At the forest edge was an undergrowth of low ferns and blessed shade. Taller trees towered above her. On their branches were clusters of a red fruit she didn’t recognize. Her mouth was too dry to salivate, but her tongue clicked against her teeth.

She glanced back the way she had come. The mango tree and its raft of vegetation was just a scrap of driftwood, broken, rotten, seaweed clinging to it, now washed up on this shore. She could see the unmoving form of an anthro — Patch or Crest — lying inert on the broken, salt-crusted foliage. And beyond the raft the sea rolled, huge, eternal, blue gray, reaching as far as she could see to a horizon of chilling geometric perfection.

Now there was a crashing tread, a great snapping of foliage. Roamer shrank back.

A giant form emerged from the forest, like a tank rolling through the undergrowth. Huge, squat, under a great bony dome of a shell, it looked like a giant tortoise — or perhaps an armored elephant — a great plated body supported by four stumpy legs. Behind it a tail swung carelessly, tipped by a spiky club. And as its small reinforced head pushed out into the light, armored eyelids blinked. This tremendous ankylosaur-like creature was a glyptodont. Roamer had never seen anything like it in Africa.

But then, this wasn’t Africa.

The giant armored monster lumbered away. Cautiously Roamer followed the glyptodont deeper into the forest. She came to a clearing, surrounded by a wall of tall, imposing trees. The floor was carpeted by aloes. Experimentally Roamer nibbled at a leaf. It was succulent, but bitter.

She moved further forward, and found the glimmer of still water. It turned out to be a shallow, reed-choked freshwater pond. At its shore browsed a pair of huge animals. They grazed on the plants at the pond’s edge with snouts like spatulas. They looked like hippos, but were actually immense rodents.

The pond was on the fringe of a broader plain. And there, dimly visible now, much stranger mysteries awaited Roamer. There were creatures that might have been horses, camel, deer, and smaller animals, like hoofed pigs. Alongside them moved a small family of dinomyids: bulky, bearlike grazers. They were giant rodents, the extravagant relations of dormice and rats. There were predators here too, creatures who ran in packs like dogs — but they were marsupials, only distantly related to placental counterparts elsewhere, shaped by convergent evolution, similarly adapted for a similar role.

From a green shadow near Roamer, a head turned, startling her. The head was upside down. Two black eyes peered at her dimly. Above the head was a huge brown-furred body, dangling from limbs that clutched a branch above. This was a sloth, a kind of megatherium.

Cautiously, Roamer crept forward, at last, to the pool. The water was muddy, greenish, warm. But when she plunged her face into it, it was the most delicious thing she had ever tasted. She sucked down great mouthfuls. Soon her shrunken belly was full, and agonizing pains shot through her, as if she were being torn apart from inside. She fell forward, crying out, and threw up almost all she had drunk. But she pushed her face back into the water and drank again.

This brackish pond was actually a sinkhole. Fifty meters deep, it had been caused by groundwater dissolving the underlying limestone. There were many such sinkholes in the area, aligned along great, deep-buried faults in the rocks.

Seen from the air, the sinkholes would have formed a huge semicircle some hundred and fifty kilometers across. The arc of sinkholes marked a boundary fault of the ancient, long-buried Chicxulub crater, the rest of which stretched under the Gulf of Mexico’s shallow waters and sediments. This was the Yucatan Peninsula.

Expelled by an African river, riding westward currents, Roamer’s raft had crossed the Atlantic.

Nowhere on Earth was truly isolated.

Everywhere was connected by the ocean currents, some of which covered as much as a hundred kilometers a day. The great currents were like conveyor belts that bore flotsam around the world. In later times, inhabitants of Easter Island would burn logs of American redwood, washed ashore after a journey of five thousand kilometers. People living on coral atolls in the deep Pacific would make tools from stones embedded in the roots of stranded trees.

With the flotsam traveled animals. Some insects rode the surface of the water itself. Other creatures swam: Westward currents could carry leatherback turtles across the Pacific from their feeding ranges near Ascension Island to breeding grounds in the Caribbean.

And some animals rode across the oceans on impromptu rafts — oceanic odysseys undertaken not by choice or design, but by the vicissitudes of chance, just as had befallen Roamer.

The Atlantic, which had been widening since the shattering of Pangaea, was still much narrower than in human times: no more than five hundred kilometers wide at its narrowest point. It was not an impossible distance, a traverse that could be survived even by fragile forest creatures like Roamer, with luck. Such crossings were improbable. But they were possible, given the outflows of mighty rivers, the narrow oceans, perhaps the help of hurricane winds.

On the longest of timescales, over millions of years, the workings of chance defied human intuition. Humans were equipped with a subjective consciousness of risk and improbability suitable for creatures with a lifespan of less than a century or so. Events that came much less frequently than that — such as asteroid impacts — were placed, in human minds, in the category not of rare, but of never. But the impacts happened even so, and to a creature with a lifespan of, say, ten million years, would not have seemed so improbable at all.

Given enough time even such unlikely events as ocean crossings from Africa to South America would inevitably occur, over and again, and would shape the destiny of life.

Thus it was now. In the trees that towered above Roamer there was not a single primate — not one, not in all the continent, for her remote cousins, other children of Purga, had succumbed to extinction here millions of years ago, beaten by the rodents’ competitive pressure.

So, in this place where a world had ended, where differently evolved creatures foraged through different forests, a new life was starting, a new line of Purga’s great family. From just three survivors — given enough time, and the slow plastic working of their genetic material — would radiate a whole spectrum of new species.

By any standards the New World monkeys would be successful. But on this crowded jungle continent, the fate of Roamer’s grandchildren would be quite different from those of her sister’s in Africa. There, the primates, molded cataclysmically by the shifting climate, would rapidly develop new forms. There, Purga’s line would continue — through the apes — its slow shaping toward humanity. Even the later monkeys who Roamer so resembled would diversify away from the forest, finding ways to live in savannah, mountain plateaus, and even deserts.

Here it would be different. On a more equable continent, it would always be too tempting to stay in the vast rain forests.

Roamer’s grandchildren would never leave the trees. They would never grow much smarter than they were now. And they would play no part in the future destiny of mankind — save as pets, or prey, or objects of scientific curiosity.

But all that lay in the unimaginable future.

Roamer already felt remarkably revived by her brief time in the green and the water she had drunk. She looked around. In the undergrowth she saw a splash of red, and she stumbled that way. She found a fruit, unfamiliar, but fat and soft-skinned. She bit into it. As she munched on the flesh, juice burst out and dribbled over her fur. It was the cleanest, sweetest thing she had ever tasted.