She had the sense of movement from her left, which was impossible, as there was just a wall there. Someone was approaching, from no direction. Alison stumbled away, seeing a woman in a hoodie and a long skirt walk into her flat from…there, like someone stepping through a mirror. Alison heard her own cracked whimper and hated it. But it gave her the strength to move, and she lashed out at the woman with the blade. The Plug-ugly woman just gently tapped her wrist, which was enough to send the knife clattering to the floor.

She looked into the broad, cheerily hideous face of the woman who’d once been a Belgian importer. Someone from another country, another people. An inhabitant of there. Knowing what she did, Alison thought, How do they walk unseen amongst us, when they’re not human at all? But then she met the woman’s eyes and could read genuine sympathy there, agent to agent, spy to spy.

Soon after, Simms was roaming the floor of Alison’s flat, crying unhappily at being left alone.

Interlude: The Utopians

Excerpt from Other Edens: Speculative Evolution and Intelligence by Professor Ruth Emerson of the University of California

You’re quiet, now, subdued by what we’ve shown you. How many times can you watch the world end, after all, even if it’s not your world?

Perhaps a happy ending now. But I cannot guarantee you’ll take much joy in it.

How long, after all, does it take for life to generate sentience, for sentience to generate a civilization that can command its world enough to be proof against disaster? Each branching timeline that we’ve followed is set into motion later than the last. In each new timeline, the previous lords of Earth are suppressed in favour of a new might-have-been, for otherwise the new hopes would be precluded by the old rulers’ presence.

This one diverges in what you might know as the Carboniferous era. Because for some reason palaeontologists ceased naming geological periods after obscure Welsh tribes and holiday regions on the English coast and became ruthlessly pragmatic about things. Carboniferous: coal-forming. In your world these forests give you your industrial revolution. You will burn their bones in three hundred and sixty million years’ time, choking on your own technological advancement. But you’ll be grinning through black teeth because of all it allows you to accomplish.

But not here. Nobody will ever burn the compressed carbon of these trees.

In the Carboniferous of this Earth, as in yours, the land goes mad for forests. This is a land of giants: millipedes six feet long, dragonflies with three-feet wingspans, armoured amphibians that rut and fight and bellow like water buffalo along the riverbanks. But the megafauna is not our focus.