They win, proving their mastery of the world beyond all doubt. They calm the volcanos; they cool the Earth; they expand the icecaps even past where they were before. Their species, after terrible losses, looks forward to a new golden age where anything is possible. A thousand thousand species have been lost to the fires, to anoxic seas, to drought or to the newly minted ice, but Earth’s icthyan lords remain.

Their golden age lasts only a generation, in which they expand in all optimism to every corner of the Earth that is habitable to them. And those corners only grow more numerous as the years go on, so it seems to most of them that they are greater than they ever were, the iceberg coasts creeping closer to the tropics every year.

Their greatest scientists raise concerns, but for the longest time nobody wants to listen to the truth. Only when the ice is halfway to the equator do they leave off their long age of optimism and accept that they have only traded one problem for another. The processes they have set in motion have a global momentum too. They cannot just be turned off or even reined in. Every year is a little colder; every winter the ice reaches with longer arms, every summer it gives less ground in its retreat.

They do everything they can in a vain quest for equilibrium, but this new fate—which they built themselves—is too much for them to undo. And perhaps there was never a time when they could have escaped. Perhaps they only ever had the choice between two deaths.

There comes a time when the Earth can only beget more ice, the white sheen of it casting back the sunlight, so that the frozen land beneath is never warmed. The world enters the greatest ice age since the freezing times that came before the Ediacaran epoch. In your timeline, the Permian extinction killed perhaps eighty or ninety per cent of all species in the world. On these Devonians’ Earth, at the end, there is nothing left beyond extremophile bacteria and algae—whatever can live in and off the ice. The forests, the fish, the insects: all the rich tapestry of life has either burned or frozen. And yet the ice-makers are not gone, not quite.