Evolution

by Stephen Baxter

  Buried deep, Purga began a spectacular journey lasting millions of years. As continents collided, the land was uplifted, bearing all its entombed passengers like some vast ocean liner riding a swell. Heat and compressing forces fractured and twisted the rocks. But erosion continued, a relentless, destructive force balancing Earth’s creative uplift. Eventually this land became an angular landscape of plateaus, mountains, and desert basins.

  At last the erosion cut through the mass grave that had swallowed Purga’s bones. As the rock crumbled away bits of fossil bone emerged into the light, corpses bobbing to the surface, waking from a sixty-five-million-year slumber.

  Almost all of Purga’s bones were lost, flashing to dust in geological instants, all that patient chthonic preservation wasted. But in 2010 a remote descendant of Purga’s would pick out a blackened shard in a wall of gray rock, just beneath a strange layer of dark clay, and recognize it for what it was, a tiny tooth.

  But that moment lay far in the future.

  CHAPTER 4

  The Empty Forest

  Texas, North America. Circa 63 million years before present.

  I

  Through the endless forest, Plesi climbed.

  Squirrel-like, she scampered up a scaly trunk and along a fat branch. Though it was close to noon, the light was dappled, uncertain. The canopy was high above her, the floor lost in green layers far below. The forest was silent save for the rustling of leaves in the warm breeze and the call of the canopy birds, those colorful cousins of the vanished dinosaurs.

  It was a world forest. And it belonged to the mammals — including the primates, like Plesi.

  She glanced back along her branch. There were her two pups — both daughters — who she thought of as Strong and Weak. About half Plesi’s size, they clung to the angle of branch and tree. Even now, Strong was pushing Weak subtly aside. In some species the runtish Weak might have been allowed to die. But Plesi’s kind bore few young, and in an uncertain and dangerous world, all of them had to be cared for.

  But Plesi could not protect her pups forever. They were both weaned now. Though they had learned to seek out the fruit and insects that inhabited this, their birth tree, they must learn to be more adventurous — to move out into the forest, to seek out their own food.

  And to do that, they had to learn to jump.

  Hesitantly, scrambling at the scaly surface of the branch, Plesi tensed, and leapt.

  Plesi was a plesiadapid: she belonged, in fact, to a species that would one day be called carpolestid. Plesi closely resembled her remote grandmother, Purga. Like Purga, she looked something like a small squirrel, with a low-slung body like a large rat’s, and a bushy tail. Though a true primate, Plesi retained Purga’s claws rather than nails, her eyes did not face forward, and her brain was little developed. She still even had the big night-vision eyes that had served Purga so well in the time of the dinosaurs.

  The most significant development of primate bodies since Purga’s time was in the teeth; Plesi’s was a species adapted to husk fruit, as would be the possums of Australia, much later. It was a necessary response, if the primates were to find something to eat. Few animals of this time fed off leaves. In an equable world where tropical or paratropical forests spread far from the equator, there was little seasonal variation, and here in Texas the trees did not shed their leaves regularly. In fact, the trees loaded their leaves with toxins and chemicals to make them bitter or poisonous to curious mammalian tongues.

  But still, since Purga, there had been little innovation in the primate line — even across two million years. It was the same for many other lineages. Long after the great impact, it was as if the emptied world had been shocked into stasis.

  Plesi landed on her target branch without difficulty.

  Her two pups were still huddled hesitantly against the tree trunk, and they made the mewling calls of babies. But, though the calls tugged at her, Plesi only raised her head and twitched her snout. She tried to encourage the pups to follow her by nibbling the fruit that clustered on this new tree.

  At last the pups reacted. To Plesi’s surprise it was the little one, Weak, who came forward first. She scampered to the end of the branch — nervous, hesitant, but showing good balance. She raised her tail and tensed her muscles — she backed off nervously, preened the fur of her face — and then, at last, she jumped.

  She overjudged slightly. She came tumbling out of the air and collided with her mother, making Plesi hiss in protest. But her agile hands and feet soon gripped the lumpy bark, and she was safe. Trembling, Weak scampered to her mother and buried her face in her belly, seeking a nipple that was now dry. Plesi let her suckle, rewarding her with comfort.

  But now there was a blur of movement from the other tree. Strong, left behind, suddenly lunged forward, her immature feet slipping on the bark. And — without looking carefully, without trying to use her innate skills to estimate the distance — she leapt into the air.

  Fear prickled inside Plesi.

  Strong made the branch, but she landed too hard. Immediately she slid backwards. For a heartbeat she hung there, her small hands scrabbling uselessly at the bark, her hind legs waving. And then she fell.

  Plesi saw her tumble in the air, wriggling, her white underbelly exposed, her hands and feet clutching at nothing. Even now Strong made the peeping cry of a lost infant. Then she fell into the leaves, and in a moment she was gone, taken by the green below, which swallowed all the forest’s dead.

  Plesi clung to her branch, shuddering. It had happened so quickly. One young lost, one runtish weakling left. It was not to be borne. She hissed her defiance into the menacing green.

  And, leaving Weak clinging piteously to the trunk of the tree, Plesi began to descend, down toward the green, down to the ground.

  At last she reached the lower story of branches, and looked down into an oasis of light.

  This was one of the endless forest’s few clearings. Within the last few months, an ancient canopy tree had fallen, eaten from within, wrecked by a random lightning strike. When it had crashed down it had cut a swath through the dense foliage. This clearing would not last long. But for now the plants of the undergrowth, like those hardy survivors, the ground ferns, were taking the opportunity to germinate, and the forest floor here was unusually lush and green. And already saplings were sprouting, beginning a ruthless vegetable race to steal the light and plug that hole in the canopy.

  The forest was an oddly static place. The great canopy trees competed with each other to trap as much sunlight as they could. In the gloom of the lower levels, the light was too weak to support growth, and the floor was customarily littered by dead vegetable matter and the bones of any animals or birds unlucky enough to fall. But under the silent ground, seeds and spores abided — waiting centuries, even millennia if necessary, until the day came when chance opened up a gap in the canopy, and the race to live could begin.

  Plesi slithered down a buttressing root and reached the ground. Under the broad fronds of a ground fern she scuttled uneasily through a patch of direct sunlight. The solid ground, with no give or sway, felt very strange to her, as peculiar as the shuddering of an earthquake would have felt to a human.

  There were other animals here in the clearing, drawn by the prospect of novel pickings. There were frogs, salamanders, and even a few birds, flapping across the air in bright bursts of color, seeking insects and seeds.

  And there were mammals.

  There were creatures like raccoons but more closely related to the hoofed animals of the future, and scurrying insectivores whose descendants would include the shrews and the hedgehogs. Here was a taeniodont, like a small, fat wombat. It grubbed in the soil, expert at digging out roots and tubers. None of the grubbing creatures in this clearing would have been familiar to a watching human. They were furtive, odd, ungainly, almost reptilian in their behavior, forever looking over their shoulders, like petty thieves expecting the return of the householder.

  These mammals were holdove
rs from the Cretaceous. Then, it had been as if the whole Earth had been a vast city, shaped for the needs of its owners, the dinosaurs. But now the dominant inhabitants were gone, the great buildings erased, and the only creatures left alive were the urban species who had lived in the drains and sewers, subsisting on garbage.

  But the recovering Earth had become a very different place from the dreamy Cretaceous. The Earth’s new forests were much more dense now. There were no great herbivores: The sauropods had gone, and the elephants lay far in the future. There were no animals big enough to topple these trees, to smash clearings and corridors and make parklike savannah. In response the vegetation had gone crazy, filling the world with greenery of a density and profusion not seen since the first animals had walked onto the land.

  But it was an oddly bare stage. In these thick jungles there were no more predatory dinosaurs — but neither were there yet jaguars, leopards, tigers. Practically all of the forest’s inhabitants were small, tree-dwelling mammals like Plesi. For an extraordinary span of time — for millions of years — the animals would cling to their Cretaceous habits, and no mammal species would grow to even moderately large sizes. They still contented themselves with the darkness and the corners of the empty world, nibbling on insects, eschewing any evolutionary innovations more spectacular than a new set of teeth.

  Like long-term prisoners, the survivors of the impact were institutionalized. The dinosaurs were long gone — but for the mammals, habits ingrained over a much longer span, a full hundred and fifty million years of incarceration, were not so easy to give up.

  But things were changing.

  At last Plesi heard the quiet mewling of her young.

  At the edge of the clearing Strong was huddled, pathetically, in a kind of nest of browned fronds. After she had fallen out of the tree and tumbled into the clearing, at least she had had the sense to seek cover. But she was far from safe: a large, scarlet-bellied predatory frog was watching her, an absent curiosity in its blank eyes. When she saw Plesi, Strong dashed forward and fell on her mother. She tried to find Plesi’s nipples, just as her sister had, but Plesi snapped at her, denying her comfort.

  Plesi was deeply disturbed. A carpolestid who was strong in the nest but who had no instinct for the trees — who lacked even the sense to keep silent when exposed — had poor survival prospects. Suddenly Strong didn’t look so strong after all. Plesi felt an odd impulse to find a mate, to breed again. For now, though, she merely nipped at Strong’s flank with her sharp incisor teeth, and led the way back toward the tree from which she had descended.

  But she had gone no more than a few body lengths when she froze.

  The predator’s blank eyes fixed Plesi with lethal calculation.

  The predator was an oxyclaenus.

  He was a sleek, four-footed, dark-furred animal: long-bodied, stout-legged, he looked like an outsized weasel, though his face and muzzle were more reminiscent of a bear’s. But he was related to neither weasel nor bear. In fact he was an ungulate, an early member of that great family that would one day include the hoofed mammals like pigs, elephants, horses, camels, even the whales and dolphins.

  This oxy might have seemed clumsy, slow, even unfinished to an eye used to cheetah or wolf. But his kind had learned to stalk prey through the sparse undergrowth of the endless forest. He could even climb, pursuing his quarry into the lower branches of the trees. In this archaic time, this oxy had little competition.

  And, as he looked on Plesi’s timorous, flattened form, two cold questions dominated the oxy’s mind: How will I trap you? And, How good will you be to eat?

  Plesi lay flat against the floor, quivering, her whiskers twitching, her small, sharp teeth bared. But she was equipped with instincts honed over a million centuries at the feet of the dinosaurs. And in the cold calculus of her mind, a reassessment of risk was beginning. In this open place she could not hide. She could not reach a tree to clamber out of the oxy’s grasp. Surely if she tried to outrun him he would trap her easily with one of those cruel claws.

  Only one option was left.

  She arched her back, opened her mouth and hissed, so violently that she spattered the oxy with her spittle.

  The oxy flinched at the unexpected aggression of this tiny creature. But she is no threat. The oxy, angered, quickly recovered his composure, and prepared to call Plesi’s bluff.

  But Plesi had vanished into the undergrowth. She had never meant to attack the oxy, only to gain a precious second of time. And she had left Strong behind.

  The young carpolestid, transfixed by the carnivore’s stare, flattened herself against the ground. The oxy crushed Strong with his paw, snapping the little primate’s spine. Strong, flooded with pain, turned on her attacker, seeking to gouge his flesh with her teeth. In her final moments Strong discovered something like courage. But it did her no good.

  The oxy played with the crippled animal for a while. Then he began to feed.

  As the world recovered, so its changing conditions shaped its living inhabitants.

  The mammals were beginning to experiment with new roles. The ancestors of the true carnivores, which would eventually include the dogs and cats, were still small, ferretlike animals, busy, opportunistic general feeders. But the oxyclaenus had begun to develop the specializations of mammalian predators to follow: vertical legs for sustained speed, strong permanent teeth anchored by double roots and with interlocking cusps designed to shred meat.

  It was all part of an ancient pattern. All living things worked to stay alive. They took in nourishment, repaired themselves, grew, avoided predators.

  No organism lived forever. The only way to counter the dreadful annihilation of death was reproduction. Through reproduction, genetic information about oneself was passed on to one’s offspring.

  But no offspring was identical to its parents. At any moment each species contained the potential for much variation. But all organisms had to exist within a frame of habitability set by their environment — an environment, of weather, land, and living things, which they shaped in turn. As survival was sought with ruthless ferocity, the frame of the environment was filled up; every viable variation of a species that could find room to survive was expressed.

  But room was at a premium. And competition for that room was relentless and unending. Many more offspring were born than could possibly survive. The struggle to exist was relentless. The losers were culled by starvation, predation, disease. Those slightly better adapted to their corner of the environment inevitably had a slightly better chance of winning the battle for survival than others — and therefore of passing on genetic information about themselves to subsequent generations.

  But the environment could change, as climates adjusted, or as continents collided and species, mixed by migrations over land bridges, found themselves with novel neighbors. As the environment, of climate and of living things, changed, so the requirements of adaptation changed. But the principle of selection continued to operate.

  Thus, generation by generation, the populations of organisms tracked the changes in the world. All the variations of a species that worked in the new frame were selected for, and those that were no longer viable disappeared, sinking into the fossil record, or into oblivion altogether. Such turnovers were unending, a perpetual churn. As long as the “required” variation lay within the available genetic spectrum, the changes in the population could be rapid — as rapid as human breeders of domesticated animals and plants would find as they strove for their own ideas of perfection in the creatures in their power. But when the available variation ran out, the changes would stall, until a new mutation came along, a chance event caused, perhaps, by radiation effects, that opened up new possibilities for variation.

  This was evolution. That was all there was to it: It was a simple principle, based on simple, obvious laws. But it would shape every species that ever inhabited the Earth, from the birth of life to the last extinction of all, which would take place under a glowering sun, far in the future.<
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  And it was working now.

  It was hard.

  It was life.

  Plesi had made an unspoken bargain with the oxy: Take my child. Spare me. Even as she clambered back through layers of green and into the safety of the trees, seeking her surviving daughter, that dreadful stratagem still echoed in her mind.

  That, and a feeling that came from deep within her cells, a thought she might have expressed as: I always knew it was too good to be true. The teeth and claws weren’t gone. They were just hiding. I always knew they’d come back.

  Her instinct was right. Two million years after the uneasy truce imposed by the dinosaurs’ death, the mammals had started to prey on each other.

  That night Weak, bewildered, terrified herself, watched her mother twitch and growl in her sleep.

  CHAPTER 5

  The Time of Long Shadows

  Ellesmere Island, North America. Circa 51 million years before present.

  I

  There was no true morning during these long days of Arctic summer, no authentic night. But as the clouds cleared from the face of the climbing sun, and light and warmth slanted through the trees’ huge leaves, a mist rose from the swampy forest floor, and Noth’s sensitive nostrils filled with the pleasing scent of ripe fruit, rotting vegetation, and the damp fur of his family.

  It felt like a morning, like a beginning. A pleasing energy spread through Noth’s young body.

  His powerful hind legs folded under him, his fat tail upright, he squirmed along the branch to get closer to his family — his father, his mother, his new twin sisters. Together, the family groomed pleasurably. The nimble fingers of their small black hands combed through fur to pick out bits of bark and fragments of dried baby shit, even a few parasitic insects that made a tasty, blood-filled treat. There was some loose fur, but the adult adapids had already lost most of last year’s winter coat.