pit in the ground, outside the encampment. The boy’s stiffening body was bundled inside, and the debris from the pit was hastily kicked back in, leaving a discolored patch of dirt.

It was functional — but it was a ceremony, of sorts. People had been sticking bodies in the ground for three hundred thousand years. Once it had been an essential way of disposing of waste: When you could expect to grow old and die in the same place you were born, you had to keep it clean. But now people were nomadic. Mother’s folk would be gone from here soon. They could have just dumped the boy’s body and let the scavengers take it, the dogs and birds and insects; what difference would it have made? And yet they still buried, as they always had. It had come to seem the right thing to do.

But no words were spoken, no marker was left, and the others dispersed quickly. Death was as absolute as it had always been, deep back down the lineages of hominids and primates: death was a termination, an end of existence, and those who had gone were as meaningless as evaporated dew, their very identities lost after a generation.

But it wasn’t that way for Mother. No, not at all.

In the days that followed that brutal ending and efficient burial, she returned again and again to the patch of ground that held her son’s bones. Even when the upturned ground began to fade in color, and the grass began to spread over it, still she remembered exactly where that hole’s ragged edges had been, and could imagine how he must be lying, there deep in the earth.

There was no reason for him to have gone. That was what plagued her. If she had seen him fall, or drown, or be trampled by the herds, then she could have seen why he died, and perhaps could have accepted it. Of course she had seen disease afflict many members of the tribe. She had watched many people die of causes no one could name, let alone treat. But that only made things worse: If someone had to die, why Silent? And if blind chance had killed him — if someone so close could be taken so arbitrarily — then it could happen to her, at any time, anywhere.

It couldn’t be accepted. Everything had a cause. And so there must be a cause of Silent’s death.

Alone, obsessing, she retreated into herself.

Soon after the time of Pebble and Harpoon had come an interglacial, an interval of temperate climes between the long, icebound millennia. The bloated ice caps had melted, and the seas had risen, flooding the lowlands and deforming the coastlines. But, twelve thousand years after Pebble’s death, this latest great summer drew to a close. A savage cooling cut in. The ice began to advance once more. As the ice sucked the humidity out of the air, it was as if the planet were drawing in a great, dry breath. Forests shrank, grasslands spread, and desertification intensified once more.

The Sahara, cupped in its mighty Himalayan rain shadow, was not yet a desert. Wide, shallow lakes lay across its interior — lakes, in the Sahara. These bodies of water waxed and waned, and sometimes dried out completely. But at their greatest extents they were full of fish, crocodile, and hippos. Around the waters gathered ostriches, zebra, rhinos, elephants, giraffes, buffalo, and various antelope — and animals that modern eyes would not have thought so characteristically African, like oxen, Barbary sheep, goats, and asses.

Where there was water, where there was game, there were people. This was the environment that cradled Mother’s people. But it was a marginal place, the skim of life shallow. People had to work hard to survive.

And people were still scattered remarkably thin.

No humans had yet moved out of Africa. In Europe and across Asia, there were only the heavy-browed robusts and, in places, the older forms, the skinny walkers. America and Australia were still empty altogether.

Even in Africa people were thin on the ground. The more mobile, trade-based way of life that had been born with Harpoon and her kind had not been a uniform blessing. Ever since the move out of the forests, hominids had been vulnerable to trypanosomes — the parasites that caused sleeping sickness — carried by the clouds of tsetse flies that followed the savannah’s ungulate herds. Now such diseases were spreading. The people’s trading networks had proven very effective at exchanging goods, cultural innovations, and genes — but also at transmitting pathogens.

And, culturally, things weren’t happening.

Pebble would have recognized almost everything in Mother’s camp. People still split stone flakes off prepared cores, and still wrapped hide around their bodies, tied in place with bits of sinew or leather. Even their language was still a formless jabber of concrete words for things, feelings, actions, useless for transmitting complex information.

Across seventy thousand years these people — humans with as modern a body plan, even as modern a brain, as any twenty-first-century citizen — had scarcely made a single innovation in their technology or techniques. It had been a time of stupefying passivity, stunning stasis. After all this time, people were just another tool-using animal in the ecology, like beavers or bowerbirds, still little more than glorified chimps. And, bit by bit, they were losing their battle to survive.

Something was missing.

She could just walk off into the dust, alone.