The Earths I have described all faced the great dying: some suffered more than others, some controlled it, some were spectacular in the failure of their control. But on all of them, one species had the chance to understand what was happening to their world, for good or ill. When death came calling they were at least in a position to try and bar the door.

What is this grand dying, though? It is a long, slow convulsion of the Earth, like the coils of a constrictor. It is the continents of the world coming together to make a single mass of land, the interior of which dries into a desert that expands and expands. It is two million square kilometres of lava flow, in a place that will be Siberia, which raises dust clouds that block the sun. A stifling-hot age of darkness that lays waste to all plant life on Earth. It is acid rain that falls in such toxic abundance that the shells of molluscs dissolve in the waves. Hot algal blooms scour the oceans of oxygen, before dying and becoming bloated mats of decay a hundred kilometres across. It is global warming on a colossal scale—as the volcanos pump out carbon dioxide and the seas vomit forth still more killer gases. The Earth tried to kill us in our cradle, but the timelines you have seen survived it, or at least bequeathed a relic of themselves to the future.

But there are dead Earths out there, withered branches on the tree of life. There are also worlds where life recovered but never became sentient again. Here, dumb beasts move through the ruins of civilizations that were just becoming self-aware. Then the world turned and ground them beneath its wheels like a juggernaut.

There was a world where giant snail-like molluscs were the winners in the Carboniferous swamps and shallow seas. Their glistening paths through ferns and horsetails formed strange sigils of occult significance. Occult, because we can never truly know what they meant. The acid of the seas and the rains devoured their shells and they died in agony, intelligent enough to understand the magnitude of what was happening to them. And after the dying times, no other mind arose to look up into their sky—as they had with their stalked eyes—and wonder about the patterns of the stars.