But still, since Purga, there had been little innovation in the primate line — even across two million years. It was the same for many other lineages. Long after the great impact, it was as if the emptied world had been shocked into stasis.

Plesi landed on her target branch without difficulty.

Her two pups were still huddled hesitantly against the tree trunk, and they made the mewling calls of babies. But, though the calls tugged at her, Plesi only raised her head and twitched her snout. She tried to encourage the pups to follow her by nibbling the fruit that clustered on this new tree.

At last the pups reacted. To Plesi’s surprise it was the little one, Weak, who came forward first. She scampered to the end of the branch — nervous, hesitant, but showing good balance. She raised her tail and tensed her muscles — she backed off nervously, preened the fur of her face — and then, at last, she jumped.

She overjudged slightly. She came tumbling out of the air and collided with her mother, making Plesi hiss in protest. But her agile hands and feet soon gripped the lumpy bark, and she was safe. Trembling, Weak scampered to her mother and buried her face in her belly, seeking a nipple that was now dry. Plesi let her suckle, rewarding her with comfort.

But now there was a blur of movement from the other tree. Strong, left behind, suddenly lunged forward, her immature feet slipping on the bark. And — without looking carefully, without trying to use her innate skills to estimate the distance — she leapt into the air.

Fear prickled inside Plesi.

Strong made the branch, but she landed too hard. Immediately she slid backwards. For a heartbeat she hung there, her small hands scrabbling uselessly at the bark, her hind legs waving. And then she fell.

Plesi saw her tumble in the air, wriggling, her white underbelly exposed, her hands and feet clutching at nothing. Even now Strong made the peeping cry of a lost infant. Then she fell into the leaves, and in a moment she was gone, taken by the green below, which swallowed all the forest’s dead.

Plesi clung to her branch, shuddering. It had happened so quickly. One young lost, one runtish weakling left. It was not to be borne. She hissed her defiance into the menacing green.

And, leaving Weak clinging piteously to the trunk of the tree, Plesi began to descend, down toward the green, down to the ground.

At last she reached the lower story of branches, and looked down into an oasis of light.

This was one of the endless forest’s few clearings. Within the last few months, an ancient canopy tree had fallen, eaten from within, wrecked by a random lightning strike. When it had crashed down it had cut a swath through the dense foliage. This clearing would not last long. But for now the plants of the undergrowth, like those hardy survivors, the ground ferns, were taking the opportunity to germinate, and the forest floor here was unusually lush and green. And already saplings were sprouting, beginning a ruthless vegetable race to steal the light and plug that hole in the canopy.

The forest was an oddly static place. The great canopy trees competed with each other to trap as much sunlight as they could. In the gloom of the lower levels, the light was too weak to support growth, and the floor was customarily littered by dead vegetable matter and the bones of any animals or birds unlucky enough to fall. But under the silent ground, seeds and spores abided — waiting centuries, even millennia if necessary, until the day came when chance opened up a gap in the canopy, and the race to live could begin.

Plesi slithered down a buttressing root and reached the ground. Under the broad fronds of a ground fern she scuttled uneasily through a patch of direct sunlight. The solid ground, with no give or sway, felt very strange to her, as peculiar as the shuddering of an earthquake would have felt to a human.

There were other animals here in the clearing, drawn by the prospect of novel pickings. There were frogs, salamanders, and even a few birds, flapping across the air in bright bursts of color, seeking insects and seeds.

And there were mammals.

There were creatures like raccoons but more closely related to the hoofed animals of the future, and scurrying insectivores whose descendants would include the shrews and the hedgehogs. Here was a taeniodont, like a small, fat wombat. It grubbed in the soil, expert at digging out roots and tubers. None of the grubbing creatures in this clearing would have been familiar to a watching human. They were furtive, odd, ungainly, almost reptilian in their behavior, forever looking over their shoulders, like petty thieves expecting the return of the householder.

These mammals were holdovers from the Cretaceous. Then, it had been as if the whole Earth had been a vast city, shaped for the needs of its owners, the dinosaurs. But now the dominant inhabitants were gone, the great buildings erased, and the only creatures left alive were the urban species who had lived in the drains and sewers, subsisting on garbage.

But the recovering Earth had become a very different place from the dreamy Cretaceous. The Earth’s new forests were much more dense now. There were no great herbivores: The sauropods had gone, and the elephants lay far in the future. There were no animals big enough to topple these trees, to smash clearings and corridors and make parklike savannah. In response the vegetation had gone crazy, filling the world with greenery of a density and profusion not seen since the first animals had walked onto the land.

But it was an oddly bare stage. In these thick jungles there were no more predatory dinosaurs — but neither were there yet jaguars, leopards, tigers. Practically all of the forest’s inhabitants were small, tree-dwelling mammals like Plesi. For an extraordinary span of time — for millions of years — the animals would cling to their Cretaceous habits, and no mammal species would grow to even moderately large sizes. They still contented themselves with the darkness and the corners of the empty world, nibbling on insects, eschewing any evolutionary innovations more spectacular than a new set of teeth.

Like long-term prisoners, the survivors of the impact were institutionalized. The dinosaurs were long gone — but for the mammals, habits ingrained over a much longer span, a full hundred and fifty million years of incarceration, were not so easy to give up.

But things were changing.

At last Plesi heard the quiet mewling of her young.

At the edge of the clearing Strong was huddled, pathetically, in a kind of nest of browned fronds. After she had fallen out of the tree and tumbled into the clearing, at least she had had the sense to seek cover. But she was far from safe: a large, scarlet-bellied predatory frog was watching her, an absent curiosity in its blank eyes. When she saw Plesi, Strong dashed forward and fell on her mother. She tried to find Plesi’s nipples, just as her sister had, but Plesi snapped at her, denying her comfort.

Plesi was deeply disturbed. A carpolestid who was strong in the nest but who had no instinct for the trees — who lacked even the sense to keep silent when exposed — had poor survival prospects. Suddenly Strong didn’t look so strong after all. Plesi felt an odd impulse to find a mate, to breed again. For now, though, she merely nipped at Strong’s flank with her sharp incisor teeth, and led the way back toward the tree from which she had descended.

But she had gone no more than a few body lengths when she froze.

The predator’s blank eyes fixed Plesi with lethal calculation.

The predator was an oxyclaenus.

He was a sleek, four-footed, dark-furred animal: long-bodied, stout-legged, he looked like an outsized weasel, though his face and muzzle were more reminiscent of a bear’s. But he was related to neither weasel nor bear. In fact he was an ungulate, an early member of that great family that would one day include the hoofed mammals like pigs, elephants, horses, camels, even the whales and dolphins.