Earth in this era was very different, David said. Tectonic drift had brought all of the continents into a single giant assemblage, the largest landmass in the history of the planet. The tropical areas were dominated by immense deserts, white the high latitudes were scoured by glaciation. In the continental interior the climate swung wildly between killing heat and dry freezing.

And this already fragile world was hit by a further calamity; a great excess of carbon dioxide, which choked animals and added greenhouse heating to an already near-lethal climate.

“Animal life in particular suffered: almost knocked back to the level of pond life. But for us it’s nearly over, Bobby; the excess cee-oh-two, is drawing back into where it came from: deep sea traps and a great outpouring of flood basalts in Siberia, gases brought up from Earth’s interior to poison its surface. And soon that monstrous world continent will break up.

“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.”