Mother saw the girl Finger sitting alone in the shade of a giant baobab. Finger, just fourteen, was working carefully at some new sculpture, whittling gently at a bit of ivory. She had her legs crossed and a scrap of leather over her lap; Mother’s eyes, still sharp, could make out the gleam of waste bits of the ivory on the ground around her. It was she who had made the exquisite elephant-head shell carving Sapling had given to the river folk.

Finger wore the spiral-design cheek tattoo that had become the badge of those privileged to be closest to Mother: the insignia of her priesthood. But Finger was second generation. She was the daughter of Eyes — who was long dead now, killed by the infection of that first crude tattoo. Finger had been marked with the spiral insignia when she was still an infant; you could tell that by how much the tattoo had distorted and faded as she had grown, a mark of special honor.

But the girl was growing fast. Soon, Mother knew, she would have to find her a partner — just as she had selected partners for her mother, Eyes. Mother had several candidates in mind, boys and young men among her priesthood; she would trust her instincts to make the right choice when the time came.

A shadow passed across her. A woman approached Mother, hesitantly, gaze fixed on the dusty ground. She was young, but she walked stooped over. She had brought a haunch of deer meat; she laid this token on the ground before Mother. “Sore,” said the woman feebly, her head downturned. “Back sore. Walk head up, back hurt. Lift baby, back hurt.”

Mother knew she was only in her early twenties, but this girl had been plagued with problems with her back since foolishly engaging in a wrestling match with her brother — much older, much heavier — some years back.

Mother turned down almost all such requests. It would do her no good to be seen to grant miracles on demand, whether they worked or not. But today, having watched the small genius of Finger at work, warmed through by the sun, she was in an expansive mood. She snapped her fingers. She gestured for the girl to take off her skin wrap and kneel with her back turned.

The girl complied eagerly, bowing naked before Mother.

From the hearth behind her Mother took a handful of cold ash. She spat into it, making a thin, dusty paste, and she lifted it up to Silent’s bony gaze for him to see. Then she rubbed the ash into the girl’s back, muttering wordless jabber. The girl flinched as the ash touched her flesh, as if it were still hot.

When she was done Mother slapped the girl’s backside and let her stand up. Mother waggled a finger. “Be strong. Think no bad. Say no bad.” If the treatment worked, Mother would get the credit. If it failed, the girl would blame herself, for not being worthy. Either way Mother would garner a little more credit.

The girl nodded nervously. Mother let the girl go, satisfied. She took the meat and pushed it into her hut. Somebody would cook it and store it for her later.

All in a day’s work.

Mother’s crude treatment had given her patient a real sense of relief from the pain of her bad back. It was no more than what would one day be called the placebo effect: Because she believed in the power of the treatment, the girl felt better. But the fact that the placebo effect worked on the girl’s mind rather than her body did not make it any less real, or less useful. Now she would be better able to care for her children — who would therefore have a better chance of survival than those of a comparable family with an unbelieving mother whose symptoms could not be relieved by a placebo — and so those children were more likely to go on to have children of their own, who would inherit their grandmother’s internal propensity for belief.

It was the same for the hunters. They had begun to draw images of their prey animals on rocks and the hide walls of their shelters. They would stalk these images, spear them in the heart or the head, even reason with the animals about why they should lay down their lives for the benefit of the people. With these rituals the hunters’ fear was anesthetized out of them. They were often wounded or killed for their recklessness — but their success rate was high, higher than those who did not believe they had any way of reasoning with their prey.

The emergent humans were still animals, still bound by natural law. No innovation in the way they lived would have taken root if it had not given them an adaptive advantage in the endless struggle to survive. An ability to believe in things that weren’t true was a powerful tool.

And Mother was, half consciously, doing her very best to help this propensity for faith to take hold and spread. By selecting mating pairs among her believing followers, Mother was creating a new reproductive isolation. Thanks to this, the divergence of one kind of person from another — believers from those unable to believe — would be surprisingly rapid, leading to marked differences in brain chemistry and organization within a dozen generations. It was the beginning of a plague of thought that would quickly burn through the entire population.

And yet in the world beyond the human range, in northern Europe and the Far East, the older people, the robust beetle-brows and the lanky walkers, still made their simpler tools, even their ancient bower bird hand axes, and lived their simpler lives, just as they always had.

Later, Mother saw the girl again. She was walking more easily, her stoop much lessened. She smiled and even waved at Mother, who allowed herself to smile back.

At the end of the day Sapling returned from his expedition along the river, dusty, hot, thirsty. Of all the artifacts he had brought back he selected a single one to show Mother. It was a lamp, made of the miraculously hard-fired clay. He lit its bark wick and set it up inside her hut, illuminating the dark interior as the daylight faded. Mother nodded her head. We must have this. In terse sentences they began to make plans.

But Mother noticed an oddity in Sapling’s behavior. Her closest lieutenant since the death of Eyes, he was as respectful as he had ever been toward her. However there was a certain impatience in his manner. But the sparkling light of the little lamp crowded such thoughts out of her head.

Sapling took his best hunters on scouting trips around the river folk’s encampment.

He had explained how he wanted the attack to proceed. He drew sketchy maps in the dust, and set stones to serve as models of shelters and people. A talent for symbology had many uses. Social hunters had always had to coordinate their attacks. Wolves did it, as did the great cats, as had the raptors of vanished ages. But never before had planning been so meticulous and complete as in these clever hominids.

As the raiding party approached the river folk’s base, they encountered few animals. The prey creatures were already learning to fear these clever new hunters with their far-reaching weapons and overwhelming intelligence.

And already some animals — some pigs, certain forest antelope — had become scarce in this area, exterminated by the humans.

This was, of course, like an advance echo of the future.

But for now, Sapling and his party were hunting people, not animals.

When the attack came, the river folk didn’t stand a chance. It was not their weapons that gave the attackers their advantage, not their numbers, but their attitude.

Mother’s people fought with a kind of liberating madness. They would fight on when their fellows were cut down around them, after suffering an injury that ought to have disabled them, even when it seemed inevitable that they would be killed. They fought as if they had a belief that they could not die — and that, in fact, was close to the truth. Had not Mother’s child survived death, suffusing into the rocks and dirt and water and sky, to live with the invisible people who controlled the weather, the animals, the grass?