Evolution

by Stephen Baxter

  There was a sudden flurry of activity, a great clash like a bone breaking. Two of the great goat-elephants, males, had begun a dispute. Their heads bobbed and swayed atop long giraffelike necks, and their horns, elaborately curling before their faces, clashed like baroque swords.

  Remembrance cowered deep into the shade of her acacia. As the great herbivores began to mill around her, disturbed by the battle, she wasn’t so safe. This tree, trunk and all, could be smashed up and devoured in a few heartbeats.

  And now the watchful predators took advantage of the confusion.

  A pack of them erupted from cover. Lean and vulpine, with long, powerful shanks and thickly padded feet, they were more rats. Working closely together, they moved wedgelike to separate one older goat-elephant from the rest of the herd. His huge horn-tusks chipped and scarred by a lifetime of battles, this big male bellowed his rage and fear and began to run. The rats settled into the pursuit, running closely together.

  These rat derivatives were like dogs, yet they were not dogs. Their characteristic rodents’ incisors had been subtly modified from teeth designed for processing seeds and insects into blades with stabbing points. Their rear molars were like shears, well equipped for shredding meat. And they moved more closely than any dog pack had ever run, with a liquid, slithering power. But, like a dog pack, their basic strategy was to chase the goat-elephant until he was exhausted.

  Soon the prey and his pursuers had passed out of sight.

  The goat-elephants settled down once more to their drinking and fighting — though some of them turned their great heads to the place where the old one had stood, remembering his absence.

  Remembrance took the opportunity to creep forward.

  The water was scum laden. But she scooped it up in her hands and let it trickle into her mouth, leaving her palms and fingers coated with fine green slime.

  From the water, two yellow eyes watched her with abstract instinct. It was a crocodile, of course. These ancient survivors had ridden out the human apocalypse as they had survived so many before: by living off the gruesome brown food chain of the dying lands, by burrowing into the welcoming mud in drought. And even now no animal, no pig or rabbit or primate, no fish or bird, reptile or amphibian — not even the rodents — had managed to dislodge the crocodiles from their watery kingdom.

  Remembrance shuddered, and backed off from the water’s edge.

  A new predator stalked over the bluff toward the lake. Again Remembrance scurried for cover, screened by the huge, impassive bodies of a herd of duck-billed goats.

  This predator was more rodent stock; in fact it came from a kind of mouse. But its behavior was not like any dog or cat’s. It came to the edge of the water, and lifted itself up on its massive hind legs. The herbivores at the water’s edge cowered away. But the mouse-raptor had no interest in the creatures milling before it. With lordly dismissal it dipped its ferocious muzzle to taste the water. Then it stalked back to dry land where it used its small, feeble-looking hands to pluck at the grass, as if testing it.

  It looked like one of the great carnivorous dinosaurs of the Cretaceous days. Its forearms were small, its tail was thickened for balance, and its hind legs were awesomely powerful machines of muscle and bone. Its incisors had developed into ferocious slashing weapons, to be deployed by thrusts of the heavy head. The mouse-raptor was a land shark, like a tyrannosaur, a body design rediscovered and made devastatingly effective. And yet this arrogant creature retained the small ears and brown fur of the diminutive rodents from which it had derived.

  The mouse-raptor seemed satisfied with the water and the grass. It squealed, spat, and drummed its tail on the ground. From the distance there was a series of answering calls, drums, and cries.

  More mouse-raptors approached the lake. They fanned out over a swath of grassland, sniffing the air. A few kits ran around the legs of the adults, wrestling and nipping at each other with the ancient playful curiosity of predators.

  When they had gathered, the adult mouse-raptors turned, opened their throats, and set up a kind of synchronized wailing. In response, a herd of another kind of animal came lumbering toward the water.

  These were big creatures, as big as the goat-elephants. Nervously they huddled together, querulously jostling. But even as they stumbled toward the water, under the apparent guidance of the mouse-raptors, they cropped hastily at the grass under their feet.

  Their bodies were coated with sparse fur. Their heads were crested, their skulls shaped to allow anchorage for the tremendous cheek muscles that worked their immense lower jaws: Their heads looked rather like those of robust pithecines, in fact. Their ears, plastered back over their massive skulls, were huge and veined, great radiator fins designed to extract waste heat from their huge bodies. Though their hind legs were massive, enabling them to support their weight, they had the peculiar wrong-way-bending look of the rabbit-gazelles: legs meant for fleeing.

  These animals were ugly, elephantine. But they had not descended from goat or pig. They had forward-looking eyes under heavy browridges, huge dark eyes that peered at the world, baffled and fearful. They walked on all fours, but they supported themselves on the folded knuckles of their hands, a posture that had once been called knuckle-walking.

  Like Remembrance, their ancestors had once been human.

  Remembrance waited until the big dull animals had settled to their drinking, jostling querulously, their ears spreading in the cooling air of the afternoon. Then she crept away.

  It had taken millions of years for the great rebound of life to be completed.

  Today, to the north of Remembrance’s tropical forest, a great band of temperate woodland and grassland marched around the Earth, stretching from Europe-Africa across Asia to North America. Here more rabbit types browsed the cool foliage, while things like hedgehogs and pigs worked the undergrowth. In the trees there were birds and squirrels — and many, many bats. This diverse group of mammals had continued to proliferate and diverge, and now there were some nocturnal flyers who had lost their eyes altogether, others who had learned to compete with the birds for the richer pickings of the day.

  Further north still, coniferous forests grew, evergreen trees whose spiky leaves were always ready to take advantage of the sun’s thin ration of light. Browsing animals lived on the young twigs and needles in the summer, and on bark, mosses, and lichen the rest of the year. Many of them were goats. Especially common were the hadrosaur-like duckbill forms. Their predators included the ubiquitous mice and rats — but there were also carnivorous squirrels and huge birds of prey that seemed to be trying to emulate the pterosaurs of the oxygen-rich Cretaceous skies.

  On the northern fringes of the continents a belt of tundra had formed. Here the descendants of pigs and goats cropped the thin foliage of summer, and huddled together to endure the winter. Like the vanished mammoths, some of these creatures had grown huge, the better to retain their warmth, until they were great round boulders of flesh. On the tundra the predatory rats had grown their incisors into huge stabbing instruments, the better to penetrate those thick layers of fur and fat. They looked something like the saber-toothed cats of earlier times. There were even populations of migrant bats who had learned to subsist on the vast swarms of insects that formed during the brief tundra spring.

  None of these new species, of course, would ever bear a human name.

  There was one key difference in this latest recovery of life, compared to its last great trauma after Chicxulub. The rodents had not evolved until some ten million years after the comet impact. This time, though, when the days of recovery came, the rodents were everywhere.

  Rodents were formidable competitors. They were born with incisors ready to gnaw. These great teeth were deep-rooted in powerful jaws: Once rats had been able to gnaw through concrete. Their teeth enabled them to eat food hard and tough enough to be inaccessible to other mammals. But the rodents’ ability to proliferate and adapt was more fundamental. Rodents lived fast and bred young. Even among
the giant species like the rat-cheetahs, females had short gestation times and produced large litters. Many of those kits would die, but every one of those dead babies was raw material for the relentless processes of adaptation and selection.

  Given empty spaces to fill, the rodents evolved quickly. In the grand recovery after the disappearance of man, the rodents had been the big winners. By now, on land at least, Earth could be described as a kingdom of rats.

  All this had left little room for the descendants of humans.

  Crowded out by increasingly ferocious and confident rodents, the posthumans had given up the strategy — superior intelligence — that had brought them such success, and disaster. They had retreated, seeking sheltering niches and passive strategies. Some had become small, timid, fast-breeding runners. They were like vermin. Some even burrowed into the ground. Remembrance’s folk had returned to the ancestral trees, but now the rats were invading even that ancient shelter.

  The elephantine humans had tried another approach: becoming so huge they were protected simply by their immense size. But this had not been entirely successful. You could tell that from the design of their gazellelike back legs. Elephants could not have run very quickly, but they had not needed to; in their day no predator existed that could have taken on a full-grown proboscidean. Facing the power of the rodent predator families, the elephantine posthumans had had to retain the power to flee.

  But even this had not been enough.

  The mouse-raptors were social creatures. Their sociability was deep rooted, reaching back to the colony structures of the marmots and prairie dogs, which had lived in hierarchical “towns” of millions of animals. They scouted, seeking prey or water. They kept sentry watch for each other. They hunted cooperatively. They communicated: The adults called to each other continually with cries, squeals, and the drummings of those powerful tails that sent long-range shudderings through the ground.

  For the posthumans, the sociability of these raptors made them simply too effective as predators. The numbers of the big herbivores had steadily dwindled.

  But that was bad for the raptors too. And so, in time, the elephantines and the mouse-raptors had developed a kind of symbiosis. The mouse-raptors learned to protect the herds of slow-witted elephantines. Their presence would deter other predators. By their behavior and signals they could warn the elephantines of other dangers, such as fires. They could even guide them to water and good grazing.

  All the raptors asked for in return was to take their share of meat.

  The elephantines passively accepted all this. They had no choice. And over enough time, selection had shaped the elephantines to fit the new conditions. If the raptors chased away the other predators for you, why be fast? And if they did your thinking for you, why be smart?

  As their bodies had bulked up, the people’s minds had shriveled, casting off the burden of thought. They were like domesticated chickens, whose brains had been sacrificed to make longer guts and a more effective digestive system. It wasn’t so bad when you got used to it. Under the mouse-raptors’ unthinking guidance their numbers had even increased. It wasn’t so bad, so long as you turned away when your mother or your sister or your child was taken.

  Not such a bad life, to be farmed by rodents.

  The light began to leak out of the sky. So Remembrance found another stand of acacias, and crawled gingerly into the branches of the tallest tree. It would have to do. At least she was off the ground.

  As the light died, so the stars appeared — but it was a crowded sky.

  The sun, in its endless swimming around the Galaxy, was now passing through a wisp of interstellar dust and gas, a wisp mighty enough to span light years. Human astronomers had seen this coming. It was the vanguard of a mighty bubble blown in the gas by an ancient supernova explosion, and at its heart was a region where stars were being built. And so the new sky was spectacular, full of bright, hot new stars.

  But there was nobody on Earth who might understand any of this. Remembrance spent a sleepless night listening to the squeals, thrums, and roars of predators, while unnamed constellations drifted over the sky.

  III

  The first few hundred asteroids the astronomers discovered had orbited in their orderly belt between Mars and Jupiter, comfortably far from Earth. These space rocks had been a curiosity, nothing but a theoretical challenge to students of the origins of the solar system.

  It had been quite a shock when Eros was discovered.

  Eros was found to sail within Mars’ orbit — in fact at its nearest to Earth, it came to within less than a quarter of the closest approach of Mars and Earth. Later, more asteroids were found that actually crossed the orbit of Earth, making them candidates for eventual collision with the planet.

  Eros, that first rogue, was never forgotten. As long as people cared about such things, the asteroid became a kind of mute hero among its kind, better known than any other.

  At the beginning of the twenty-first century Eros was the target of the first space probe to orbit an asteroid. The probe was called NEAR, for Near Earth Asteroid Rendezvous. At the end of the mission the probe was made to land gently on the asteroid’s ancient ground. Those first astronomers had given their asteroid the romantic name of the Greek god of love. There was much talk of how the probe NEAR had “kissed” the target rock, and the press had been predictably excited that the contact had occurred only a little before Valentine’s Day.

  But under the circumstances the asteroid’s name could not have been more inappropriate.

  It had long been believed that Eros, with its eccentric orbit taking it endlessly across the orbit of Mars, was in no danger of collision with the Earth. In fact, it seemed much more likely to collide with Mars itself.

  But Mars was gone.

  And, over long enough periods, as it responded to the subtle tweaks of the planets’ gravitational pulls, the spinning of the sun, and its own intricate, intrinsic dynamic instabilities, the orbit of the asteroid evolved. One million years after the demise of mankind, Eros had sailed close to Earth — very close, close enough to be visible to a naked eye, had anyone been looking.

  Some twenty-nine million years after that, it was coming closer still.

  Stuck in her acacia tree, Remembrance itched. She scrabbled at her fur, hunting for the ticks and bugs that loved to feast on your blood, or lay their irritating eggs under your skin. But there were places she couldn’t reach, like the small of her back, and naturally the bugs congregated there.

  It was a painful reminder of how alone she was. As language had declined, the habit of grooming had returned to serve its old function of social cement. (It had never really gone away anyhow.) But Remembrance had had no grooming since before her last sleep, when she had huddled with her mother in her nest.

  Hot, itchy, hungry, thirsty, lonely, Remembrance waited in her acacia stand until the sun had once more climbed high in the sky.

  Then, at last, she clambered down.

  The elephant people and their rodent keepers had gone. Across the empty, dust-strewn grassland, little stirred. The silence was as heavy as the heat. Through dusty haze, she could see a dark smudge to the east that might have been a herd of elephantine pigs or goats, or even hominids. To the west there was a little pocket of motion, a glimpse of brown fur. Perhaps it was a predatory rat with her kits.

  To the north, where the mountains loomed purple, she could see that splash of dull greenery. She still had no other impulse than to make straight for the forest’s alluring comfort.

  Naked, her hands empty, she set off across the plain, slumping every now and again to let her knuckles carry some of her weight. She was a tiny figure crossing a huge, bare landscape, accompanied by nothing but the shadow under her feet.

  She found no water, nothing to eat save handfuls of sparse grass. As she lumbered on, she was increasingly distracted by thirst. The silence settled still more heavily. Soon it was as if there were nothing in her life but this walk, as if her memories of a life of
green and family were as meaningless as her dreams of falling.

  She found herself walking down a shallow slope into a broad bowl of land kilometers across. Before this great depression she hesitated.

  A valley was incised across the heart of the bowl — a valley once cut by a river — but even from here she could see that the valley was dry. The vegetation was different from that in the plain beyond. There were no trees here, few shrubs, and only occasional splashes of grass green. Instead, there was a broad mass of rustling violet leaves.

  To distrust anything new was a good rule of thumb. But this great bowl lay right across her path, cutting her off from her forested slope, still far away. She could see there were no animals here, no herbivores, no prowling predators.

  So she set off, wary, watchful.

  The belt of violet purple turned out to be flowers growing in thick clumps, some tall enough to reach her waist, amid spindly, pale blades of grass. She walked on until she was surrounded by the clamoring purple. But there was still no water.

  Once there had been a city here. Even now, so long after the city’s fall, the soil was so polluted that only metal-tolerant plants could survive here — such as the violet-petaled copper flowers that waved over the soil.

  Eventually the purple flowers grew thinner. At the very heart of this strange place she came to the river’s shallow bank. The channel was dry, filled only with drifting dust: Ancient geological shifts had long since diverted away the water that had cut this channel. Remembrance clambered down the eroded banks and tried digging into the dusty substrate, but there was no moisture to be had here either.