The Light of Other Days

by Arthur C. Clarke

“Just remember this: life survived. In fact, our ancestors survived. Fix on that. If not, we wouldn’t be here.” As Bobby studied the flickering mix of reptile and rodent features that centred in his vision, he found that idea cold comfort. They moved beyond the extinction pulse into the deeper past.

  The recovering Earth seemed a very different place. There was no sign of mountains, and the ancestors clung to life at the margins of enormous, shallow inland seas that washed back and forth with the ages. And, slowly, after millions of years, as the choking gases drew back into the ground, green returned to planet Earth.

  The ancestor had become a low-slung, waddling creature, covered with short dun fur. But as the generations fluttered past, her jaw lengthened, her skull morphing back, and at last she seemed to lose her teeth, leaving a mouth covered with a hard, beak-like material. Now the fur shrank away and the snout lengthened further, and the ancestor became a creature indistinguishable, to Bobby’s untrained eye, from a lizard.

  He realized, in fact, that he was approaching so great a depth in time that the great families of land animals — the turtles, the mammals and the lizards, crocodiles and birds — were merging back into the mother group, the reptiles.

  Then, more than three hundred and fifty million years deep, the ancestor morphed again. Her head became blunter, her limbs shorter and stubbier, her body more streamlined. Perhaps she was amphibian now. At last those stubby limbs became mere lobed fins that melted into her body.

  “Life is retreating from the land,” David said. “The last of the invertebrates, probably a scorpion, is crawling back into the sea. On land, the plants will soon lose their leaves, and will no longer be upright. And after that the only form of life left on land will be simple encrusting forms…”

  Suddenly Bobby was immersed, carried by his retreating grandmother into a shallow sea.

  The water was crowded. There was a coral reef below, stretching into the milky blue distance. It was littered with what looked like giant long-stemmed flowers, through which a bewildering variety of shelled creatures cruised, looking for food. He recognized nautiloids, what looked like a giant ammonite.

  The ancestor was a small, knife-like, unremarkable fish, one of a school which darted to and fro, their movements as complex and nervous as those of any modern species.

  In the distance a shark cruised, its silhouette unmistakable, even over this length of time. The fish school, wary of the shark, darted away, and Bobby felt a pulse of empathy for his ancestors.

  They accelerated once more: four hundred million years deep, four hundred and fifty.

  There was a flurry of evolutionary experimentation, as varieties of bony armour fluttered over the ancestors’ sleek bodies, some of them appearing to last little more than a few generations, as if these primitive fish had lost the knack of a successful body plan. It was clear to Bobby that life was a gathering of information and complexity, information stored in the very structures of living things — information won painfully, over millions of generations, at the cost of pain and death, and now, in this reversed view, being shed almost carelessly.

  …And then, in an instant, the ugly primeval fish disappeared. David slowed the descent again.

  There were no fish in this antique sea. The ancestor was no more than a pale worm-like animal, cowering in a seabed of rippled sand.

  David said, “From now on it gets simpler. There are only a few seaweeds — and at last, a billion years deep, only single-celled life, all the way back to the beginning.”

  “How much further?”

  He said gently. “Bobby, we’ve barely begun. We must travel three times as deep as to this point.”

  The descent resumed.

  The ancestor was a crude worm whose form shifted and flickered — and now, suddenly, she shrivelled to a mere speck of protoplasm, embedded in a mat of algae.

  And when they fell a little further, there was only the algae.

  Abruptly they were plunged into darkness.

  •

  “Shit,” Bobby said. “What happened?”

  “I don’t know.”

  David let them fall deeper, one million years, two. Still the universal darkness persisted.

  At last David broke the link with the ancestor of this period — a microbe or a simple seaweed — and brought the viewpoint out of the ocean, to hover a thousand kilometres above the belly of the Earth.

  The ocean was white: covered in ice from pole to equator, great sheets of it scarred by folds and creases hundreds of kilometres long. Beyond the icy limb of the planet a crescent Moon was rising, that battered face unchanged from Bobby’s time, its features already unimaginably ancient even at this deep epoch. But the cradled new Moon shone almost as brightly, in Earth’s reflected light, as the crescent in direct sunlight.

  Earth had become dazzling bright, perhaps brighter than Venus — if there had been eyes to see.

  “Look at that,” David breathed. Somewhere close to Earth’s equator there was a circular ice structure, the walls much softened, a low eroded mound at its heart. “That’s an impact crater. An old one. That ice covering has been there a long time.”

  They resumed their descent. The shifting details of the ice sheets — the cracks and crumpled ridges and lines of dune-like mounds of snow — were blurred to a pearly smoothness. But still the global freeze persisted.

  Abruptly, after a fall of a further fifty million years, the ice cleared, like frost evaporating from a heated window. But, just as Bobby felt a surge of relief, the ice clamped down again, covering the planet from pole to pole.

  There were three more breaks in the glaciation, before at last it cleared permanently.

  The ice revealed a world that was Earth-like, and yet not. There were blue oceans and continents. But the continents were uniformly barren, dominated by harsh ice-tipped mountains or by rust-red deserts, and their shapes were utterly unfamiliar to Bobby.

  He watched the slow waltz of the continents as they assembled themselves, under the blind prompting of tectonics, into a single giant landmass.

  “There’s the answer,” David said grimly. “The supercontinent, alternately coalescing and breaking up, is the cause of the glaciation. When that big mother breaks up, it creates a lot more shoreline. That stimulates the production of a lot more life — which right now is restricted to microbes and algae, living in inland seas and shallow coastal waters — and the life draws down an excess of carbon dioxide from the atmosphere. The greenhouse effect collapses, and the sun is a little dimmer than in our times.”

  “And so, glaciation.”

  “Yes. On and off, for two hundred million years. There can have been virtually no photosynthesis down there for millions of years at a time. It’s astonishing life survived at all.”

  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.

  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 shrivelled and melted, and a new ocean blanketed the Earth: an ocean of molten rock, hundreds of metres 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 atmos
phere of rock vapour, forty or fifty kilometres 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 vapour.”

  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 vapour. 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 there 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?”

  “Oh, the comet? A real monster. The size of modern-day asteroids like Vesta or Pallas, perhaps five hundred kilometres 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.”