Cautiously Harpoon clambered into the hut. Watching Pebble, she sat on the trampled ground. Her eyes were bright in the gloom.

Slowly, Pebble sat before her.

At length Harpoon pushed her slim hand under a bark blanket and pulled out a handful of baobab fruit. She held it out to Pebble. Hesitantly he took it. For long heartbeats they sat in silence, representatives of two human subspecies, with not a word, not a gesture in common.

But at least they weren’t trying to kill each other.

After that day Pebble felt increasingly uncomfortable in his home, with his people.

The stringy folk seemed to accept him. The tall man who had found him in the hut, Ko-Ko — for Pebble would always think of his cries of “Ko, ko!” “Get away!” — never quite trusted him, that was clear. But Harpoon seemed to take to him. They worked tools together, she showing off the subtle skills of her delicate fingers, he his immense strength. They peered across the sea at the rich island that still tantalized Pebble.

And they tried to work out each other’s vocabulary. It wasn’t easy. There were many words, such as directional terms like “west,” which Pebble’s ancestors had never needed.

He even went hunting with her.

These newcomers were by preference scavengers or ambush hunters. With their lithe but feeble frames they used guile rather than brute strength to make their kills, and their weapons of choice were hurled, not thrust. But they grew to welcome Pebble’s mighty contributions during the closing stages of a kill, when the prey had to be finished off at close quarters.

Meanwhile, the two kinds of people started a new kind of relationship. They did not fight, nor did they ignore each other, the only two ways people had had to relate to each other before.

Instead, they traded. In exchange for the fruits of the sea and some of their artifacts, such as their massive thrusting spears, Pebble’s folk began to receive bone tools, meat from the interior, marrow, skins, and exotic items like honey.

Despite the obvious benefits of the new relationship, many of Pebble’s folk felt uneasy. Hands and Seal had inquisitively explored the possibility of the new tools. Dust, aging quickly, seemed sunk in apathy. But Cry was unremittingly hostile to the new people — and to Harpoon in particular. This wasn’t the way things were done.

These were, after all, an immensely conservative people, people who moved house only when forced to by an Ice Age. But they traded anyhow, for the advantages were undeniable.

Harpoon had been able to hold back Ko-Ko from killing Pebble because, to these people, a stranger wasn’t necessarily a threat. You had to be able to think that way if you were going to trade.

For hominids, that was a brand-new way of thinking. But then Harpoon’s kind was only five thousand years old.

There had been a band of people, not unlike Pebble’s, who had lived on a beach, not unlike this one, on the eastern shore of southern Africa. The beach was crowded by thick, buff-colored sedimentary rocks. The vegetation was unique to that part of the world, an antique flora recalling Roamer’s days, dominated by bushes and trees covered by big, thistly flowers. It was a rich place to live. The sea was productive, offering mussels, barnacles, fish, seabirds. In places the forest came right down to the shore, echoing with the calls of monkeys and birds, and in the grassy glades there was game in abundance: black rhinos, springboks, wild pig, elephant, as well as long-horned buffalo and giant horses.

Here Harpoon’s ancestors had had a home base close to the sea. Like Pebble’s folk, they had lived there for generations beyond counting, their bones lying thick in the earth. From here they would work across the landscape, never traveling more than a few kilometers from home.

But then, with terrible suddenness, the climate collapsed. The ocean rose, and flooded their ancestral home. Just like Pebble’s group they had been forced to flee. And like Pebble’s folk, lost in a crowded land, they had had nowhere to go.

Every step they took away from the lands they had known left them more baffled and confused. Many of them died. Many infants, in the arms of starving refugee mothers, failed to live much past birth.

At last, desperate, starving, they were forced along a riverbank. They reached the river’s mouth, where mangroves grew thick. Here they could stay, because it was a place nobody else wanted. Much of the floor was covered with an oily brown water, through which slid crocodiles. Damp, fetid, unhealthy, it was a kingdom of lizards, snakes, and insects, many of which, even the marching ants, seemed to conspire to drive out the people.

There was food to be had: water lily roots, shoots, and stems. Even mangrove fruits were palatable to the starving. But there was scarcely any meat. And there was no stone anywhere with which to make tools. It was as if they were trying to live on a great soaked-through mat of vegetation.

Stranded out of their environment, the people might have died out within a generation, if they hadn’t adapted.

It had started innocently. A woman, Harpoon’s remote grandmother, had wandered as far as she could up the river valley and on to drier land. Here, on the floodplains and in the seasonal swamps, the well-watered, silty soil supported many annuals, herbs, legumes, vines, lilies, and arrowroots. After years in the swamp she had grown adept at using crude wooden tools and her bare hands to harvest food from soggy, unpromising terrain. She had already filled her belly, and was gathering clumps of roots to take home to her children.

Then she came upon the stranger. The man, from another group further upriver, was using a knife of basalt to skin a rabbit. The two of them stared at each other, one with meat, the other with roots. They might have fled, or tried to kill each other. They did not.

They traded: meat for roots. And they went their separate ways.

A few days later the same women returned to the same spot. Again the man returned. Scowling, suspicious, their tongues mutually incomprehensible, they traded again, this time shellfish and barnacles from the river’s mouth for a couple of basalt knives.

That was how it began. The people of the swamp, unable to find everything they needed to stay alive in the scrap of land they had inherited, exchanged the produce of the sea, the swamp, and the floodplain for meat, skin, stone, and fruit from the interior.

After a couple of generations they migrated out, and began a new kind of life. They became true nomads, following the great natural highways, the coasts and the inland water courses. And everywhere they went they traded. As they moved, so they fissured and spread, and tentative trading networks grew. Soon it was possible to find bits of shaped rock hundreds of kilometers from where they had been formed and seashells deep in the interior of the continent.

Living like this was a challenge, though. Trading meant building up a new kind of map of the world. Other people were no longer just passive features of the landscape, like rocks and trees. Now a track had to be kept of who lived where, what they could offer, how friendly they were — and how honest. There was a ferocious pressure on the swamp people to get smarter, fast.

The design of their heads changed drastically. Their skulls enlarged to make room for bigger brains. And changing diets and lifestyles had a dramatic effect on their faces. No longer used to chew tough, uncooked food or to treat leather, their teeth became more feebly rooted. As chewing muscles withered, the upper tooth row shrank back. The lower jaw was left jutting, and the face tilted back, so that these hominids lost the last trace of their ancient apelike muzzles. The declining muzzle and ballooning foreheads provided new anchoring surfaces for the muscles of the face, and the old projecting browridges disappeared.