“Too late now,” said Torr reasonably. And so it was; Ejan could see that the three brothers had turned away and were paddling evenly downstream, toward the river’s mouth. “Come, Ejan,” Torr said. “Let’s fish.”

When they had reached an open stretch of deep water the brothers took their net of woven flax and slipped into the water. The brothers swam apart until the net was stretched out, then Ejan hooked his big toe into the net’s lower margin to open it out vertically. They had turned the net into a fence across the current; it was about fifteen meters long. The brothers began to swim forward, sweeping the water.

Languidly flowing, the water was warm on Ejan’s skin, muddy, murky with green life.

After about fifty meters they swam together, closing the net. Their haul was not great — the fish had indeed been scared off today — but there were a few fat specimens that they threw into the canoe. They took care to release the smallest, most immature fish; nobody would eat a morsel when he could wait and take a fat adult in a few months. They pulled the net taut and prepared to swim upstream once more.

But now a cry went up from the shore, an eerie wail.

Ejan turned to Torr. “Mother.”

“We must go back.”

They lodged their net over a tree stump; there it could wait. They scrambled back into the canoe, turned it, and thrust it back into the tangle of drifting debris that lined the riverbank.

When they got back to the encampment, they found their sisters trying to comfort their distraught mother. The three brothers hadn’t even got out of sight of the shore before a tidal surge had smashed their fragile craft. None of them had been seen since; all of them had drowned.

Never again would Osa, Born, or Iner lash their canoes to Ejan’s.

Ejan pushed his way through his siblings to his mother, and laid his hand on her shoulder. “I will make that journey,” he said. “For Osa and the others. And I will not die trying.”

But his mother, her graying hair ragged, her eyes blurred with tears, only wailed more loudly.

Ejan was a remote descendant of Eyes and Finger, acolytes of the original Mother of Africa.

After Mother, the progress of mankind was no longer limited by the millennial pace of biological evolution. Now language and culture were themselves evolving at the speed of thought, feeding back on themselves, becoming ever more complex.

Not long after Mother’s death, a new exodus from Africa had begun, a great diffusion of people, in all directions. Ejan’s folk had gone east. Following the ancient footsteps of Far’s walker kind, they had worked their way along the southern fringe of Eurasia, following the coastlines and archipelagos. Now there were people strung out in a great strip from Indonesia and Indochina, through India and the Middle East, all the way back to Africa. And as the populations slowly grew, there had been a gradual colonizing push out of those beachheads along the inland waterways into the interior of the great continent.

Ejan and Torr were the product of the purest strand of coastal wanderers, those who had kept up their seashore migrations generation after generation. To exploit the riches of the rivers, estuaries, coastal strips, and offshore islands, these people had gradually honed their skills in boat-building and fishing.

But now they faced a quandary. On this archipelago, off the southwestern corner of the Asian landmass, they had traveled as far as they could go: They had run out of land. And the place was getting crowded.

There were opportunities to go further; everybody knew that.

Though the latest glaciation had yet to reach its deepest cold, the sea level had already dropped hundreds of meters. In the coastal reshaping that resulted, the islands of Java and Sumatra had been joined with Southeast Asia to form a great shelf, and much of Indonesia had become a long peninsula. Similarly Australia, Tasmania, and New Guinea had been merged into a single mighty mass.

In this unique, temporary geography, there were places where the Asian landmass was separated from greater Australia by only a hundred kilometers or so.

Everybody knew the southern land was there. Brave or unfortunate sailors, washed far from the coast and the offshore islands, had glimpsed it. Nobody knew its true extent, but over the generations enough travelers’ tales had accumulated for everyone to be sure that this was no mere island: this was a new land, extensive, green, rich, with a long and abundant coast.

To get there would be quite a feat. The people had gotten this far by island-hopping, moving through reasonably equable seas from one scrap of land to another, each clearly visible, one from the next. Moving from this last island to the southern lands — passing out of sight of land altogether — would be a challenge of a different order.

But still, to open up a new world, all it would take was for somebody bold enough to attempt the crossing. Bold enough, smart enough — and lucky.

Ejan took many days to select the tree he wanted.

With Torr at his side he walked through the fringes of the forest, studying sterculias and palms. He would stand beneath the trees, eyeing the lines of their trunks, tapping on their bark with his fist to detect any inner defect.

At last he selected a palm — very fat, very true, its trunk a bulky unblemished pillar. But it was a long way from his band’s settlement. Not only that, the palm was a long way from any riverbank; they wouldn’t even be able to float it home.

Torr thought about complaining about this, but when he saw the set expression on Ejan’s small face he kept his counsel.

First the brothers felled the palm with their stone axes. Then they briskly stripped the bark off the trunk. The exposed wood was perfect, as Ejan had hoped, and very hard under his hand.

They hiked back to the encampment to enlist help to bring back the trunk. Though there was a great deal of sympathy for the loss of their three brothers, nobody relished the prospect of such a long and difficult haul through the forest. In the end it was only family members — only Ejan, Torr, and their three sisters — who returned to the felled palm.

When he had gotten the palm back to camp Ejan immediately got to work. Sliver by sliver he hollowed out the stem of the trunk, taking care to leave the pith intact at stem and stern. He used stone axes and adzes — quickly blunted, yet equally quickly knapped.

Torr helped for the first couple of days. But then he drew away. As the oldest remaining sibling, responsibility now lay heavily on him, and he devoted himself to the basic chores of the family, to staying alive.

After a few days Ejan’s youngest sister, Rocha, brought him a small net bag full of dates. He set the dates down on the stern platform he was carving into the wood, and absently pushed them into his mouth while he worked.

Rocha, fifteen years old, was small, dark, slim — a quiet, intense girl. She walked around the trunk, seeing what he had done.

The hollow now extended through much of the trunk’s length. The trunk’s broad base would be the prow, and Ejan was leaving a broad platform here on which a harpooner could stand. A smaller flat seat at the stern would accommodate the helmsman. It was remarkable to see a boat emerging from the wood. But the great notch Ejan was digging into the trunk was still heartbreakingly shallow, the surfaces rough and unfinished.

Rocha sighed. “You are working so hard, brother. Osa used to put together a raft in a day, two at most.”

He straightened up. He wiped sweat from his brow with his bare arm, and dropped another worn ax blade. “But Osa’s raft killed him. The ocean between us and the southern land is not like the placid waters of the river. No raft is strong enough.” He ran his hand along the inside of the hollow. “In this canoe I will be tucked safely inside the craft. So will my belongings. Even if I capsize, I will not be harmed, for the boat will easily be righted. Look here.” He rapped on the trunk’s exterior. “This trunk is very hard on the outside, but the pith is light inside. The wood is so buoyant it cannot even sink. This is the best way to make the crossing, believe me.”