Evolution

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.

The anatotitans converged on a grove of cycads. The cycads’ mature leaves were thick and spiny, but their fresh growth, protected by a crown of older leaves, was green and luscious. So the anatotitans rose up on their heavy hind legs and cropped the new growth. As their great feet fell back on the undergrowth of ferns, clouds of insects rose up. The phalanx of titans would leave the cycads smashed and broken. Though the anatotitans would scatter seeds for future growth far from here, the vegetation would take a long time to recover from the devastation they caused.

There was noise everywhere: the mighty foghorn honks of the duckbills, the bellows of the armored dinosaurs, the screeching of birds, the leathery flapping of the huge flocks of pterosaurs. And, under it all, there was the ugly, unstructured roar of a female tyrannosaur, the area’s top predator: All of these animals were within her domain, and she was letting them, and any competitor tyrannosaur, know about it.

The scene might have reminded a human of Africa. But though there were great herbivores to fill the roles of antelopes, elephants, hippos, and wildebeests, and predators who hunted like lions, cheetahs, and hyenas, these animals were more closely related to birds than to any mammal. They preened, displayed, fought, and nested with oddly rapid motions fueled by the rich oxygen of the thick air. The smaller, more lithe dinosaurs that ran or stalked through the undergrowth would have seemed surreal: There was nothing like these bipedal runners in human times. And there was no sight in twenty-first century Africa like the two ankylosaurs who now began to mate, backing their rear ends together with the most exquisite care.

It was a landscape of giants, in which Purga was a lost, helpless figure, utterly irrelevant. But to the west, Purga made out a storey of denser forest, layer on layer of it rising up toward the distant volcanoes.

Purga had run the wrong way, coming to this place of the sea. She was a creature of the forest and the dirt; that was where she must go. But to get there she had to cross the open plain — and evade all those mountainous feet. With trepidation she slid down the sand bank.

But now she glimpsed stealthy movement through low ferns. She hurried beneath an immature araucaria and flattened herself against the ground.

A raptor. Standing as still as a rock, it was studying the jostling anatotitans. It was a deinonychus, something like a featherless, flightless bird. But it was as still as a crocodile. The raptor had only a faint scent — its skin was not as glandular as mammals’ — but there was a dry pungency in the air, a spiciness that filled Purga with a sense of peril.

It was very close to Purga. If it caught Purga the raptor would, of course, kill her in a second.

A bird was climbing into the tree above her. Its feathers were bright blue and it had claws on its wing bones and teeth in its beak. This creature was a relic of ancient times, of archaic linkages between birds, crocodiles, dinosaurs. The bird was climbing to feed its brood of fat, squawking chicks. Apparently it had not seen the raptor.

But for now the raptor was stalking larger prey.

The raptor watched the anatotitan herd with blank, hawklike eyes, its only calculation was which of the titanic herbivores might serve it as Prey. If necessary, it would harass the herd, seeking to make one of them peel away and thereby become vulnerable.

But that proved unnecessary.

One of the adult titans fell behind the rest. This female, walking tiredly, was more than seventy years old. Her growth had continued all her life, and now she was the largest in the herd — one of the largest of her kind anywhere, in fact. Now she dipped a heavy head into the scummy water of a shallow pond.

The raptor began to stalk steadily, silently, toward the old titan. Purga cowered in the shelter of her araucaria.

The raptor was three meters tall — compact, agile, with slim legs capable of high-speed running and a long stiff tail for balance. It had a huge claw on each hind limb; while the raptor walked, its toes lifted the claws up and clear of the ground.

The raptor wasn’t so smart. Its brain was small — no larger than a chicken’s or a guinea fowl’s. And it was a solitary hunter; it wasn’t smart enough to hunt in a pack. But it didn’t need to be.

The great anatotitan still had no idea of the danger it was in.

The raptor erupted from cover. It spun in the air, and its grime-crusted hind claws flashed cruelly. The strikes were made well.

Blood gushed. Bellowing, the anatotitan tried to back away from the water. But the titan’s black entrails slid out of immense, deep belly wounds, steaming. At last she caught her forefeet in the slippery mess. With a sound like thunder she slid forward on her chest. And then, with a spasm, the great hind legs collapsed, rolling the great bulk of her body onto its side.

One of the other anatotitans looked back and lowed mournfully, a deep noise that made the ground under Purga tremble. But the herd was already moving on.

The raptor, panting rapidly, waited for the titan to weaken.

The dinosaurs had first emerged more than a hundred and fifty million years ago, in a time of hot dry climates more welcoming to reptiles than mammals. In those days the continents were fused into the single vast Pangaean landmass, and the dinosaurs had been able to spread across the planet. Since then, continents had fissioned, danced, and whirled, and bands of climate had shifted across the planet. And the dinosaurs had evolved in response.

Dinosaurs were different.

They did not hunt like the mammalian killers of later times. Their cold blood meant they were poor at sustaining speed for long distances; they could never be endurance hunters, running down their prey like wolves. But they had versatile, high-pressure hearts. And the design of their bodies had much in common with birds’: This raptor’s neck bones and torso contained a duct system that drew the air through its lungs, and oxygen could be supplied to its tissues at a tremendous rate. It was capable of short sprints, and could pour a great deal of energy into its attacks.

Dinosaur hunts were events of stillness, of ambush and silence and motionlessness, broken by brief bursts of savage violence.

Mammals were not poorly evolved compared to the dinosaurs. The product of her own track of tens of millions of years of evolution, Purga was exquisitely adapted for the niche in which she made her living. But the brutal facts of energy economics kept mammals caged in the neglected corners of a dinosaur world. Overall, a dinosaur killer made better use of energy than mammals: This raptor could run like a gazelle but it rested like a lizard. It was that combination of energy efficiency and lethal effectiveness that had kept the dinosaurs supreme for so long.

The raptor was something like a huge, ferocious bird, perhaps. Or something like a souped-up crocodile. But it was not truly like those animals. It was like nothing seen on Earth in human times, something no human eye would ever witness.

It was a dinosaur.

This raptor’s preferred way of killing was to burst out of cover and slash at its prey, inflicting wounds that were savage but often nonlethal. The prey might flee, but it would be weakened by raking wounds to its legs and flanks — or hamstringing — blood loss and shock would result. The raptor had poor dental hygiene — its breath stank ferociously — and its bite passed on a mouthful of bacteria. The raptor would follow, perhaps attacking again, perhaps just following the scent of the stinking, infected wounds, until weakness disabled the prey.

Today this raptor had been lucky; it had disabled its victim with a single blow. All it had to do now was wait until the titan was too weak to do the raptor any harm. It could even take its food while its prey was still alive.

The raptor would not trouble with such small fry as Purga while such a giant meal awaited it. Moving cautiously, watchfully, Purga left the shelter of the fern, and scurried across the scrubby floodplain, through the devastated track left by the anatotitan herd, until she reached the security of the trees.

For the first time in four billion years, heat had touched the Devil’s Tail. Fragile ice sculptures older than Earth were quickly lost.

Gases boiled through fissures in the crust. Soon a shining cloud of dust and gas the size of the Moon had gathered around the comet. The wind from the sun, of light and sleeting particles, made the gas and dust stream behind the falling comet nucleus in tails millions of kilometers long. The twin tails were extremely tenuous, but they caught the light and began to shine.

For the first time, uncomprehending eyes on Earth made out the approaching comet.

Spitting, rotating, its dark nucleus founting gases with ever greater vigor, the Devil’s Tail swam on.

III

Another long, hot Cretaceous day wore away.

Purga slept through the day, her new family curled around her. She slept even when her pups suckled. The snug burrow floor was littered with the primates’ soft fur — and it smelled, indubitably, of Purga, of her new mate, and of the three pups who were half of herself.

Purga’s mate gave himself no name, and nor did Purga name him, any more than she named herself. But if she had — in recognition that he could never be the first in her life — she might have called him Second.

As Purga slept, she dreamed. Primates already had brains large and complex enough to require self-referential cleansing. So she dreamed of warmth and darkness, of flashing claws and teeth, and of her own mother, huge in her memory.

Purga, like all mammals, was hot-blooded.

All animal metabolisms were based on the slow cellular burning of food in oxygen. The first animals to colonize the land — gasping fish, driven from drying rivulets, using swim bladders as crude lungs — had had to rely on metabolic engines designed for swimming. In those first land-walkers the metabolic fires had glowed dimly. Still, their decisive move onto the land had been successful; and now and into the future every animal — mammals, dinosaurs, crocodiles, and birds, even snakes and whales — would use a variant of the same ancient tetrapod body plan of four legs, a backbone, ribs, fingers, and toes.

But some two hundred million years before Purga’s birth, certain animals had begun to develop a new kind of metabolism. They had been predators, driven by selection to burn food more briskly in order to improve their luck in the chase.

It had meant a complete redesign. These ambitious predators needed more food, a higher rate of digestion, a more efficient system of waste elimination. All this had raised their metabolic rate, even when resting, and they had had to increase the size of heat-producing organs like the heart, kidneys, liver, and brain. Even the working of their cells had speeded up. In the end a new and stable high body temperature had been set.

The new hot-blooded bodies had had an unplanned advantage. Cold-bloods relied on drawing heat from the environment. But the hot-bloods did not. They could operate at peak efficiency in the cool of night, when the cold-bloods had to rest, or in extreme heat, when cold-bloods would have to hide. They could even prey on cold-bloods — frogs, small reptiles, insects — at times like dawn and dusk, when those slow movers were vulnerable.

But they could not topple the dinosaurs from their thrones; the dinosaurs’ supreme energy efficiency saw to that.

Purga’s dreams were disturbed by the immense stomping of the dinosaurs as they went about their incomprehensible activities in the world of day above. The ground would shake as if in an earthquake, and bits of the burrow walls crumbled and fell around the dozing family. It was as if the world was full of walking skyscrapers.

But there was nothing to be done about any of that. To Purga the dinosaurs were a force of nature, as beyond her control as the weather. In this huge, dangerous world, the burrow was home. The thick earth protected the primates from the heat of the day, and sheltered the still-naked pups from the night’s chill: The earth itself was Purga’s shelter against dinosaur weather.

And yet, at the back of her small mind, there was a tiny chapel of memory, a reminder that this was not her first home, not her first family — a lingering warning that she could lose all this, too, in another instant of light and flashing claws and teeth.