The contrast of this little island of civilization with the centuries-wide sea of ignorance and toil that surrounded it was so striking that David felt a reluctance to leave it.

“You’re taking a risk to come here,” he said.

Bobby’s face, hovering above the ’Shroud, was like an eerie mask, illuminated by the frozen smile of his distant grandmother. “I know that. And I know you’ve been helping the FBI. The DNA trace.”

David sighed. “If not me, somebody else would have developed it. At least this way I know what they’re up to.” He tapped his SoftScreen. A border of smaller images lit up around the image of the grandmother. “Here. WormCam views of all the neighbouring rooms and the corridors. This aerial view shows the parking lot. I’ve mixed in infrared recognition. If anybody approaches…”

“Thanks.”

“It’s been too long, brother. I haven’t forgotten the way you helped me through my own crisis, my brush with addiction.”

“We all have crises. It was nothing.”

“On the contrary… You haven’t told me why you’ve come here.”

Bobby shrugged, the movement inside his ’Shroud a shadowy blur. “I know you’ve been looking for us. I’m alive and well. And so is Kate.”

“And happy?”

Bobby smiled. “If I wanted happy, I could just turn on the chip in my head. There’s more to life than happiness, David. I want you to take a message to Heather.”

David frowned. “Is it about Mary? Is she hurt?”

“No. No, not exactly.” Bobby rubbed his face, hot in his SmartShroud. “She’s become one of the Joined. We’re going to try to get her to come home. I want you to help me set it up.”

It was disturbing news. “Of course. You can trust me.”

Bobby grinned. “I know it. Otherwise I wouldn’t have come.”

And I, David thought uneasily, have, since we last met, discovered something momentous about you.

He looked into Bobby’s open, curious face, lit up by a day two millennia gone. Was this the time to hit Bobby with another revelation about Hiram’s endless tinkering with his life — perhaps, indeed, the greatest crime Hiram had committed against his son?

Later, he thought. Later. There will be a moment.

And besides, the WormCam image still glowed on the ’Screen, enticing, alien, utterly irresistible. The WormCam in all its manifestations had changed the world. But none of that mattered, he thought, compared to this: the power of the technology to reveal what had been thought lost forever.

There would be time enough for life, for their complex affairs, to deal with the unshaped future. For now, history beckoned. He took the joystick, pushing it forward; and the Roman buildings evaporated like snowflakes in the sun.

Another brief blur of migrations, and now here was a new breed of ancestor: still with the characteristic strawberry hair and blue eyes, but with no trace of the Romanesque nose.

Around the flickering faces David glimpsed fields, small and rectangular, worked by ploughs drawn by oxen, or even, in poorer times, by humans. There were timber granaries, sheep and pigs, cattle and goats. Beyond the grouped fields he saw earthwork banks, making the area into a fort — but abruptly, as they sank. deeper into the past, the earthworks were replaced by a cruder wooden palisade.

Bobby said, “The world’s getting simpler.”

“Yes. How did Francis Bacon put it?… ‘The good effects wrought by founders of cities, law-givers, fathers of the people, extirpers of tyrants, and heroes of that class, extend but for short times: whereas the work of the Inventor, though a thing of less pomp and show, is felt everywhere and lasts forever.’ Right about now the Trojan War is being fought with bronze weapons. But bronze breaks easily, which is why that war lasted twenty years with comparatively few casualties. We forgot how to make iron, so we can’t kill each other as efficiently as we used to…”

The earnest toil in the fields continued, largely unchanging from generation to generation. The sheep and cattle, though domesticated, looked like much wilder breeds.

A hundred and fifty generations deep, and the bronze tools gave way, at last, to stone. But the stone-worked fields were little changed. As the pace of historical change slowed, David let them fall faster. Two hundred, three hundred generations passed, the fleeing faces blurring one into the other, slowly moulded by time and toil and the mixing of genes.

But soon it will mean nothing, David thought bleakly — nothing, after Wormwood Day. On that dark morning all of this patient struggle, the toil of billions of small lives, will be obliterated; all we have learned and built will be lost, and there may not even be minds to remember, to mourn. And time’s wall was close, much closer even than the Roman spring they had glimpsed; so little history might be left to play itself out.

Suddenly it was an unbearable thought, as if he had imaginatively absorbed the reality of the Wormwood for the first time. We must find a way to push it aside, he thought. For the sake of these others, the old ones who stare out at us through the WormCam. We must not lose the meaning of their vanished lives.

And then, suddenly, the background was a blur once more.

Bobby said, “We’ve become nomads. Where are we?”

David tapped a reference panel. “Northern Europe. We forgot how to do agriculture. The towns and settlements have dispersed. No more empires, no cities. Humans are pretty rare beasts, and we live in nomadic groups and clans, settlements that last a season or two at best.”

Twelve thousand years deep, he paused the scan.

She might have been fifteen years old, and there was a round sigil of some kind crudely tattooed onto her left cheek. She looked in rude health. She carried a baby, swaddled in animal hide — my remote great-uncle, David thought absently — and she was stroking its round cheek. She wore shoes, leggings, a heavy cloak of plaited grasses. Her other garments seemed to have been stitched together from strips of skin. There was grass stuffed into her shoes and under her hat, presumably for insulation.

Cradling her baby, she was walking after a group of others: men, women with infants, children. They were making their way up a shallow, sloping ridge of rock. They were walking casually, easily, a pace that seemed destined to carry them many kilometres. But some of the adults had flint-tipped spears at the ready: presumably as a guard against animal attack rather than any human threat.

She topped the ridge. David and Bobby, riding at their grandmother’s shoulder, looked with her over the land beyond.

“Oh, my,” David said. “Oh, my.”

They were looking down over a broad, sweeping plain. In the far distance, perhaps the north, there were mountains, dark and brooding, striped with the glaring white of glaciers. The sky was crystal blue, the sun high.

There was no smoke, no tracery of fields, no fencing. All the marks made by humans had been erased from his chill world.

But the valley was not empty.

…It was like a carpet, thought David: a moving carpet of boulder-like bodies, each coated in long red-brown fur that dangled to the ground, like the fur of a musk ox. They moved slowly, feeding all the while, the greater herd made up of scattered groups. At the near fringe of the herd, one of the young broke away from its parent, incautiously, and began to paw at the ground. A wolf, gaunt, white-furred, crept forward. The calf’s mother broke from the pack, curved tusks flashing. The wolf fled.

“Mammoth,” David said.

“There must be tens of thousands of them. And what are they, some kind of deer? Are those camels? And — oh, my God — I think it’s a sabre-toothed cat.”

“Lions and tigers and bears,” David said. “Do you want to go on?”