Later, she dug. The rain still fell, and the ground was boggy, but by digging close to the tough roots of the ferns she was able to construct a satisfactory burrow. She began to relax, for the first time since the impact — perhaps for the first time since the crazed troodon had begun to pursue her.

Life had nothing more to ask of Purga. One of her pups had survived, and would breed, and through her the great river of genes would pass on, on into an unknowable future. And it was an irony that in former times she would surely already have succumbed to predation by now: It was the great emptying of the world that had preserved her life — a few extra months won at the expense of uncounted billions of creatures.

As content as it was possible to be, she settled to sleep in a cocoon of earth that still smelled of the great burning that had ended a world.

The planet was filling up with fast-breeding, short-lived creatures. Already almost all of Earth’s population had been born into the new era, and had known nothing but ash, darkness, and carrion. But as Purga slept her hind legs convulsed and her front paws scrabbled at the ground around her. For Purga, one of the last creatures on the planet to remember the dinosaurs, the terrible lizards still stalked, at least in her dreams.

There came a morning when she did not wake, and the little burrow became her coffin.

Soon a blanket of sediment, deposited by the ocean, covered over the vast impact crater. The great geological deformation was eventually hidden under a layer of limestone a thousand meters thick.

Of the Devil’s Tail itself, nothing remained but traces. The nucleus had been destroyed in the first seconds of the impact event. Long before Earth’s skies cleared, the last remnants of the coma and the glorious tail — the tenuous body of the comet, now cut from its tiny head — blew away in the wind from the sun.

But still the comet had left a kind of memorial. In the boundary clay would be found tektites — bits of Earth that had been blasted into space and returned, melted into glassy dewdrop shapes like tiny space capsules by their re-entry into the air — as well as fragments of quartz and other minerals, shocked into strange glassy configurations by the impact energy. There were shards of crystalline carbon, normally formed only deep in Earth’s interior, but baked on the surface in those few ferocious seconds: tiny diamonds, littering an ash of burned Cretaceous forests and dinosaur flesh. There were even traces of amino acids, the complex organic compounds once delivered by long-vanished comets to rocky Earth, the compounds that had enabled life to emerge here: a wistful present from a visitor who had come too late.

And as the dust clouds finally cleared and the chill dispersed, the comet’s final gift to the Earth came into play. Vast amounts of carbon dioxide, baked out of the limestone of the shattered seabed, now lingered in the air. A savage greenhouse effect kicked in. The vegetation, striving to recover, struggled to cope. The first millennia were times of swamps, of marshes and rotting bogs, where dead vegetation choked lakes and rivers. All over the world coal was laid down in great seams.

At last, though, as spores and seeds blew around the world, new plant communities blossomed.

Slowly, Earth turned green.

Meanwhile, time worked on Purga’s tiny remains.

Within hours of her death, blowflies had laid eggs in her eyes and mouth. Soon flesh flies were dropping larvae on her skin. As maggots burrowed into the little corpse, so the gut bacteria that had served her all her life burrowed out. Intestines burst. The contents began to rot other organs, and the cadaver liquefied, with a powerful stink, like cheese. This attracted carnivorous beetles and flies.

In the days after her death, five hundred types of insects feasted on Purga’s corpse. Within a week, there was nothing left but her bones and teeth. Even the great DNA molecules could not survive long. Proteins broke down into their individual building blocks; amino acids in turn decayed into mirror-image forms.

Just a few days after that, a flood of acidic water swept away the little hollow. Purga’s bones were dumped in a shallow depression half a kilometer away, jumbled with the bones of raptors, tyrannosaurs, duckbills, and even troodons: enemies made equal in the democracy of death.

With time, more layers of mud were laid down by floods and bank-bursting rivers. Under pressure, the layers of silt turned to rock. And, in her rocky tomb, Purga’s bones were further transformed, as mineral-rich water was forced into their every pore, filling them with calcite, so they became things of rock themselves.

Buried deep, Purga began a spectacular journey lasting millions of years. As continents collided, the land was uplifted, bearing all its entombed passengers like some vast ocean liner riding a swell. Heat and compressing forces fractured and twisted the rocks. But erosion continued, a relentless, destructive force balancing Earth’s creative uplift. Eventually this land became an angular landscape of plateaus, mountains, and desert basins.

At last the erosion cut through the mass grave that had swallowed Purga’s bones. As the rock crumbled away bits of fossil bone emerged into the light, corpses bobbing to the surface, waking from a sixty-five-million-year slumber.

Almost all of Purga’s bones were lost, flashing to dust in geological instants, all that patient chthonic preservation wasted. But in 2010 a remote descendant of Purga’s would pick out a blackened shard in a wall of gray rock, just beneath a strange layer of dark clay, and recognize it for what it was, a tiny tooth.

But that moment lay far in the future.

Through the endless forest, Plesi climbed.

Squirrel-like, she scampered up a scaly trunk and along a fat branch. Though it was close to noon, the light was dappled, uncertain. The canopy was high above her, the floor lost in green layers far below. The forest was silent save for the rustling of leaves in the warm breeze and the call of the canopy birds, those colorful cousins of the vanished dinosaurs.

It was a world forest. And it belonged to the mammals — including the primates, like Plesi.

She glanced back along her branch. There were her two pups — both daughters — who she thought of as Strong and Weak. About half Plesi’s size, they clung to the angle of branch and tree. Even now, Strong was pushing Weak subtly aside. In some species the runtish Weak might have been allowed to die. But Plesi’s kind bore few young, and in an uncertain and dangerous world, all of them had to be cared for.

But Plesi could not protect her pups forever. They were both weaned now. Though they had learned to seek out the fruit and insects that inhabited this, their birth tree, they must learn to be more adventurous — to move out into the forest, to seek out their own food.

And to do that, they had to learn to jump.

Hesitantly, scrambling at the scaly surface of the branch, Plesi tensed, and leapt.

Plesi was a plesiadapid: she belonged, in fact, to a species that would one day be called carpolestid. Plesi closely resembled her remote grandmother, Purga. Like Purga, she looked something like a small squirrel, with a low-slung body like a large rat’s, and a bushy tail. Though a true primate, Plesi retained Purga’s claws rather than nails, her eyes did not face forward, and her brain was little developed. She still even had the big night-vision eyes that had served Purga so well in the time of the dinosaurs.

The most significant development of primate bodies since Purga’s time was in the teeth; Plesi’s was a species adapted to husk fruit, as would be the possums of Australia, much later. It was a necessary response, if the primates were to find something to eat. Few animals of this time fed off leaves. In an equable world where tropical or paratropical forests spread far from the equator, there was little seasonal variation, and here in Texas the trees did not shed their leaves regularly. In fact, the trees loaded their leaves with toxins and chemicals to make them bitter or poisonous to curious mammalian tongues.