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, beaklike 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, knifelike, unremarkable fish, one of a school which darted to and fro, their movements as complex and nervous as those of any modem 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 armor 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 wormlike 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 shriveled 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 kilometers 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 kilometers 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 dunelike 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 Earthlike, and yet not. There were blue oceans and continents. But the continents were uniformly barren, dominated by harsh icetipped 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 freefloating.
Bobby said, ‘They don’t look like seaweed to me.’
‘My God,’ David said, startled. ‘They look like ediacarans. Multicelled 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 multicelled 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 mere were hints of more complex, multicelled 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.’