The two of them descended once more into the belly of the ocean, and allowed the DNA trace to focus their attention on an undistinguished mat of green algae. Somewhere here was embedded the unremarkable cell which was the ancestor of all the humans who ever lived.

And above, a small shoal of creatures like simple jellyfish sailed through the cold blue water. Farther away, Bobby could make out more complex creatures: fronds, bulbs, quilted mats attached to the seafloor or free-floating.

Bobby said, “They don’t look like seaweed to me.”

“My God,” David said, startled. “They look like ediacarans. Multi-celled life-forms. But the ediacarans aren’t scheduled to evolve for a couple of hundred million years. Something’s wrong.”

They resumed their descent. The hints of multi-celled life were soon lost, as life shed what it had painfully learned.

A billion years deep and again darkness fell, like a hammer blow.

“More ice?” Bobby asked.

“I think I understand,” David said grimly. “It was a pulse of evolution — an early event, something we haven’t recognized from the fossils — an attempt by life to grow past the single-celled stage. But it’s doomed to be wiped out by the snowball glaciation, and two hundred million years of progress will be lost… Damn, damn.”

When the ice cleared, a further hundred million years deep, again there were hints of more complex, multi-celled life forms grazing among the algae mats: another false start, to be eliminated by the savage glaciation, and again the brothers were forced to watch as life was crushed back to its most primitive forms.

As they fell through the long, featureless aeons, five more times the dead hand of global glaciation fell on the planet, killing the oceans, squeezing out of existence all but the most primitive life-forms in the most marginal environments. It was a savage feedback cycle initiated every time life gained a significant foothold in the shallow waters at the fringe of the continents.

David said, “It is the tragedy of Sisyphus. In the myth, Sisyphus had to roll the rock to the top of the mountain, only to watch it roll back again and again. Thus, life struggles to achieve complexity and significance, and is again and again crushed down to its most primitive level. It is a series of icy Wormwoods, over and over. Maybe those nihilist philosophers are right; maybe this is all we can expect of the universe, a relentless crushing of life and spirit, because the equilibrium state of the cosmos is death…”

Bobby said grimly, “Tsiolkovski once called Earth the cradle of mankind. And so it is, in fact the cradle of life. But -”

“But,” said David, “it’s one hell of a cradle which crushes its occupants. At least this couldn’t happen now. Not quite this way, anyhow. Life has developed complex feedback cycles, controlling the flow of mass and energy through Earth’s systems. We always thought the living Earth was a thing of beauty. It isn’t. Life has had to learn to defend itself against the planet’s random geological savagery.”

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 the 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 sponge-like 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, kilometres 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 shrivelling 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 raw 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 shrivelled, 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 kilometre 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.