And at last the sphere had converged on this system, this world- amid a handful of others- as the putative origin. It had found a world of organic chemistry and creatures interesting in their way. But it was a dying world, overheated by its sun, the life-forms restricted to the fringes of a desert continent. There was no sign of organized intelligence.

And yet, here and there, the ancient rocks of the supercontinent had been marked deliberately, it seemed to the sphere, with cuts and gouges and great pits. Once there had been mind here, perhaps. But if so it was vanished from these wretched, crawling creatures.

The sphere represented a new order of life. And yet it was like a child, wistfully seeking its lost father. The last traces of the Martian robots' original blueprint, assembled by long-dead NASA engineers in computer laboratories in California and New England and much modified since, had been lost. It was somehow appropriate that this greatest, and strangest, of all of mankind's legacies should have been created entirely accidentally- and that those created should have been abandoned to their fate.

There was nothing more to be learned here. With an equivalent of a sigh, the sphere leapt to the stars. The small world dwindled behind it.

Ultimate huddled in the dirt until the scavenging siblings had finished feeding. Then she stumbled away, clutching her baby, not even noticing that the sphere had vanished.

III.

Ultimate kept heading west, away from the borametz quarry.

At night she wedged herself with her infant into crevices between rocks, trying to emulate the comforting enclosure of the Tree's cocoon. She ate whatever she could find- half-desiccated toads and frogs buried in the mud, lizards, scorpions, the flesh and roots of cacti. She fed the child a chewed-up pulp of meat and vegetable matter. But the child spat out the coarse stuff. She was still missing her belly-root, and she mewled and complained.

Ultimate walked, and walked and walked.

She had no strategy in mind save to keep moving, to keep her infant out of the chemical clutches of the Tree, and to wait to see what turned up. If her thinking had been sophisticated enough she might have hoped to find more people, somewhere she could stay, maybe even a community that lived independently of the Trees.

It would have been a futile hope, for there were no more such communities anywhere on Earth. She didn't know it, but she had nowhere to go.

The land began to rise slowly. Ultimate found herself walking on coarse sand and gravel fans.

After half a day of this she came to a place of low, smooth-shouldered hills. She could see how these eroded stumps went marching off to the horizon, to north and south, for kilometer after kilometer, all the way to the dust-laden horizon and beyond. She was walking through the remnant of a once-great mountain chain, thrown up by the ancient stitching together of the continents. But the dust-laden winds of New Pangaea had long since worn down the mountains to these meaningless stumps.

When she looked back she could see her own footprints, accompanied by the scraping of knuckles, marked by the messy places where she had stopped to feed or make waste or sleep. They were the only trail through these silent hills.

It took her two days to cross the mountains.

After that the land began to descend again.

On the plain, a little more vegetation grew. There were spiky trees with gnarled branches and clumps of needlelike leaves, like bristlecone pines. Around their roots sheltered a few leaping mice- hardy rodent survivors, ferocious conservers of water- and many, many lizards and insects. She chased tiny things like geckos and iguanas, and munched their flesh. But on this looser ground Ultimate had to be cautious, watching for rat-mouths embedded in the ground, and for the quivering, invisible mass of an ambush hunter.

As the land descended further, the view to the west opened up. She saw a great plain. Beyond a kind of coastal fringe, the land was white, white as bone, a sheet that ran all the way to a knife-sharp, geometrically flat horizon. A thin wind moaned in her face. On its breath, she could already taste the salt. Nothing moved, as far as she could see.

She had come to a fragment of the dying inland ocean. There was still water out there- it took a long, long time to dry out a sea- but it was a narrowing strip of water so saline it was all but lifeless, and it was framed by this great white rim of exposed salt flats, a sheet that extended to her horizon.

Tucking her baby's face into the fur on her breast, Ultimate continued her dogged descent.

She reached the place where the salt began. Great parallel bands showed where water had once lapped. She scooped up a bit of the salty dirt and licked it. She spat out the bitter stuff immediately. There was vegetation here, tolerant of the salty soil. There were small, spiky yellow shrubs that looked like the desert holly and honeysweet and spurge that had once clung to life in the Californian deserts of North America. Experimentally she broke off a bit of the holly's foliage and tried to chew it, but it was too dry. Frustrated, she hurled the bit of twig away over the salt.

And then she saw the footprints.

Curious, she fit her own feet into the shallow indentations in the ground. Here had been toes, here a scuffing that might have been caused by a resting knuckle. The prints could not have been made recently. The mud was baked hard as rock, and her own weight left no mark.

The prints set out, straight as an arrow, across the salt pan, marching on toward the empty horizon. She followed for a step or two. But the salt was hard and harsh and very hot, and when it got into minor cuts and scrapes on her feet and hands it stung badly.

The footprints did not turn back. Whoever had made them had not returned. Perhaps the walker had been intent on reaching the ocean proper, on walking all the way across North America: After all, there were no barriers now.

She knew she could not follow, not into the belly of this dead sea.

And it would have made no difference even if she had. This was New Pangaea. Wherever she went she would have found the same crimson ground, the same searing heat.

She stayed on the desolate, silent beach for the rest of the day. As it descended the overheating sun grew huge, its circular shape quivering. Its harsh light turned the salt plain a washed-out pink.

This had been the last significant journey ever undertaken by any of her ancient, wandering lineage. But the journey was over. This parched, dead beach had been the farthest point of all. The children of humanity had done with exploring.

As the light failed she turned away and began to walk up the sloping ground. She did not look back.

In the years after Ultimate's death Earth would spin on, ever more slowly, its waltz with its receding Moon gradually running down.

And the sun blazed ever brighter, following its own hydrogen logic.

The sun was a fusion furnace. But the sun's core was becoming clogged with helium ash, and the surrounding layers were falling inwards: The sun was shrinking. Because of this collapse the sun was getting hotter. Not by much- only by around 1 percent every hundred million years- but it was relentless.

For most of Earth's history, life had managed to shield itself from this steady heating-up. The living planet used its "bloodstream"- the rivers and oceans and atmosphere and the cycling rock, and the interactions of trillions of organisms- to remove waste and restore nutrients to where they were needed. Temperature was controlled by carbon dioxide- a vital greenhouse gas, and the raw material for plants' photosynthesis. There was a feedback loop. The hotter it got, the more carbon dioxide was absorbed by the weathering rocks- so the less greenhouse effect there was- and so the temperature was adjusted back down. It was a thermostat that had kept the Earth's temperature stable for eons.

But as the sun got hotter, so more carbon dioxide got trapped in the rocks, and the less there was available for the plants.

Eventually, fifty million years after Ultimate's time, photosynthesis itself began to fail. The plants shriveled: grasses, flowers, trees, ferns, all gone. And the creatures that lived off them died too. Great kingdoms of life imploded. There was a last rodent, and then a last mammal, a last reptile. And after the higher plants had disappeared, so did the fungi and slime mold and ciliates and algae. It was as if evolution had, in these final times, reversed itself, and life's hard-won complexity was shed.

At last, under a blazing sun, only heat-loving bacteria could survive. Many of them had descended with little modification from the earliest life-forms of all, the simple methane eaters who had lived before poisonous oxygen was spread into the atmosphere. For them it was like the good old days before photosynthesis: the arid plains of the last supercontinent were briefly streaked with gaudy, defiant colors, purples and crimsons draped like flags over the eroded rocks.

But the heat climbed, relentlessly. The water evaporated, until whole oceans were suspended in the atmosphere. At last some of the great clouds reached the stratosphere, the atmosphere's upper layer. Here, assailed by the sun's ultraviolet rays, the water molecules broke up into hydrogen and oxygen. The hydrogen was lost to space- and with it the water that might have reformed. It was as if a valve had been opened. Earth's water leaked rapidly out into space.

When the water was gone, it got so hot that the carbon dioxide was baked out of the rocks. Under air as dense as an ocean, the dried seabeds grew hot enough to melt lead. Even the thermophiles wilted. It was the last extinction event of all.

But on rocky ground as hot as the floor of an oven, the bacteria had left behind desiccated spores. In these toughened shells, virtually indestructible, the bacteria, dormant, rode out the years.

There were still convulsions as asteroids and comets sporadically fell onto the parched land, more unremarked Chicxulubs. Now there was nothing left to kill, of course. But as the ground flexed and rebounded, huge quantities of rock were hurled into space.

Some of this material, taken from the edge of each impact zone, was not shocked, and therefore it reached space unsterilized. That was how the bacterial spores left Earth.

They drifted away from Earth and, propelled by the gentle, persistent pressure of sunlight, they created a vast diffuse cloud around the sun. Encysted in their spores, bacteria were all but immortal. And they were hardy interplanetary travelers. The bacteria had coated their DNA strands with small proteins that stiffened the helical shapes and fended off chemical attack. When a spore germinated it could mobilize specialized enzymes to repair any DNA damage. Even some radiation damage could be fixed.

The sun continued its endless circling of the Galaxy's heart, planets and comets and spore cloud and all.

At last the sun drifted into a dense molecular cloud. It was a place where stars were born. The sky was crowded here; dazzling young stars jostled in a great swarm. The fiercely hot sun with its ruined planets was like a bitter old woman intruding in a nursery.

But, just occasionally, one of the sun's spaceborne spores would encounter a grain of interstellar dust, rich with organic molecules and water ice.

Battered by the radiation of nearby supernovae, a fragment of the cloud collapsed. A new sun was born, a new system of planets, gas-stuffed giants and hard, rocky worlds. Comets fell to the surface of the new rocky planets, just as once Earth had been impact-nourished.

And in some of those comets were Earthborn bacteria. Only a few. But it only took a few.

Still the sun aged. It bloated to monstrous proportions, glowing red. Earth skimmed the diffuse edge of the swollen sun, like a fly circling an elephant. The dying giant star burned whatever it could burn. The final paroxysms lit up the great shell of gas and dust that lingered around the sun. The solar system became a planetary nebula, a sphere shimmering with fabulous colors, visible across light-years.

These glorious spasms marked the final death of Earth. But on a new planet of a new star, the nebula was just a light show in the sky. What mattered was the here and now, the oceans and the lands where new ecosystems assembled, where creatures' changing forms tracked changes in their environment, where variation and selection blindly worked, shaping and complexifying.

Life always had been chancy. And now life had found ways of surviving the ultimate extinction event. In new oceans and on strange lands, evolution had begun again.

But it had nothing to do with mankind.

Exhausted, dust-laden, her body covered by a hundred minor scrapes, bruises and prickles, her baby cradled in her arms, Ultimate limped to the center of the ancient quarry.

The land seemed beaten flat, with the sun poised above, a great glowing fist. And at first glance there was no sign that anything still lived on this desert world, none at all.

She approached the Tree itself. She saw the big pendulous folded-over shapes of cocooned people, inert and black. The Tree stood there, silent and still, neither reproving nor forgiving her small betrayal.

She knew what she had to do. She found a folded-up ball of leaves. Carefully she prized the leaves open, shaping them into a makeshift cradle. Then she placed her baby carefully inside.

The baby gurgled and wriggled. She was comfortable, here in the leaves; she was happy to be back with the Tree. But already, Ultimate saw, the belly-rope had snaked into its orifice in the child's stomach. And white tendrils were pushing out of pores in the cradling leaves, reaching out for the baby's mouth and nose, ears and eyes.

There would be no pain. Ultimate had been granted that much knowledge, and comfort. Ultimate stroked the child's furry cheek one last time. Then, without regret, she folded up the leaves and sealed them up.

She clambered off the ground, found her own favorite cocoon, and snuggled inside, neatly closing up the big leathery leaves around her. Here she would stay until a better time: a day miraculously cooler and moister than the rest, a time when it might be possible for the Tree to release Ultimate from this protective embrace, to send her out into the world once more- even to seed her belly with another generation of people.

But there would never be another impregnation, never another birth, never another doomed child.

One by one the cocoons would shrivel as their inhabitants, sealed in green, were absorbed back into the bulk of the borametz- and in the end the borametz itself, of course, would succumb, thousands of years old, tough and defiant to the last. The shining molecular chain that had stretched from Purga through generations of creatures that had climbed and leapt, and learned to walk, trod the dirt of another world, and grown small again, and mindless, and returned to the trees- at last that great chain was broken, as the last of Purga's granddaughters faced an emergency she could not withstand.

Ultimate was the last mother of all. She couldn't even save her own child. But she was at peace.

She stroked the belly-root and helped it worm its way into her gut. The Tree's anesthetic and healing chemicals soothed her aching body, closed her small wounds. And as psychotropic vegetable medications washed away the sharp, jagged memory of her lost baby, she was filled with a green bliss that felt as if it would last forever.

It wasn't such a bad way for the long story to end.

Epilogue.

There had been a sighting of another band of feral kids, this time on Bartolome Island. So Joan and Lucy had loaded up the nets and tasers and hypo rifles, and here they were limping over the Pacific in their sun-powered launch.

The flat equatorial sunlight reflected off the water onto Joan's pocked skin. She was fifty-two now but looked a good deal older, such was the damage that had been done to her skin, not to mention her hair, by the environment she had endured since Rabaul. But Lucy had met very few truly old old people in the course of her short life, and she had few points of comparison; to her, Joan was just Joan, her mother, her closest companion. people in the course of her short life, and she had few points of comparison; to her, Joan was just Joan, her mother, her closest companion.

The day was bright, the few clouds high and streaky. The sun beat hard on the big solar cell sail spread over Lucy's head. Still, they had packed their heavy-duty ponchos, and every few minutes the women glanced at the sky, fearful of rain that might wash down more of the high dust onto them, the toxic, sometimes radioactive grit that had once been fields and cities and people that was now wrapped around the planet like a thin gray blanket.

And, as always, Joan Useb talked, and talked.

"I always had a soft spot for the British, you know, God rest them. In their heyday they didn't always behave well, of course. But the human story of the Galapagos was otherwise pretty unhappy: mad Norwegian farmers, Ecuadorian prison camps, everybody eating the wildlife as fast as they could. Even the Americans used the islands as bombing ranges. But all the Brits did to the Galapagos was send over Darwin for five weeks, and all they took away was the theory of evolution."

Lucy let Joan's chatter wash through her head, these random echoes from a world she had never known.

Frigate birds wheeled overhead, pursuing the launch as they had pursued the fishing vessels and tourist boats that had once plied these waters. They were great gaunt black-feathered birds that always reminded Lucy of nothing so much as the pterosaurs of her mother's books and fading printouts. In the water she thought she saw a sea lion, perhaps attracted by the buzz of the launch's electric motor. But these cute mammals were rare now, poisoned by the toxic garbage that still circulated through the sluggish oceans.

The Galapagos were a bunch of volcanic cones that had been thrust a few million years ago above the surface of the Pacific, here on the equator, a thousand kilometers west of South America. Some of them were little more than a jumble of volcanic boulders, piled up one on top of the other. But others had undergone their own geological evolution. On Bartolome, for instance, the softer outer shells of the older cones had worn away, and the stubborn plugs within had turned crimson red as the iron they contained had rusted. But newer lava had flooded around these older formations, fields of lava bombs, tubes, cones, like a gray-black lunar sea washing around the feet of the stubborn old monuments.

But there was life, here on these new, half-formed islands: Of course there was, a scrap of life that had once been the most famous in the world.

She saw a bird standing gaunt on a small promontory. It was a flightless cormorant: scruffy and black, a thing of stubby useless wings and oily feathers. Standing alone on its bit of volcanic rock, it peered out to sea- patient and still, like so much of the wildlife in this predator-free place, as if waiting for something.

"Ugly, ugly," Joan murmured. "These islands, the birds and animals. Wonderful, of course, but ugly. Islands have always been great laboratories of evolution. The isolation. The emptiness, populated by a handful of species who raft or fly in, and then radiate into all the empty niches. Like that cormorant. That's how far you get in three million years, apparently: halfway between a pelican and a penguin. Give it another few megayears, though, and those useless wings will have become genuine flippers, the feathers properly waterproof, and I wonder what they will become then then? No wonder Darwin's eyes were opened here. You can see see selection working." selection working."

"Mother-"

"You understand all that, of course." She grimaced, her masklike face twisting. "You know, the fate of the old is to turn into one's own parents; this is just the way my mother used to speak to me. No conversation that didn't turn into a lecture."

They pulled into the shore by a shallow beach. The launch grounded itself, and Lucy hopped out, her sandaled feet crunching on the coarse black sand. She turned back to help her mother, and then the two of them made the launch fast and briskly hauled out their gear.

As Joan began to set up the traps, Lucy took a couple of the hypo rifles and went patrolling along the beach.

The beach itself was an eerie place. The black lava sand was littered by equally black rocks. Even the sea was made to look black, like a sea of oil, by the darkness of its bed. In the distance she could make out mangrove, trees capable of exploiting the salty water, a splash of green against the mineral black and red.

And marine iguanas were lined up here like fat meter-long sculptures, their expressionless faces turned to the sun. They were themselves black, so dark and still it took a second glance to recognize them as living things and not an eerie formation of rope lava. Stranded here on Darwin's laboratory after rafting across with tortoises and turtles, the iguanas' ancestors had been dry-land creatures, tree climbers. They were gradually adapting to living off algae they strained out of seawater. But they would spit out the excess water- the air was filled with their hawking; the little jets spouting from their mouths sparkled in the sunlight- and they had to rely on the heat of the sun to bake the thin repast in their stomachs.

Lucy kept her rifle ready. If feral kids were around, it paid to be wary.

During the scramble for places on the last few boats back to the mainland, kids had been dumped here by desperate parents. The weak ones had quickly died, leaving their bones to litter the beaches and rocky outcrops, like the bones of sea lions and iguanas and albatrosses. But some of the kids had survived. In fact the word "kids" was a misnomer, for they had already been here long enough to spawn a second generation, children who had grown up knowing nothing but these barren bits of rock and the endless oceans, kids even more wordless and without culture than their parents. Feral kids, without tools, with only a rudimentary language- and yet human, capable of being cleaned up and educated.

And also capable of taking a bite out of your leg.

Joan's traps were simple: nothing much more than concealed nets and snares, baited with rich-smelling spicy food. When she had set them up, she and Lucy settled down out of sight in the shade of an outcrop of tuff- crumbling, easily eroded lava- and prepared to wait for the feral children.

Since Rabaul, life had been hard for Joan and her daughter- but then it had been hard for everyone on the planet. Even though her grand empathetic project had been crushed, Joan hadn't stopped working. With wide-eyed little Lucy in tow, she had retreated here, to Galapagos.

Paradoxically these fragile islands had been relatively well preserved through the greater global catastrophe. Once seventeen thousand people had lived here, mostly emigrants from mainland Ecuador. Before Rabaul there had been a constant friction between the needs of this growing, resentful population and the unique wildlife, nominally preserved by Ecuador's national park legislation. But the islands had always been fed by the mainland. When everything had spun apart after Rabaul, when the ships had stopped coming, most of that population had fled back home.

So the islands, largely free of people- and their companions, the rats and the goats, and their waste products, sewage and oil- had, in their modest way, begun to prosper again.

Joan and Lucy- and a handful of others, including Alyce Sigurdardottir until her death- had settled in the ruins of what had once been the Charles Darwin Research Station on Santa Cruz, and, with the locals who remained, had devoted themselves to helping the creatures that had so intrigued Darwin himself through the unfolding extinction.

For a time there had been communications. But then the high-altitude electronics-busting bombs, fired off at the height of the messy multipolar wars, had wrecked the ionosphere. And when the last satellites were shot out of the sky, that had been the end of TV, even speech radio. Joan had long maintained a regime of listening, as long as their sets and power lasted. But it had been years since they had heard anything.

No radio, then. No contrails in the sky, no ships on the horizon. There was no outside world, for all intents and purposes.

They were getting used to the isolation. You always had to remember that when something wore out it was gone forever. But the supplies left behind by those vanished thousands- tools and clothing and batteries and torches and paper and even canned foods- would sustain this little community of fewer than a hundred for their lifetimes and beyond.

The world might be ending- but not here, not yet. yet.