So it was now. Joan Useb's intuition, long ago, had been right: One way or another, the future for mankind had been cooperation, with one another and the creatures around them. But she could never have foreseen this, the final expression of that cooperation.

The Tree, a remote descendant of the borametz of Remembrance's time, had taken the principle of cooperation and sharing to its extremes. Now the Tree could not survive without the termites and other insects that brought nutrients to its deep roots, and the furry, bright-eyed mammals who brought it water, food, and salt, and planted its seeds. Even its leaves, strictly speaking, belonged to another plant that lived on its surface and fed on its sap.

But likewise the symbiotes, including the posthumans, could not have survived without the succor of the Tree. Its tough leaves sheltered them from predators, from the harsh heat of the climate, even from the once-in-a-century rainstorms. Sap was delivered through the belly-roots, just as the Tree took back its nutrients by the same conduits: infants were not breast-fed but were swaddled by the Tree, nurtured by these vegetable umbilicals. The sap, drawing on the deepest groundwater, sustained them through the mightiest supercontinental droughts- and, laden with beneficent chemicals, the sap healed their injuries and illnesses.

The Tree was even involved in human reproduction.

There was still sex- but only homosexual sex, for there was only one gender now. Sex served only for social bonding, pleasure, comfort. People didn't need sex for breeding anymore, not even for the mixing of genetic material. The Tree did it all. It took body fluid from one "parent" in its sap and, circulating it through its mighty bulk, mixed it and delivered it to another.

People still gave birth, though. Ultimate herself had given birth to the infant that now lay in its leafy cradle. That heritage, the bond between mother and child, had proved too central to give up. But you no longer fed your child, by breast or otherwise. All you had to give your child was attention, and love. You no longer raised it. The Tree did all that, with the organic mechanisms in its leafy cocoons.

Of course there was still selection, of a sort. Only those individuals who worked well with the Tree and with each other were enfolded and allowed to contribute to the circulating stream of germ material. The ill, the weak, the deformed, were expelled with vegetable pitilessness.

Such a close convergence of the biologies of plant and animal might have seemed unlikely. But given enough time, adaptation and selection could turn a wheezing, four-finned lungfish into a dinosaur, or a human or a horse or an elephant or a bat- and even back into a whale, a fishlike creature, once again. By comparison hooking up people and trees with an umbilical connection was a trivial piece of re-engineering.

In the myths of vanished humanity there had been a kind of foreshadowing of this new arrangement. The Middle Ages' legends of the Lamb of Tartary had spoken of the Borametz, a tree whose fruit was supposed to contain tiny lambs. All of mankind's legends were forgotten now, but the tale of the Borametz, with its twining of animal and plant, found strange echoes in these latter days.

But there were costs, as always. Their complex symbiosis with the Tree had imposed a kind of stasis on the postpeople. Over time the bodies of Ultimate and her kind had specialized for the heat and aridity, and had simplified and become more efficient. Once the crucial linking was made, Tree and people became so well adapted to each other that it was no longer possible for either of them to change quickly.

Since the snaking umbilicals had started to worm their way into posthuman bellies, since people had first huddled in the protective enclosure of borametz leaves, two hundred million years had worn away unmarked.

But even now, even after all this time, the symbiotic ties were weak compared to more ancient forces.

In its slow vegetable way, the Tree had concluded that for now the people could not afford another baby. Ultimate's infant was being reabsorbed, her substance returning to the Tree.

It was an ancient calculation: in hard times it paid to sacrifice the vulnerable young, and to keep alive mature individuals who might breed again in an upturn.

But the infant was almost old enough to feed herself. Just a little longer and she would have survived to independence. And this was Ultimate's baby this was Ultimate's baby: the first she had had, perhaps the only one she would ever be permitted to have. Ancient drives warred. It was a failure of adaptation, this battling of one instinct against another.

It was a primordial calculus, an ancient story told over and over again, in Purga's time, in Juna's, for uncounted grandmothers lost and unimaginable in the dark. But for Ultimate, here at the end of time, the dilemma hurt as much as if it had just been minted in the fires of hell.

It took heartbeats to resolve. But in the end the tie of mother to child defeated the bonds between symbiotes. She dug her hands into the cottony stuff and dragged her baby from the cocoon. She pulled the belly-root from the infant's gut, and bits of white fiber from her mouth and nose. The child opened her mouth with a popping sound, and turned her head this way and that.

Cactus watched, astonished. Ultimate stood there panting, her mouth open.

Now what? Standing there holding the baby- in defiance of the Tree that had given her life- Ultimate was out on her own, beyond instinct or experience. But the Tree had tried to kill her baby. She had had no choice.

She took a step away from the Tree. Then another step. And another.

Until she was running, running past the place where she had dug the salt- the sphere was gone now, faded from her memory- and she kept running, her baby clutched in her arms, until she came to the walls of the quarry, up which she scrambled in a flash.

She looked back into the great pit, its floor studded with the lowering, silent forms of the borametz trees. And here came Cactus, running after her with a defiant grin.

II.

The land was bare. There were a few stubby trees, and shrubs with bark like rock and leaves like needles, and cacti, small and hard as pebbles and equipped with long toxin-laden spines. Protecting their water, these plants were little balls of aggression, and Ultimate and Cactus knew better than to tackle such risky fare until it was essential to do so.

You had to watch where you put your feet and hands.

There were pits in the desert's crimson floor. They were bright red, a little like flowers, barely visible against the red soil, but with knots of darkness at their centers. Foolish lizards and amphibians, and even the occasional mammal, would tumble unwarily into these waiting traps- and they would not emerge, for these pits were mouths.

These deadly maws belonged to creatures that lived in narrow burrows under the ground. Hairless, eyeless, their legs reduced to scrabbling finlike stubs with sand-digging claws, they were rodents, among the last remnants of the great lineages that had once ruled the planet.

This time of openness and lack of cover did not favor large predators, and the survivors had been forced to find new strategies. The frantic activity and sociability of their ancestors long abandoned, these burrowing rat-mouths spent their lives in holes in the ground, waiting for something to fall into their mouths. Shielded from the excesses of the climate, moving from their burrows only when driven to mate, the rat-mouths had slow metabolisms and very small brains. They made few demands of life, and in their way were content.

But for creatures as smart as Ultimate and Cactus, the rat-mouths weren't hard to avoid. Side by side, the companions moved on.

The companions came to a little gully. It was nearly choked: the rainstorm had filled it with pebbles and stones. But there was still a trickle of silty runoff water. Ultimate and Cactus crouched down, Ultimate shielding her baby, and they pushed their faces into the water, sucking at it gratefully.

Ultimate found green here, in the damp. It was a kind of leaf, prostrate, dark, slightly crimped. Its form was very ancient, too primitive even to have the wherewithal to grow up toward the light. It was actually a descendant of a liverwort, all but unchanged by the passage of time, a barely modified copy of one of the first plants ever to colonize land- a land that had not looked so different from this harsh place. The times had come around, and the liverwort found room to live. Curious, Ultimate plucked the leaf from the rock it clung to, chewed it- it was waxy, sticky- and kissed her baby, letting bits of the leaf trickle into her mouth. The baby chomped with a sucking noise, her little eyes rolling.

Close to one of those pebblelike cactuses, Ultimate spotted a beetle, silvery-backed, toiling to push a dried-up pellet of dung along a miniature crevasse. Ultimate briefly considered making a grab for the beetle.

But as the beetle passed the shade of the cactus, a tiny crimson form shot out of the darkness. It was a lizard, smaller than Ultimate's little finger, and its head was a lot smaller than the beetle itself. But nevertheless the lizard clamped its jaws on the toiling beetle's rear end. Ultimate could hear the miniscule crunch: The beetle waved its legs and antennae, but it could not get away. The lizard, its burst of energy expended, spread great sail-like fans from its neck and legs. The cooling fans made the lizard look twice its resting size, though its red color gave it good camouflage against the Pangaean dust. Saved from overheating, it began the slow, luxurious process of sucking out the beetle's salty vitals from within its carapace.

But it wasn't to be given that luxury. From nowhere a small bird came running on to the scene. Black-feathered, its wings vestigial stubs hidden beneath its skin, it was flightless. Without hesitation, and with lethal precision, the bird lunged at the lizard with a yellow beak full of tiny teeth. The lizard released the beetle and tried to squirm away under the cactus, its sail-fans folding. But the bird had hold of one fin and it pulled the lizard back into the light, shaking its tiny body.

The mutilated beetle crawled away- only to be scooped up by Cactus's little paw and delivered to her mouth.

There were plenty of birds around; that great, ancient lineage was much too adaptable not to have found a place even in this harsh, much-changed world. But few birds flew nowadays. Why fly when there was nothing to flee, nowhere to go that wasn't exactly the same as here here? So the birds had taken to the ground, and in the great shriveling, had adopted many forms.

Meanwhile, disturbed by the bird's attack, more lizards erupted from under the cactus. There were many of them, all of them smaller than the sail-fan caught by the bird, smaller than Ultimate's own fingernail. They were so tiny, Ultimate saw, they had to clamber over pebbles and irregularities in the dirt as if they were hills and valleys. They scurried in all directions, disturbed from their daily slumber, and made for cover in the rocks and pebbles.

Ultimate watched, fascinated.

As new Pangaea's great drying had continued, the larger species had fallen away. In the barren emptiness of the supercontinent there was nowhere for a creature the size of Ultimate to hide, still less a gazelle or a lion. On the largest scales the ancient game of predator and prey had broken down.

But on smaller scales, a new ecology proliferated. Under Ultimate's feet there were holes in the rocks, crevasses in the sand, holes in borametz tree trunks, tangles in root systems. Even in the flattest landscape there was topography where you could hide from predators, or wait to ambush prey, or simply hole up and avoid the rest of the world- if you were small enough.

But if the world of the smallest scales was still rich in opportunity, it was a world largely excluded from the hot-blooded kind.

All hot-bloods had to maintain their high body temperatures. But there was a limit to how much insulating hair and fat you could grow before you became an immobilized little puff-ball, how fast a pulse you could sustain. The last of the shriveling mole folk, their tiny hearts rattling heroically, had been as small as a centimeter. But a centimeter-scale was still huge. There was plenty of room below this, plenty of ways to live.

But all these niches were taken by insects and reptiles and amphibians. Small and skinny, the cold-bloods hid from the heat of the sun and the chill of the night, under rocks and in the shade of trees and cacti. In a handful of dirt it was possible now to find tiny, perfectly formed descendants of frogs and salamanders, snakes- and even the endlessly enduring crocodiles. There were even tiny lungfish, silvery little creatures hastily adapted for the land as the inland waters had dried. This largest of continents was dominated by the smallest of animals.

Without the Tree's support, such large hot-blooded mammals as Ultimate's postpeople could never have survived so long. They were like throwbacks to easier times, out of place in this marginal environment. As the Earth warmed relentlessly, as the great desiccation continued, even the Tree-based communities shrank back and died, one by one. And yet they were here: And yet here was Ultimate, the latest link in a great chain that now passed back through a hundred million grandmothers, morphing and changing, loving and dying, back to Purga herself, and into the formlessness of the still deeper past beyond.

Ultimate and Cactus watched the tiny scramblings in the dirt. Then, hooting, the posthumans fell on the scrambling lizards. Most of them were too small to catch- you would close your hand around them only to see them squirming out the other side- and even when Ultimate managed to cram one into her mouth it was too small a morsel to be satisfying.

But they didn't need to eat the lizards. They were playing. Even now you could have fun. But in the silence of New Pangaea their whoops and hooting cries echoed from the bare rocks, and as far as could be seen they were the only large creatures moving, anywhere.

The sunset came quickly.

The air was scrubbed clean of dust by the rains. As soon as the sun touched the horizon, darkness striped across the flattened land, small ridges, dunes, and pebbles casting shadows tens of meters long. The light in the sky faded from blue to purple, quickly sinking to black at the zenith. It was like a sunset on the airless Moon.

Ultimate and Cactus huddled together, the baby between their bodies. Every night of her life Ultimate had spent in the Tree's enfolding vegetable embrace. Now the shadows were like raptor fingers reaching out for them.

But as the temperature fell, so Ultimate's adaptations to the desert came into operation.

Her skin was actually hot to the touch. During the day her body stored heat, in layers of fat and tissue. In the cooler air of night, her body was able to radiate a lot of the heat back to the environment. If she had not been able to perform this trick of refrigeration she would have had to lose the heat by sweating- and that would have used up water she couldn't afford to waste. And Cactus and Ultimate were breathing, deeply and slowly. That way a maximum amount of oxygen was extracted from each lungful of air, and a minimum amount of water was lost. Meanwhile Ultimate's body was manufacturing water from the carbohydrates in the food she had eaten. She would finish the night with more water in her body's stores than when she had started.

But still, for all this remarkable physiological engineering, the two of them could do little but sit and endure the night, breathing slowly, lapsing into a kind of dull half dream as their bodies' functioning slowed to a crawl.

While above them a bewildering sky unfolded.

Ultimate had a grandstand perspective view of the Galaxy. The huge spiral arms were corridors of brightness that spanned the sky, studded with pinpricks of sapphire-blue young stars and ruby-red nebulae. At the center of the disk was the Galactic core, a bulge of yellow-orange stars like the yolk of a fried egg: the light had taken twenty-five thousand years to travel here to Earth from that crowded core.

In human times the sun had been embedded in the body of the huge flat disk, so that the Galaxy had been seen edge-on, its glory diminished by the obstructing dust clouds that littered the disk. But now the sun, following its slow orbit around the core, had sailed out of the Galaxy's plane. Compared to the random scattering of a few thousand lamps that had marked man's sky, this was like glimpsing the lights of a hidden city.

Ultimate cowered.

A bony hook rose in the sky. It was the Moon, of course, an old Moon, tonight a narrow crescent. The same patient face that had peered down on Earth since long before the birth of man was all but unchanged across half a billion years. And yet this thin crescent Moon shone more brightly over the new supercontinent than it had over the more equable lands in the past. For the Moon shone by reflected sunlight- and the sun had grown brighter.

Had she known where to look Ultimate might have made out a dim smudge in the sky away from the Galaxy's disk, easily visible on the clearest nights. That remote smudge was the great galaxy known as Andromeda, twice the size of its neighbor. It was still a million light-years distant from Earth's Galaxy- but in human times it had been twice as far away as that, and even then it had been visible to the naked eye.

Andromeda and the Galaxy were heading for a collision, still another half-billion years distant. The two great star systems would pass through each other like mingling clouds, with direct collisions between stars rare. But there would be a vast gush of star formation, an explosion of energy that would flood the disks of both galaxies with hard radiation. It would be a remarkable, lethal light show.

But by then there would be little left alive on Earth itself to be troubled by the catastrophe. For the brightening of the sun was life's final emergency.

Morning came with its usual stark suddenness. Scuttling lizards and insects disappeared into the nooks and crannies where they would ride out the day, waiting for the richer opportunities of evening.

The baby mewled. Her fur stuck up in clumps, and the pucker where the belly-root should sit looked inflamed. She kept up her complaints, her bulbous little head turning to and fro, until Ultimate had chewed some more liverwort and dribbled it into her mouth. Cactus, too, was grumbling, picking dirt and bits of dried shit from her fur.

This morning it didn't seem such a good idea to be out here in the middle of nowhere, so far from home. But as she held her baby Ultimate knew she had to stay away from the Tree- stay away, or lose her baby. She clung to that one irreducible fact.

Ultimate and Cactus began to work their random way across the landscape, heading roughly away from the quarry. Just as they had yesterday, they ate where they could- though they found no water- and they avoided the rat-mouths and other hazards.

And, at some point past noon, when the sun had begun its climb down the sky, Ultimate suddenly found herself facing the sphere once again.

She had forgotten it existed. It did not occur to her to wonder how such an immense object might have gotten here here from from there, there, in the quarry. in the quarry.

Cactus showed no interest, once she had figured out that you couldn't eat the sphere. She passed on, grumbling to herself, picking bits of crimson dust out of her fur.

Her baby asleep in her arms, Ultimate walked up to the sphere's purple-black bulk. She sniffed it and, this time, tasted it. Again that unidentifiable electric tang subtly thrilled her. She lingered, somehow drawn. But the sphere offered her nothing.

But suddenly Cactus was howling, thrashing on the ground. Ultimate whirled, crouching. Cactus's left leg was somehow pinned, and blood spurted from her foot- and Ultimate heard the crunch of bone, as if poor Cactus's limb had been taken in some vast mouth.

But there was no mouth to be seen.

No teeth and claws held Cactus. But slashes appeared on her chest and torso, dripping with startlingly bright blood, as if out of nowhere. Still she fought. She swung her fists, kicked, tried to bite even as she screamed. She was landing blows- Ultimate could hear the meaty sound of flesh being struck, and there were peculiar bits of discoloration in the air over Cactus, purple and blue. And the blood itself was starting to outline her assailant in crimson splashes. Ultimate could make out a long cylindrical torso, stubby legs, a wide, snapping mouth.

But Cactus was losing her fight. Her legs and upper body became trapped under the shimmering mass. She turned to Ultimate, and reached out her hand.

Instincts warred in Ultimate. It might have been different if she could have imagined how Cactus was feeling, the mortal fear that flooded through her. But Ultimate could not; empathy had been lost in mankind's great shedding, along with so much else.

She had hesitated too long.

That great blurred mass raised itself up and came crashing down on Cactus. A thicker, richer blood gushed from the helpless posthuman's mouth.

Ultimate's shock evaporated. With a squeal of terror she turned and ran, her squealing baby clutched to her chest, her feet and her free hand clattering over the dusty ground. She kept going until she came to an eroded ridge of crimson rock.

She flung herself to the ground, and looked back. Cactus was still. Ultimate could make out nothing of the vast transparent thing that had destroyed her. But new creatures had emerged, as if from nowhere. They looked like frogs, with sprawling bodies, leathery amphibian skin, splayed, clawed feet, and wide mouths equipped with needle-sharp teeth for rending and gouging. Already the first of them had opened up Cactus's chest and was feasting on the still-warm organs within.

The invisible predator had done its job. It lay exhausted in a pool of Cactus's blood. It was too weary even to feed itself, and it relied on scraps brought to it by its greedy siblings. The meat could be seen being shredded by its grinding teeth and then passing into its gullet and stomach, where digestive processes would begin to absorb and transform it.

As the world had emptied and been eroded flat, the lack of cover was the killer. In a landscape like a pool table, you just couldn't hide a one-ton salamander, even if it was painted as red as the rocks. That was why most of the big animals had quickly disappeared, outcompeted by their smaller cousins.

But these creatures had adopted a novel strategy: the ultimate camouflage. The great redesign had taken many tens of millions of years.

Invisibility- or at least transparency- had been a strategy adopted by some fish in earlier times. There were transparent substitutes for most of the body's biochemicals. A substitute had to be found for hemoglobin, for example, the bright red protein in blood cells that combined with oxygen to transport that vital substance through the body.

Of course no land-going creature could ever be truly invisible. Even in these arid times all animals were essentially bags of water. If you were actually immersed in water- where those long-extinct fish had once swum- something approaching true invisibility could be achieved. But light moved differently through air and water; in the air the final land-going "invisible" actually looked like a big bag of water sitting in the dirt.

Still, it worked pretty well. As long as you kept still you were hard to see- just a mistiness, a slight distortion here and there that might easily be mistaken for a bit of heat shimmer. You could huddle against a rocky outcrop, ensuring that you presented only your least visible angles to any prey. You even had fur, transparent-like fiber-optic cable, which transmitted bits of background color to baffle your prey further.

But even so, few species had adopted the stratagem, for invisibility was a blight.

Every invisible was blind, of course. No transparent retina could trap light. On top of that the creature's biochemistry, limited by the use of transparent substances, was a lot less efficient. And there was no shielding, even for its innermost parts, from the ferocious light, heat, and ultraviolet radiation from the sun, or from the cosmic radiation that had always battered the planet despite its great shield of magnetism. Its organs were transparent, but not transparent enough to let through all the damaging radiation.

Already Cactus's killer was in agony, and soon the cancers developing in its transparent gut would kill it. And it was neotenous. It would die without reaching puberty. None of the invisible kind had ever lived long enough to breed true, nor would their genetic material, damaged by radiation, ever have been able to produce a viable offspring.

Sickly, helpless from birth, these wretched creatures began dying before they emerged from their eggs.

But that didn't matter, not from the point of view of the genes, for the family benefited.

This amphibian species had reached a compromise. Most of its young were born as they always had been. But perhaps one in ten was born invisible. Like the sterile workers in a hive, the invisible lived through its brief, painful life and died young, all for a single purpose: to retrieve food for its siblings. Through them- through their offspring, not its own- the invisible's genetic legacy would live on.

It was an expensive strategy. But it was better to sacrifice one in ten of each generation to a brief life of agony than to succumb to extinction.

The presence of food in its stomach and waste in its lower gut made the invisible easy to spot, of course. So when they were hungry again its siblings would starve it, waiting for all the waste to pass out of its system, rendering it as transparent as possible. And then they would set it to work once more, under the lethal sun, hoping to have it snatch one more meal for them before it died.

The sphere had made its own observations of these events.

The sphere was a living thing, and yet it was not. It was an artifact- and yet it was not that either. The sphere had no name for itself, or for its kind. Yet it was conscious.

It was one of a great horde that now spanned the stars, in a great belt of colonization that swept around the Galaxy's limb. And yet the sphere had come here, to this ruined world, seeking answers.

Memories stretched deep. Among the sphere's kind, identity was a fluid thing, to be split and shared and passed on through components and blueprints. The sphere could think back, deep through thousands of generations, but it was a memory trail that ended in mist. The replicating hordes had forgotten where they came from.

In its way, the sphere longed to know. How How had this great star-spanning swarm of robots first originated? Had there been some form of spontaneous mechanical emergence, cogs and circuits coming together on some metallic asteroid? Or had there been a Designer, some had this great star-spanning swarm of robots first originated? Had there been some form of spontaneous mechanical emergence, cogs and circuits coming together on some metallic asteroid? Or had there been a Designer, some other, other, who had brought the progenitors of these swarming masses into being? who had brought the progenitors of these swarming masses into being?

For a million years the sphere had studied the distribution of the replicators through the Galaxy. It wasn't easy, for the great disk had rotated twice since the origin of its kind, and the stars had swum about, smearing the robot colonists across the sky. Great mathematical models had been built to reverse that great turning, to restore the stars as they once must have been, to map back the replicators' half-forgotten expansion.