Finally Third turned on the pup — Last, for she was Purga’s last surviving child. Purga couldn’t see Third. But with her whiskers and well-developed hearing she could sense her mate approaching the pup, step by step, mouth wide, as if stalking a centipede.

Third was angry, confused, frightened, and very, very hungry. But what he was doing made a certain sense. After all, there was nothing to eat here. If the flesh of the pup kept the adults alive a little longer, long enough for them to produce another litter, the genetic program would be fulfilled. The calculations were relentlessly logical.

Perhaps in other times Purga would have submitted to Third’s aggression, even helped him finish off the pup. But Purga’s life had already been long for her kind, and she had suffered a series of extraordinary events: the destruction of her first home, her dogged pursuit by Wounding Tooth — and now the nightmare of the comet impact and her stranding in this world of cold and silence.

The imperatives resolved themselves. She bit Third savagely on the thigh, and scrambled past him to stand alongside her daughter.

Last was just as confused as the others. But she figured out that her mother was defending her from some kind of attack by her father. And so she stood with Purga and bared her teeth at Third. For a full half-minute the burrow was filled with hissing and the sound of tiny paws scraping the ground aggressively; three sets of whiskers filled the space between the primates, each of them waiting for the other to strike.

In the end it was Third who backed off. He gave up quite suddenly, abandoning his aggressive posture and curling up alone in a corner of the burrow. Purga stood with her daughter until the anger and aggression had drained out of her system.

It was this final incident that changed the balance of the forces in Purga’s mind.

They couldn’t stay here, for they would starve, or freeze, if they didn’t kill each other first. They had to go out, regardless of what mysterious dangers lurked in the newly silent world above. Enough was enough. When her body clock next woke her, Purga pushed away the dirt that clogged the entrance to the burrow.

And emerged into the dark.

After two days, the fire in the sky had died. But now, from pole to pole, dust and ash covered the wounded Earth, a black shroud laced with wispy, yellow-white clouds of sulfuric acid. The Earth had been transformed from a starlike shining to a dismal, gloomy darkness, darker than the core of the comet which had wrought such devastation. Dust and ash: The dust was comet fragments, and sea-bottom dirt, and even volcanic debris spewed out after the immense seismic shocks that had rippled through the planet. And the ash was burned life, trees and mammals and divergent species of dinosaurs from America and China and Australia and Antarctica, burned to cinders by the global firestorms and then burned again in the pulse of superheat, now mingled together in the choked stratosphere. Meanwhile, sulfur, baked out of seabed rock in the first moments of the impact, had lingered in the air, forming sulfuric acid crystals. The high, bright acid clouds reflected away sunlight and drove the cold deeper still.

Followed by Third and Last, Purga crept cautiously away from her burrow’s mouth, whiskers twitching nervously. It was late afternoon, here in the chill heart of North America. If the sky had been clear, the sun would still have been well above the horizon. There was only the gloomiest of twilights, barely sufficient even for Purga’s huge, sensitive eyes.

She stumbled forward over bare, scorched rock. Everything was wrong. There was no scent of green growing things, nor the pungent, spicy stink of the dinosaurs, not even of their dung. Instead, she smelled only ash. The whole of the great thick green-brown layer of Cretaceous life had been burned off: even the dead leaves, even the dung, all destroyed. All that was left were minerals, lifeless dirt, and rock. It was as if Purga had been transported to the surface of the Moon.

And it was cold, a deep intense cold that quickly penetrated through diminished layers of fat to her bones.

She came to the ruins of what had been a small stand of tree ferns. She scraped at the ground with her claws, but the ground was strangely hard — and it was cold, deep cold, so cold it hurt the pads of her hands. But when she licked her hand, a slow trickle of water gathered in her mouth.

Just a few days earlier this had been a place of tropical forest and swamplands. No frost had formed here in millions of years. But now there was frost. Purga scrabbled at the ground, cramming the strange, chill stuff into her mouth. Slowly she got mouthfuls of water — and plenty of ash and dirt along with it.

She tried to dig deeper. She knew that even after the most ferocious fire there was food to be had: hardened nuts, deep-buried insects, worms. But the nuts and spores were trapped under a lid of frozen ground, too tough for Purga’s small paws.

She moved on, feeling her way through the dark with her whiskers.

She came to a shallow puddle. In fact it was the footprint of a vanished ankylosaur. Her snout hit a hard surface: brutally cold and hard as rock. The cold that stabbed through her fur was intense. She backed up hastily.

Like frost, she had never encountered solid ice before either.

More cautiously she poked at the ice with her snout and hands. She scraped and scratched — she could smell the water that was somehow hidden here, and it maddened her to be able to get no closer to it. Frustrated, she began to circle the little puddle, pushing and probing. At last she came to a place where the ankylosaur’s foot, pushing into what had been soft warm mud, had dug a somewhat deeper pit. The ice here was thin, and when she pushed at it, the surface cracked and lifted up. She jumped back, startled. The fragment of ice, upended, slid slowly into the black water. Cautiously she slid forward once more. And this time, when she tentatively dipped her snout, she found liquid water: chill, already frosting over with fresh ice, but liquid nonetheless. She sucked in great mouthfuls, ignoring the bitterness of the ash and dust that laced it.

Attracted by the sound of her drinking, Third and Last came hurrying to her side. They quickly extended the hole she had broken, jostling to slurp up the gritty water.

For the first time since the comet had struck, things had gotten better for Purga: not by much, but better.

But now something touched her shoulder: something light, cold. She yelped and turned. It was a wisp of white, already melting.

Now more flakes came drifting down out of the sky. They fell with a random, gentle movement. When a flake came close enough, she leapt up and took it in her mouth, like plucking a fly from the air. She got a mouthful of soft ice.

It was snowing.

Spooked at last beyond endurance, she turned and bolted for the security of the burrow.

The impact had hurled vaporized ocean water into the air. After weeks of suspension, it began to fall back.

There was a lot of vapor. An epochal rain fell, all over the planet.

But the rain itself brought further devastation. It was full of sulfuric acid from the ice clouds, and the impact had injected thin clouds of toxic metals into the atmosphere, metals that now rained out. Nickel alone reached twice the threshold of toxicity for plants. Runoff water washed substances like mercury, antimony, and arsenic out of the soils, concentrating them in lakes and rivers.

And so on. For years, every raindrop would be poisoned.

The great rain washed out the dust and ash. All over the world, a fine layer of blackened clay was laid down, a band of darkness that would forever show up as a punctuation in the sedimentary rocks of the future — a boundary clay, one day to be studied by Joan Useb and her mother, the last remnant of a biosphere.