In the milky light Purga’s large black eyes had fixed on a moth. The insect sat on a leaf, brown wings folded, fat and complacent. With an efficient lunge Purga caught her prey in her paws. She severed off the wings with a couple of nips of her tiny incisors. Then, with a noise like the crunch of a tiny apple, she began to munch with relish at the moth’s abdomen. For this brief moment, with food in her mouth, Purga found a scrap of contentment in her crowded, difficult life.

The moth quickly died, its sparklike awareness incapable of recording much pain.

The moth consumed, Purga moved on. There was no grass cover here — the grasses had yet to dominate the land — but there was a green covering of low ferns, mosses, ground pine, horsetails, and conifer seedlings, even a few gaudy purple flowers. Through this tangle, scuttling between scraps of cover, she was able to progress almost silently. In the dark, solitary foraging was the best strategy. Predators worked by ambush, exploiting the shadows of the night; no group could have been as invisible as a lone prowler. And so Purga worked alone.

To Purga the world was a plain picked out in black, white, and blue, lit up by the uneasy light of the comet, which shone behind high scattered clouds. Her huge eyes were not as sensitive to color as the best dinosaur designs — some raptors could make out colors beyond anything that would be visible to humans, somber infrareds and sparkling ultraviolets — but Purga’s vision worked well in the low light of night. And besides she had her whiskers, which fanned out before her like a tactile radar sweep.

Purga looked more rodentlike than primate, with whiskers, a pointed snout, and small folded-back ears. She was about the size of a small bush baby. On the ground she walked on all fours, and she carried her long bushy tail behind her, like a squirrel. To human eyes she would have seemed strange, almost reptilian in her stillness and watchfulness, perhaps incomplete.

But, as Joan Useb would one day learn, she was indeed a primate, a progenitor of that great class of animals. Through her brief life flowed a molecular river with its source in the deepest past, its destination the sea of the furthest future. And from that river of genes, widening and modifying as thousands of millennia passed, would one day emerge all of humanity: Every human ever born would be descended from the children of Purga.

She knew none of this. She didn’t give herself a name. She was not conscious like a human — or even like a chimp or monkey; her mind was more like a rat’s or a pigeon’s. Her behavior was made up of fixed patterns, controlled by innate drives that constantly shifted in balance and priority, reaching a new sum each moment. She was like a tiny robot. She had no sense of self.

And yet she was aware. She knew pleasure — the pleasure of a full belly, the safety of her burrow, the snouts of her pups as they nuzzled her belly for milk — and, in this dangerous world, she knew fear very well.

She crept among the feet of dreaming ankylosaurs. As she moved beneath the immense bellies Purga could hear the huge rumble of the dinosaurs’ endless digestion, and the air was thick with their noxious farts. With their crude teeth, all the work of processing and digesting their coarse food had to be done in the dinosaurs’ vast guts, which labored even as the ankylosaurs slept.

The ankylosaurs were herbivorous dinosaurs. But this was a time of huge, ferocious predators. So these animals, larger than African elephants, were covered with armor, a fusion of bones, ribs, and vertebrae. Great yellow-black spines were embedded in their backs. Their skulls were so heavily reinforced there was little room left for brain. Their tails ended in heavy clubs that could smash legs or skulls.

The dinosaurs were too huge for Purga to comprehend. Hers was a small world, where a fallen log or a puddle was a major obstacle, where a scorpion could be a significant predator, where a fat millipede was a rare treat. To her, the dozing ankylosaur herd was a forest of immense stumpy legs and drooping tails that had no connection to each other.

But for Purga there was a rich prize here: dinosaur dung, immense heaps of it scattered in the muddy, trampled ground. Here, in fibrous mountains of roughly digested vegetation, she might find insects, even dung beetles, laboring to destroy the tremendous turds. She burrowed into the steaming stuff eagerly.

Thus had been the role of the ancestors of humanity, all through the long dinosaur summer: relegated to the fringe of the reptiles’ great society, emerging from their burrows only at night, foraging for a living from dung, insects, and the small pickings of the forest.

But tonight the rewards were meager, the droppings watery and foul-smelling. The volcano-damaged vegetation had provided poor fodder for the ankylosaurs, and what came out the other end was of little value to Purga.

She moved across the clearing and into the forest. Here conifers towered grandly, rising to spreading mats of leaves far overhead. Among them were smaller trees a little like palms, and a few low bushes bearing pale yellow flowers.

Purga scrambled briskly into the angular branches of a ginkgo tree. As she climbed she used the scent glands in her crotch to mark the tree. In her world of night, scent and sound were more important than sight, and if others of her kind found this mark, any time within the next week, it would be a sign like a neon light, telling them she had been here, even how long ago she had passed.

It was pleasing to climb, to feel her muscles work smoothly as they hauled her high above the dangerous ground, to use the delicate balance afforded by her long tail — and, most of all, to jump, to fly briefly from one branch to another, using all her body’s equipment, her balance, her agility, her grasping hands, her fine eyes. She was forced to shelter in burrows on the ground. But everything about her had been shaped by an existence in the complex three-dimensional environment of the trees, where almost all primate species, throughout the family’s long history, would find refuge.

But the acid rain of recent months had withered the trees and undergrowth; the bark was sour, and there were few insects to be found.

Purga was perpetually hungry. She needed to consume her body weight every day: It was the price of her warm blood, and the milk she must produce for her two pups, safe in their burrow deeper in the forest. She clambered reluctantly back down the trunk of the ginkgo. Fear and hunger warring in her mind, she tried one or two more trees, but with no better luck.

But now she lifted her head, whiskers twitching, bright eyes wide, to peer into the green dark of the forest. She could smell meat: the alluring stench of broken flesh. And she heard a forlorn, helpless piping, like that of baby birds.

She scuttled away, following the scent.

In a small clearing at the base of a huge, gnarly araucaria there was a heap of roughly piled moss. At its edge, a small patch of debris-littered silt began to move. Soon the patch rose like a lid, and a small, scrawny neck poked out of the ground and through the layer of mud and debris. A beaklike mouth opened wide.

Its little head quivering, tiny scales and feathers still moist with yolk, the infant dinosaur took its first breath. It looked like an oversized baby bird.

It was the moment the didelphodon had been waiting for. This mammal, the size of a domestic cat, was one of the largest of its day. It was low slung, with a black-and-silver coat. Now it lunged forward and grabbed the chick by its thin neck, hauling it from its shell and flinging it into the air.

The chick’s life was a handful of brief, vivid impressions: the cold air beyond its cracked shell, the blurred glow of the comet, a sense of flying. But now a hot cavern opened beneath it. Its skin still smeared with yolk, the chick died instantly.