This oxy might have seemed clumsy, slow, even unfinished to an eye used to cheetah or wolf. But his kind had learned to stalk prey through the sparse undergrowth of the endless forest. He could even climb, pursuing his quarry into the lower branches of the trees. In this archaic time, this oxy had little competition.

And, as he looked on Plesi’s timorous, flattened form, two cold questions dominated the oxy’s mind: How will I trap you? And, How good will you be to eat?

Plesi lay flat against the floor, quivering, her whiskers twitching, her small, sharp teeth bared. But she was equipped with instincts honed over a million centuries at the feet of the dinosaurs. And in the cold calculus of her mind, a reassessment of risk was beginning. In this open place she could not hide. She could not reach a tree to clamber out of the oxy’s grasp. Surely if she tried to outrun him he would trap her easily with one of those cruel claws.

Only one option was left.

She arched her back, opened her mouth and hissed, so violently that she spattered the oxy with her spittle.

The oxy flinched at the unexpected aggression of this tiny creature. But she is no threat. The oxy, angered, quickly recovered his composure, and prepared to call Plesi’s bluff.

But Plesi had vanished into the undergrowth. She had never meant to attack the oxy, only to gain a precious second of time. And she had left Strong behind.

The young carpolestid, transfixed by the carnivore’s stare, flattened herself against the ground. The oxy crushed Strong with his paw, snapping the little primate’s spine. Strong, flooded with pain, turned on her attacker, seeking to gouge his flesh with her teeth. In her final moments Strong discovered something like courage. But it did her no good.

The oxy played with the crippled animal for a while. Then he began to feed.

As the world recovered, so its changing conditions shaped its living inhabitants.

The mammals were beginning to experiment with new roles. The ancestors of the true carnivores, which would eventually include the dogs and cats, were still small, ferretlike animals, busy, opportunistic general feeders. But the oxyclaenus had begun to develop the specializations of mammalian predators to follow: vertical legs for sustained speed, strong permanent teeth anchored by double roots and with interlocking cusps designed to shred meat.

It was all part of an ancient pattern. All living things worked to stay alive. They took in nourishment, repaired themselves, grew, avoided predators.

No organism lived forever. The only way to counter the dreadful annihilation of death was reproduction. Through reproduction, genetic information about oneself was passed on to one’s offspring.

But no offspring was identical to its parents. At any moment each species contained the potential for much variation. But all organisms had to exist within a frame of habitability set by their environment — an environment, of weather, land, and living things, which they shaped in turn. As survival was sought with ruthless ferocity, the frame of the environment was filled up; every viable variation of a species that could find room to survive was expressed.

But room was at a premium. And competition for that room was relentless and unending. Many more offspring were born than could possibly survive. The struggle to exist was relentless. The losers were culled by starvation, predation, disease. Those slightly better adapted to their corner of the environment inevitably had a slightly better chance of winning the battle for survival than others — and therefore of passing on genetic information about themselves to subsequent generations.

But the environment could change, as climates adjusted, or as continents collided and species, mixed by migrations over land bridges, found themselves with novel neighbors. As the environment, of climate and of living things, changed, so the requirements of adaptation changed. But the principle of selection continued to operate.

Thus, generation by generation, the populations of organisms tracked the changes in the world. All the variations of a species that worked in the new frame were selected for, and those that were no longer viable disappeared, sinking into the fossil record, or into oblivion altogether. Such turnovers were unending, a perpetual churn. As long as the “required” variation lay within the available genetic spectrum, the changes in the population could be rapid — as rapid as human breeders of domesticated animals and plants would find as they strove for their own ideas of perfection in the creatures in their power. But when the available variation ran out, the changes would stall, until a new mutation came along, a chance event caused, perhaps, by radiation effects, that opened up new possibilities for variation.

This was evolution. That was all there was to it: It was a simple principle, based on simple, obvious laws. But it would shape every species that ever inhabited the Earth, from the birth of life to the last extinction of all, which would take place under a glowering sun, far in the future.

And it was working now.

It was hard.

It was life.

Plesi had made an unspoken bargain with the oxy: Take my child. Spare me. Even as she clambered back through layers of green and into the safety of the trees, seeking her surviving daughter, that dreadful stratagem still echoed in her mind.

That, and a feeling that came from deep within her cells, a thought she might have expressed as: I always knew it was too good to be true. The teeth and claws weren’t gone. They were just hiding. I always knew they’d come back.

Her instinct was right. Two million years after the uneasy truce imposed by the dinosaurs’ death, the mammals had started to prey on each other.

That night Weak, bewildered, terrified herself, watched her mother twitch and growl in her sleep.

There was no true morning during these long days of Arctic summer, no authentic night. But as the clouds cleared from the face of the climbing sun, and light and warmth slanted through the trees’ huge leaves, a mist rose from the swampy forest floor, and Noth’s sensitive nostrils filled with the pleasing scent of ripe fruit, rotting vegetation, and the damp fur of his family.

It felt like a morning, like a beginning. A pleasing energy spread through Noth’s young body.

His powerful hind legs folded under him, his fat tail upright, he squirmed along the branch to get closer to his family — his father, his mother, his new twin sisters. Together, the family groomed pleasurably. The nimble fingers of their small black hands combed through fur to pick out bits of bark and fragments of dried baby shit, even a few parasitic insects that made a tasty, blood-filled treat. There was some loose fur, but the adult adapids had already lost most of last year’s winter coat.

Perhaps it was the gathering light that inspired the singing.

It began far away, a thin warbling of intertwined male and female voices, probably just a single mating pair. Soon more voices joined in the duo’s song, a chorus of whooping cries that added counterpoint and harmony to the basic theme.

Noth moved to the end of the branch to hear better. He peered through banks of giant leaves that angled south toward the sun, like so many miniature parasols. You could see a long way. The circumpolar forest was open, and the trees, cypress and beech, were well spaced so their leaves could catch the low Arctic sunlight. There were plenty of broad clearings where clumsy ground-dwelling herbivores rummaged. Noth’s eyes in their mask of black fur were huge — like his remote ancestor Purga’s, well adapted to the dark, but prone to dazzling in the daylight.