In their mangrove swamp the ancestors of Harpoon, no better swimmers than Pebble, had been forced to find ways of crossing crocodile-infested water. After much trial and error — with every error punishable by maiming or death — they had hit on a way of using cut mangrove logs. You could ride on such a log by lying flat on it and paddling with your hands. Through all their journeying, the skinnies had not forgotten that basic technique. And that was what Pebble had seen the children trying to do out on their bits of driftwood. At last he saw a way to get to the island.

But paddling a log across the still waters of a mangrove swamp was one thing. Mastering the choppy surface of an ocean channel was a different challenge.

After a few spectacular failures, Ko-Ko’s inventive mind had come up with the notion of strapping two logs together. That way at least you got a little more stability. But these miniature rafts were still too vulnerable to tipping over.

At last they got the logs into the water. They floated, tied together to make a stable surface.

Ko-Ko and Hands threw themselves forward, splashing heavily. They both lay flat on the logs, legs stretched behind them, and began to paddle. Slowly they pulled away from the shore. But the waves tipped the logs up and down — and eventually over, pitching both men into the water. And then the logs’ bindings came loose.

Hands came staggering back, spluttering and growling. With Ko-Ko, he hauled the logs back out of the water onto the beach.

Pebble knew that there had been no danger, for the water here was shallow enough to walk out to shore. But further away it deepened quickly — and that was where they must travel, if they were to reach the island.

So they kept working, trying different combinations, over and over.

Much had changed in Pebble’s life in seven years.

Gradually those who had come with him from Flatnose’s village faded out of the world. Hyena had never recovered from his stab wound, and they had put him in the ground. And not long after that they had had to put Dust in the ground too. Gradually Pebble’s mother had seemed to have grown fond of Harpoon, this peculiar stranger who lay with her son. But at last her growing frailty overcame her strength of will.

But where life was lost, so new life was created. His two children were close in age — six and seven years old — but they were quite different.

Sunset was the younger, at six. The boy was the result of Pebble’s reluctant union with Cry, who had continued to pursue him long after he had formed his bond with Harpoon. Sunset was squat, round, a ball of energy and muscle, and above a thick, shadowing browridge his hair was still the startling red it had been when he was born, Ice-Age-sunset red.

Sunset had brought poor Cry no pleasure, though. She had died in giving him birth, to the end protesting about the presence of the new people among them.

Pebble’s other child, Smooth, had come from Harpoon. Though she had something of her father’s chunkiness, she was much more like her mother’s kind. Already she was taller than Sunset. Every time he saw her, Pebble was struck by Smooth’s flat face, and the ridgeless brow that swept up above her clear eyes.

Pebble had had no reason to be surprised when his sexual contact with Harpoon had resulted in a child. Now, in fact, she was pregnant again. The changes between the ancestral stock and Harpoon’s generation, though they were so striking, were not yet so fundamental that the two kinds of people could not crossbreed — and indeed their hybrid children would not be mules. They would be fertile.

Thus Harpoon’s modified genes, and her new body plan and way of life, had begun to propagate through the wider population of robust folk. Thus the thread of genetic destiny would pass on through Smooth, child of human-form and robust, into the future.

As the long afternoon wore on, driven by Pebble’s determination, they kept on trying to make the logs work.

It was frustrating. They had no way of discussing their ideas. Their language was too simple for that. And even the new folk were not particularly inventive with technology, for the compartment walls in their highly specialized minds denied them full awareness of what they were doing. They weren’t able to think it through. It was something like trying to learn a new body skill, like riding a bicycle; conscious effort didn’t help. And besides the work was uncoordinated, and only progressed when somebody was passionate enough to bully the rest.

But at last, quite suddenly, Ko-Ko hit on a solution. He splashed into the water. “Ya, ya!” With frantic yells and blows, he forced the swimmers to hold on to a single log and let it float. Then he went to the far end and, swimming strongly himself, guided the log out through the choppy inshore waves to the calmer waters beyond.

Pebble watched, amazed. It worked. Rather than riding the log, they used it as a float to help these nonswimmers swim. Soon the log was so far from the shore that all he could see was a row of bobbing heads and the black stripe of the log between them.

By clinging to the log and paddling for all they were worth even the robusts, too heavy to swim, were able to cross the water, far out of their depth. It was obvious to everybody that at last they had found a way to cross the strait that had baffled Pebble for years.

Pebble hollered his triumph. His sons ran to him. He picked up Smooth and whirled her, squealing, through the sunlit air, while Sunset pulled at his legs, clamoring for attention.

The raiding party landed on a little crescent of shell-strewn sand that nestled beneath walls of eroded blue-black rock. They staggered out of the water and lay gasping on the beach. Pebble saw immediately that everybody, robust and skinny alike, had made it to the shore.

The crossing had been harder than Pebble could have imagined. He would never be able to forget that awful sensation of being suspended over the blue-black depths where unknown creatures swam. But it was over now.

And already Ko-Ko was at work. Leading by example he was having the logs hauled to the shore. The warriors — a dozen robusts, a dozen skinnies — began to unpack their gear. Some of the weapons had been carried strapped to their backs or in pouches of netting, and some — the skinnies’ long throwing spears, for example — had been tied to the logs themselves.

Harpoon stroked her belly and gazed out to sea, back the way they had come. She touched the vertical ocher stripes on Pebble’s face, just as she had the first time they had coupled. But now she wore the same ferocious marking as he did — as did all the people, skinnies and robusts alike. He grinned, and she grinned back.

United by their symbols, two kinds of people prepared to make war on a third.

A woman cried out. Pebble and Harpoon whirled. A heavy basaltic rock had fallen onto the beach, pinning a skinny woman’s leg. When the rock was pulled away, her foot was revealed, a smashed and bloody mess. She began to keen, tears streaking the ocher stripes on her cheeks.

People were jabbering, pointing at the cliffs. “Hai, hai!”

Pebble peered up, shielding his eyes. Something moved up there: a head, narrow shoulders. The rock had not fallen, Pebble realized. It had been pushed, or thrown.

So it had begun. He grabbed his thrusting spear and roared defiance, and ran along the beach. The people followed him.

A few hundred meters along, this sheltered beach gave way to a more open stretch of dunes and grassland. And on the open land Pebble saw a group of wraithlike hominids. There were more than twenty of them — women, men, children, infants. They had gathered around the carcass of a fallen eland. When they saw Pebble they stood up, their heads swiveling.

Pebble hurled himself forward, yelling.