Meanwhile, as they got smarter, they didn’t need to be as strong. Their bodies shed much of the robustness of their immediate ancestors, and reverted to something like the graceful litheness of Far’s people.

Pebble’s first impression, that Harpoon had seemed childlike, was not accidental. With the proportions of their faces and their thinned bones these new people, compared to ancestral stock, were in some ways like children arrested in their growth. Once again, under ferocious selection pressure, the genes had reached for variations that could be implemented quickly: Adjusting the comparative growth rates of skeletal features was comparatively easy.

All of these changes had been essentially complete within a few millennia. After this process Harpoon, anatomically, was all but identical to a human of Joan Useb’s time, even in her skull and the gross features of her brain. And it had been trading, a new way of dealing with other people, that had made them what they were.

But even Harpoon was not yet human.

There was a little more invention, a little more organization in her life. Her kind built hearths, for instance. But her tool kit was scarcely more advanced than that of Pebble and his ancestors. Her language was the same unstructured babbling. Much of the way she lived her life, like her sexuality, had been inherited with little change from the kinds of people who had gone before. There were still rigid barriers in her mind, a lack of connections in the neural wiring of her brain. A true human of Joan Useb’s age, stranded in this age of her ancestors, would quickly have been driven crazy by the sameness, the routine, and ritual, the lack of art and language — the boring, drab poverty of life.

And, human form or not, these folk had not been dramatically successful. Though they had spread across Africa from their origins in that southeastern swamp, their lifestyle remained marginal. It was difficult to trade if there was nobody like you to trade with. Even now the new nomads’ survival was chancy, and most extant groups, around the continent, would not survive.

The children of Harpoon were destined to pass through this bottleneck, but their genes would always bear the imprint of that narrow passage. In the future, the swarming billions who would spring from this unpromising seed would be virtually identical, genetically; every human would be a cousin.

Pebble’s relationship with Harpoon came to a head during a hunt.

One day, Pebble found himself in a blind, upwind of a herd of giant horse who cropped the long grass peacefully. The blind was just a lean-to frame of saplings, loosely woven together and covered with palm fronds and grass. Here Pebble huddled, his thrusting-spear at his side, peering out at the big, lame animal that was their target. And Harpoon was at his side. He was tense, adrenaline pumping, and the heat of the day and the sweat scent of the horse filled his head.

Suddenly he felt her fingers on his face.

He turned. Her skin seemed to glow in the green gloom. She traced the vertical ocher stripes he still wore. And then her fine fingers moved to his arm, the long-healed cuts he had inflicted there. Her every touch was startling, as if her fingers were made of ice or fire.

He ran his fingers down her arm. His fist enclosed her forearm easily, as if it were a bird’s leg. He felt he could snap the bone with a gesture. Suddenly it was just as it had been on the first day he had met her, on the beach. His mouth dried, his throat tightened.

He didn’t understand his lust: the lust that had never gone away. He thought of the clever tools she made, her long, easy stride across the ground, the food she had brought his people — and that harpoon, the exquisite harpoon point, unimaginable before he had seen it that first day. There was something in her his body wanted; the longing was unbearable.

He rolled on his back. In the rustling shade of the blind, she straddled him and smiled.

Each lump of flint was a miniature cemetery. In some long-vanished sea the corpses of crustaceans had settled into sediment, and minute glassy needles that had once formed the skeletons of sponges became the nuggets of flint embedded inside the gathering chalk seams.

Pebble had always loved the feel of flint. He turned the smooth-faced, brittle rock over in his hands, sensing its structure. Flint knappers got to know all of the stone’s subtle properties. The more a flint was exposed to the elements the more likely it was to contain fractures, caused by frost or a battering by river or ocean currents. But this flint lacked the patina of exposure. It was fresh and clean. It had only recently been dug out of its chalk matrix, after a cliff had collapsed. You couldn’t get such flint in this area, anywhere within the people’s old range. Pebble had missed good flint, in the long years on this beach, before Harpoon had walked into his life.

These days he was never more content than when working stone — or, rather, he was never less discontented.

Seven years had elapsed since his first encounter with Harpoon. At twenty-six, his body was already declining, battered and scarred by the cumulative challenges of a life that continued to be very hard, despite his people’s collaboration with the newcomers.

He had embraced Harpoon, and he had embraced the newness and changes she had brought — but those changes themselves had become bewildering. Pebble’s mind was immensely conservative. And as he grew older he increasingly relished these moments alone with the stone, when he could retreat into the recesses of his roomy mind.

But this moment of peace didn’t last.

“Hai, hai, hai! Hai, hai, hai!”

Here came his son and daughter, squat Sunset and spindly Smooth, running along the beach side by side, jabbering in the patois that had resulted as a merger of Pebble’s tongue and Harpoon’s. “Come, come, come here with us!” The children, naked, their skin crusted with salt and sweat, wanted him to come work on the logs that Ko-Ko and others were pushing into the sea.

He pretended not to hear them until they were almost on top of him. Then he grabbed them both with a roar, and all three of them rolled in the sand, wrestling. At last Pebble relented. He put aside his flint, got to his feet, and lumbered after the kids along the beach.

The morning was bright, the sun hot, and the air filled with the scent of salt and ozone. As the children flew ahead of his own lumbering gait, Smooth quickly outstripping her brother, Pebble felt briefly joyful at their youthful energy. This place would never be home to him, but it had its pleasures.

Ko-Ko, Hands, and Seal were making a kind of raft. Harpoon was here, her hands resting on a belly that was already showing a bump. She grinned fiercely as Pebble came up.

The men had cut down two stout palms from the inland forests, stripped them of their branches, and lashed them together with lianas and plaited vine. Now Hands and Seal were hauling this crude construction across the sand and down to the water. There was much straining and jabbering: “Push, push, push!” “Back back, no, back, back…” “Hai, hai!”

Pebble joined Hands and Seal at their task. Even with three of them it was hard work, and Pebble was soon sweating like the rest, his legs coated with stinging hot sand. Ko-Ko tried to help, but for sheer brute strength the robust folk had no match. And they were helped, and hindered, by the two children, and by Harpoon’s wolf companion who ran around their legs, barking.

The wolf, raised from a captured cub, was all but feral. This was just the start of a relationship longer than any other between people and animals, a relationship that would ultimately shape both species.

Pebble had never forgotten his determination to reach the island. At last, as he sat brooding on his beach, he had watched skinny youngsters playing on bits of driftwood in the water — and a connection had closed in his mind.