Pebble had found a yam vine. He bent and inspected it.

He was eight years old, naked save for smears of ocher on his barrel chest and broad face. He pulled out a little grass from around the yam’s base. This was a spot for yam, not grass, and it was best to keep it that way.

People had been here before to dig out these tubers. Perhaps he had even been here himself. At eight years old he had already covered every scrap of his people’s range, and he thought he remembered this spot, here between these eroded bluffs of sandstone.

He took his digging stick. This was a heavy pole shoved through a hole crudely bored in a small boulder. Despite the tool’s weight, he lifted it easily, and he used the mass of the boulder to ram the digger’s point into the hard ground.

Pebble was a solid slab of muscle built over a tough, robust skeleton. If Far, his long-dead, distant grandmother, had looked like a long-distance runner, Pebble might have been a junior shot-putter. His face was large, massive-featured, dominated by a great ridge of bone over his brow. He had a mountainous nose and large sinuses that gave his face an oddly puffed-out look. His teeth were flat-topped pillars of enamel. His skull, which would become considerably larger than Far’s, housed a large and complex brain — in fact comparable in size to a modern human’s — but it sat much more directly behind his face than a human’s brain would have.

When he had been born, wet from the womb, Pebble’s body had been sleek and round, inspiring an odd image in his mother’s mind, a pebble worn by a stream. Names for people still lay far in the future — with just twelve people in Pebble’s group there was no need for names — but nevertheless this boy’s mother would often look on a glistening rock in a stream, and remember her child as he had been as a baby in her arms.

Pebble, then.

In this age there were many kinds of robust folk like Pebble’s spread through Europe and western Asia. Those who inhabited Europe would one day be called Neandertals. But just as in Far’s time, most of these new kinds of people would never be discovered, let alone understood, classified, linked to a hominid family tree.

His were a strong people, though. Even at eight years old, Pebble performed work essential to his family’s survival. He wasn’t yet up to joining the adults on the hunts, but he could dig out yams with the best of them.

The wind picked up a little, bringing him the delicious scent of wood smoke, of home. He went at his work with a will.

Already his digging had broken up the earth. He plunged his hands into the dry ground and began exposing a fat tuber that looked as if it might go down a long way, perhaps as deep as two meters. He went back to his digging stick. Bits of dust and rock flew up, sticking to his sweat-covered legs. He knew what to do with yams. When he had the tuber he would cut off the edible flesh, but then replace the tuber’s stem and top in the ground so that it would regrow. His digging aided the yam in more subtle ways, too. He was loosening and aerating the soil, further fostering regrowth.

His mother would be pleased if he brought home three or four fat tubers, ready to be thrown onto the fire. And yams were useful in a lot of other ways besides eating. You could use them to poison birds and fish. You could rub their juice into your head to kill the lice that crawled there…

There was a crunching noise.

Startled, Pebble pulled back his digging stick. He leaned forward, shielding his eyes from the sun’s brightness, trying to see what was down there in the hole. It could be some deep-burrowing insect. But he could see nothing but a scrap of rust brown, like a bit of sandstone. He reached down and, his clumsy fingers stretching, grabbed the scrap and pulled it to the surface. It was a ragged-edged dome, small enough to fit in the palm of his hand. When he held it up before his face two empty eye sockets peered back at him.

It was a skull. The head of a child.

That was no great horror. Children died all the time. This was a harsh place: There was little pity to spare for the weak and hapless.

But all the children who had died within Pebble’s own short lifetime had been put in the ground close to the huts. Like all the dead, they were buried to keep the scavengers from harassing the living. Perhaps this child was long dead, then. Perhaps its people had buried it here before Pebble was born, where the yam clump grew now.

But the skull was oddly fine, light. Pebble weighed it in his hand. Its brow was a heavy lid of bone, from which a forehead sloped back almost horizontally. Pebble ran a hand over his own scalp and compared the slightly bulbous swelling of his forehead. There were tooth marks in the little cranium, he saw: precise puncture wounds inflicted by the teeth of a cat — but inflicted after the child was already dead, its body abandoned on the plain.

Pebble could not know that he was holding the remains of the Brat, brother of Far, who had lived and died not far from here. The Brat had succumbed to his infant vitaminosis and died while still a child, without issue. It would have been little comfort to the Brat if he could have known that one day, when his brief, forgotten life was already more than a million years gone, his small head would be cradled in the hand of a remote great-nephew.

And the Brat would have recognized little of this landscape, the place where he had once played.

The geological infrastructure of the Rift Valley — the plateau, the rocks, the volcanic mountains, the great sweep of the valley itself — had been left largely unchanged by time. But since Far’s time this had become a sparse, dry place. Scattered stands of acacia, leadwood, and wild laurel had replaced the denser thickets and forest pockets of the past. Even the grasslands were subtly different, great swaths of them dominated by a handful of fire-resistant species. Meanwhile, the great animal communities of the past had imploded. There was not an elephant to be seen across this great dust bowl, not an antelope or giraffe. It was as if life had crashed here. The place was depleted. Far would have been startled by its impoverishment.

But the Brat’s wretched remains had left their mark on the world: a scrap of moisture trapped in that buried, upturned skullcap had been enough to help establish the yam.

Incuriously Pebble closed his fist. The little skull was crushed to thin shards, and he let the dust fall back into the hole. He reached for his digging tool; there was still some root to be dug out.

That was when he glimpsed the strangers.

He crouched down behind a bluff, holding his breath.

They were hunters; he could see that immediately. They were following an old elephant track. Elephants walked to water, and where there was water, there would be many animals, including the medium-sized creatures like deer that people hunted by preference.

There were four of them, three men and a woman, all adults. As they walked the hunters’ legs swung powerfully, with their torsos tipped a little forward. It was a gait built for strength, not elegance or speed: the hunters had none of the fleetness of Far. Thick beards hid the men’s dark faces, and the woman had tied her long hair back with a bit of leather. Unlike Pebble this group wore clothes: just bits of hide, unsewn and tied around the body with strips of leather or plaited bark. Pebble could see the bite marks in the clothes. Leather was treated by chewing and stretching it with the teeth, and a major function of that big ridge of bone on Pebble’s brow was to provide an anchor for the jaws that must do such mighty work.

And they carried weapons: narrow wooden throwing spears, and shorter, stubby thrusting spears, great logs of hardwood with slabs of stone stuck to the end with blobs of resin and leather ties. They were giants’ weapons that a human would have had trouble lifting, let alone wielding in anger.