Here the comet swam for four and a half billion years, while on Earth continents danced and species rose and fell.

But the sun’s gentle gravity tugged. Slowly, slower than the rise of empires, the comet responded.

And it began to fall back toward the light.

Red dawn light seeped into the eastern sky. The clouds had a bubbly texture, and the sky was tinged a peculiar bruise purple. In this remote time the very air was different — thick, moist, laden with oxygen. Even the sky would have looked alien to human eyes.

Purga was still traveling, exhausted, already dazzled by the gathering light. She had wandered far from any forest. There were only scattered trees here, spaced out over a ground made green by a dense mat of low-lying ferns. The trees were cycads, tall trees with rough bark that resembled palms, squat cycadeoids looking oddly like giant pineapples, and ginkgoes with their odd, fan-shaped leaves, an already ancient lineage that would survive into the human era and beyond.

In the stillness of the predawn, nothing moved. The dinosaur herds had yet to stir, and the hunters of the night had retired to their burrows and nests — all but Purga, who was stranded in the open, all her worn nerves sparking with an apprehension of danger.

Something moved across the sky. She flattened herself against the ground and peered up.

A winged form glided high over the roof of the sky, its profile picked out cleanly by the red-gray light of the dawn. It looked like a high-flying aircraft. It was not; it was alive.

Purga’s instinctive computation relegated the pterosaur to a matter of no concern. To her the most ferocious flying creature was of much less immediate peril than the predators who might lurk under these cycads, the scorpions and spiders and ever-ravenous carnivorous reptiles, including the many, many small and savage dinosaur species.

She stumbled on, toward the gathering dawn. Soon the greenery started to thin out, and she scrambled over hard-packed dunes of reddish sand. She topped a short rise — and found herself facing a body of water, which lapped languidly to the horizon. The air smelled strange: full of salt, and oddly electric.

She had come to the northern shore of the great slice of ocean that pushed into the heart of North America. She could see vast, languid forms break the water’s surface.

And to the southeast, where the dawn light was gathering, the comet was suspended in the sky. Its head was a milky mass from which immense fountains of pearl-white gas gushed, visibly evolving as she watched. Its twin tails, streaming away from the sun, flailed around the Earth, making a confusing, billowing mass. It was like looking down a shotgun blast. The whole immense, brilliant show was reflected in the shallow sea.

Listlessly she stumbled forward, descending to a shallow, sloping beach. The shore was littered with clam shells and half-dried seaweed. She prodded at this detritus, but the seaweed was stringy, salty stuff. And she could smell the salt in the water; there was nothing to drink here.

On her little rise, Purga was increasingly exposed, as if picked out by a spotlight.

She spotted a tree fern, no more than a meter tall. She stumbled to it and began to dig at its roots, hoping to make a rudimentary burrow. But the soft sand fell back into her trenches. At last, as the ruddy sun rose above the horizon, Purga managed to dig out a hole big enough to shelter her body. She tucked her tail in behind her, put her paws over her face, and closed her eyes.

The warmth and darkness of the burrow reminded her of the home she had lost. But the smell was wrong: nothing but salt and sand and ozone and decaying seaweed, the sharp stinks of this place where land met sea. Her home burrow had smelled of herself, of that other who was her mate, of the pups who had smelled like a mixture of herself and her mate — a wonderful mélange of selves. All gone now, all lost. She felt a deep pang of regret, though her mind was not rich enough to understand why.

As she slept out the long day her legs scraped and scratched at the gritty young sand.

Cretaceous Earth was a world of ocean, of shallow seas and shore.

A giant ocean called the Tethys — like an extension of the Mediterranean — cut off Asia from Africa. Europe was little more than a scattered collection of islands. In Africa, even the mid-Sahara was an ocean floor. The world was warm, so warm there were no polar ice caps. And for eighty million years the sea levels had been rising. The post-Pangaea spreading of the continents, and the formation of huge reefs and shelves of chalk around their coasts, had pushed huge volumes of solid matter into the oceans: It had been like putting bricks into an already full bucket of water, and the brimming oceans had flooded the continents. But the vast shallow oceans were almost tideless, and their waves were gentle.

Life in the sea was richer and more varied than at any other time in Earth’s long history. Tremendous blooms of plankton filled the waters, drinking the sunlight. The plankton were the base of the ocean’s vast pyramid of eaters. And in the plankton were microscopic algae called haptophytes. After a brief free-swimming phase, the haptophytes constructed for themselves tiny, intricate suits of armor from calcium carbonate. As they died, billions of tiny corpses sank to the warm seabeds, where they settled and hardened into a complex white rock, chalk.

Eventually tremendous chalk beds, kilometers thick, would smother Kansas and the gulf coast of North America, and stretch along the southern half of England and into northern Germany and Denmark. Human scientists would call this era the Cretaceous — after creta, meaning “chalk” — for its most enduring monuments, constructed by the toiling plankton.

When the light began to seep out of the sky, Purga emerged from her shelter.

She scampered with difficulty through dry sand that yielded with every step, sometimes billowing around her belly. She was rested. But she was hungry, and confused, and pulsed with loneliness.

She came to the top of the rise she had crossed yesterday. She found herself facing a broad, gently rolling plain extending to the rising smoke-wreathed mountains to the west. Once the great American inland sea had flooded this place. But now the sea had receded, leaving a plain littered with broad, placid lakes and marshes. Everywhere there was life. Giant crocodiles cruised like gnarled submarines through the shallow waters, some of them with birds riding on their backs. There were flocks of birds, and birdlike, furry pterosaurs, some of whom built huge rafts to support their nests at the center of the lakes, far from the land-based predators.

And everywhere she looked there were dinosaurs.

Herds of duckbills, ankylosaurs, and a few gatherings of slow, clumsy triceratops clustered around the open water, jostling and fighting. Around their feet ran and hopped frogs and salamanders, lizards like iguanas and geckos, and many small, snapping dinosaurs. In the air pterosaurs and birds flapped and called. On the fringe of the forest, raptors could be seen stalking, evaluating the jostling herds.

The hadrosaurs, the duck-billed dinosaurs, were this era’s most common herbivores. Though they were larger than later mammalian equivalents like wildebeest or antelope, they walked on two legs like outsized ostriches, their strides long, their heads bobbing. Males led the way, elaborately ornamented by huge crests over their noses and foreheads. The crests acted as natural trumpets, capable of producing notes as low as a piano’s bottom register. Thus the voices of the duckbills hooted mournfully across the misty plain.

In the foreground a herd of vast anatotitans was crossing the floodplain. It was a convoy of flesh. These immense creatures looked oddly unbalanced, with powerful hind legs — each of them taller than an adult human — but comparatively spindly forelegs, and they trailed long, fat conical tails. The air was filled with their rumbles: the churning of the herbivores’ huge stomachs and the deeper growl of their voices, reaching deep into the infrasonic, deeper than any human ear could have detected, as they called reassurance to each other.