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.