Evolution (4 page)

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.

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.