At last they reached a time deeper than any of the hammer-blow glaciations.
This young Earth had little in common with the world it would become. The air was visibly thick-unbreathable, crushing. There were no hills or shores, cliffs or forests. Much of the planet appeared to be covered by a shallow ocean, unbroken by continents. The seabed was a thin crust, cracked and broken by rivers of lava that scalded the seas. Frequently, thick gases clouded the planet for years at a time-until volcanoes thrust above die surface and sucked the gases back into the interior.
When it could be seen through the thick rolling smog, the sun was a fierce, blazing ball. The Moon was huge, the size of a dinner plate, though many of its familiar features were already etched into place.
But both Moon and sun seemed to race across the sky. This young Earth spun rapidly on its axis, frequently plunging its surface and its fragile cargo of life into night, and towering tides swept around the bruised planet.
The ancestors, in this hostile place, were unambitious: generation after generation of unremarkable cells living in huge communities close to the surface of shallow seas. Each community began as a spongelike mass of matter, which would shrivel back layer on layer until a single patch of green remained, floating on the surface, drifting across the ocean to merge with some older community.
The sky was busy, alive with the flashes of giant meteors returning to deep space. Frequently-terribly frequently-walls of water, kilometers high, would race around the globe and converge on a burning impact scar, from which a great shining body, an asteroid or comet, would leap into space, briefly illuminating the bruised sky before dwindling into the dark.
And the savagery and frequency of these backward impacts seemed to increase.
Now, abruptly, the green life of the algal mats began to migrate across the surface of the young, turbulent oceans, dragging the ancestor chain-and Bobby’s viewpoint-with it. The algal colonies merged, shrank again, merged, as if shriveling back toward a common core.
At last they found themselves in an isolated pond, cupped in the basin of a wide, deep impact crater, as if on a flooded Moon: Bobby saw jagged run mountains, a stubby central peak. The pond was a livid, virulent green, and, somewhere within, the ancestor chains continued their blind toil back toward inanimacy.
But now, suddenly, the green stain shriveled, reducing to isolated specks, and the surface of the crater lake was covered by a new kind of scum, a thick brownish mat.
‘ … Oh,’ David breathed, as if shocked. ‘We just lost chlorophyll. The ability to manufacture energy from sunlight. Do you see what’s happened? This community of organisms was isolated from the rest by some impact or geological accident-the event that formed this crater, perhaps. It ran out of food here. The organisms were forced to mutate or die.’
‘And mutate they did,’ Bobby said. ‘If not.’
‘If not, then not us.’
Now there was a burst of violence, a blur of motion, overwhelming and unresolved-perhaps this was the violent, isolating event David had hypothesized.
When it was over, Bobby found himself beneath the sea once more, gazing at a mat of thick brown scum that clung to a smoking vent, dimly lit by Earth’s own internal glow.
‘Then it has come to this,’ said David. ‘Our deepest ancestors were rock-eaters: thermophiles, or perhaps even hyperthermophiles. That is, they relished high temperature. They consumed the minerals injected into the water by the vents: iron, sulphur, hydrogen … Crude, inefficient, but robust. They did not require light or oxygen, or even organic material.’
Now Bobby sank into darkness. He passed through tunnels and cracks, diminished, squeezed, in utter darkness broken only by occasional dull red flashes.
‘David? Are you still there?’
‘I’m here.’
‘What’s happening to us?’
‘We’re passing beneath the seabed. We’re migrating through the porous basalt rock there. All the life on the planet is coalescing, Bobby, shrinking back along the ocean ridges and seafloor basalt beds, merging to a single point.’
‘Where? Where are we migrating to?’
‘To the deep rock. Bobby. A point a kilometer down. It will be the last retreat of life. All life on Earth has come from this cache, deep in the rock, this shelter.’
‘And what,’ Bobby asked with foreboding, ‘did life have to shelter from?’
‘We are about to find out, I fear.’
David lifted them up, and they hovered in the foul air of this lifeless Earth.
There was light here, but it was dim and orange, like twilight in a smoggy city. The sun must be above the horizon, but Bobby could not locate it precisely, or the giant Moon. The atmosphere was palpably thick and crushing. The ocean churned below, black, in some places boiling, and the fractured seabed was laced with fire.
The graveyard is truly empty now, Bobby thought. Save for that one small deep-buried cache-containing my most remote ancestors-these young rocks have given up all their layered dead.
And now a blanket of black cloud gathered, as if hurled across the sky by some impetuous god. An inverted rain began, rods of water that leapt from the dappled ocean surface to the swelling clouds.
A century wore by, and still the rain roared upward out of the ocean, its ferocity undiminished-indeed, so voluminous was the rain that soon ocean levels were dropping perceptibly. The clouds thickened further and the oceans dwindled, forming isolated brine pools in the lowest hollows of Earth’s battered, cracked surface.
It took two thousand years. The rain did not stop until the oceans had returned to the clouds, and the land was dry.
And the land began to fragment further.
Soon bright glowing cracks in the exposed land were widening, brightening, lava pulsing and flowing. At last there were only isolated islands left, shards of rock which shriveled and melted, and a new ocean blanketed the Earth: an ocean of molten rock, hundreds of meters deep.
Now a new reversed rain began: a hideous storm of bright molten rock, leaping up from the land. The rock droplets joined the water clouds, so that the atmosphere became a hellish layer of glowing rock droplets and steam.
‘Incredible,’ David shouted. The Earth is collecting an atmosphere of rock vapor, forty or fifty kilometers thick, exerting hundreds of times the pressure of our air. The heat energy contained in it is stupendous … The planet’s cloud tops must be glowing. Earth is shining, a star of rock vapor.’
But the rock rain was drawing heat away from the battered land and-rapidly, within a few months-the land had cooled to solidity. Beneath a glowing sky, liquid water was beginning to form again, new oceans coalescing out of the cooling clouds. But the oceans were formed boiling, their surfaces in contact with rock vapor. And between the oceans, mountains formed, unmelting from puddles of slag.
And now a wall of light swept past Bobby, dragging after it a front of boiling clouds and steam in a burst of unimaginable violence. Bobby screamed-
David slowed their descent into time.
Earth was restored once again.
The blue-black oceans were calm. The sky, empty of cloud, was a greenish dome. The battered Moon was disturbingly huge, the Man’s face familiar to Bobby- save for a missing right eye … And mere was a second sun, a glowing ball that outshone the Moon, with a tail that stretched across the sky.
‘A green sky,’ murmured David. ‘Strange. Methane, perhaps? But how … ‘
‘What,’ Bobby said, ‘the hell is that?’
‘0h, the comet? A real monster. The size of modern-day asteroids like Vesta or Pallas, perhaps five hundred kilometers across. A hundred thousand times the mass of the dinosaur killer.’
‘The size of the Wormwood.’
‘Yes. Remember that the Earth itself was formed from impacts, coalescing from a hail of planetesimals that orbited the young sun. The greatest impact of all was probably the collision with another young world that nearly cracked us open.’
‘The impact that formed the Moon.’
‘After that the surface became relatively stable-but still, the Earth was subject to immense impacts, tens or hundreds of them within a few hundred million years, a bombardment whose violence we can’t begin to imagine. The impact rate tailed off as the remnant planetesimals were soaked up by the planets, and there was a halcyon period of relative quiescence, lasting a few hundred million years … and then, this. Earth was unlucky to meet such a giant so late in the bombardment. An impact hot enough to boil the oceans, even melt the mountains.’
‘But we survived,’ Bobby said grimly.
‘Yes. In our deep, hot niche.’
They fell down into the Earth once more, and Bobby was immersed in rock with his most distant ancestors, a scraping of thermophilic microbes.
He waited in darkness, as countless generations peeled back.
Then, in a blur, he saw light once more.
He was rising up some kind of shaft-like a well- toward a circle of green light, the sky of this alien, prebombardment Earth. The circle expanded until he was lifted into the light.