The great predators of this Cambrian seascape are the anomalocarids. They too have a carapace. Their bodies are pen-shaped, like squid, flanked by rippling waves of flattened fin-legs that propel them swiftly through the water. They have huge stalked eyes for spotting their prey, which in this world is most things that move. In this timeline, perhaps they are a little faster, their eyes more acute, their arms swifter to grasp than in others. Our putative ancestors, little boneless eel-things, never do well here. They cling on as sessile sea squirts and similar atavisms, but no fish develop in these seas; there are no jaws, no dorsal fins to scare the swimmers, no slow coelacanth cousin hauling itself onto land, modified swim bladder pressed into service as a makeshift lung. No tyrannosaurs, no mammoths. No us. This is not our world; this is not our story.

Nor is it the story of the anomalocarids, if it’s revenge you want. They too will pass, or at least recede into irrelevance.

To win the arms race of this Cambrian is to have the strongest shell. It is to be hauled up from the sediment by the anomalocarids’ fearsome arms, ground between their spines, gnawed at by their toothed ring of a mouth, then abandoned for flimsier prey. From such a forge, here is what will fight its way to dominance.

They are segmented beings, with a head like a crescent moon, a body of three lobes, a shell of calcite and compound eyes where each facet is a tiny tubule of stone. At the start, nothing in particular distinguishes them. They remain trundlers on the seabed for a long, long time. But they have a long, long time. They have, though they cannot know it, half a billion years on their long road to now.

When at last they take to the land, their shells fend off the killer radiation of the sun and their respiratory surfaces hold enough water for brief seaside strolls. Their articulated legs are initially only strong enough to drag their jointed bellies in the sand, but that will change. Their predators (other trilobite descendants as well as the gamely persevering anomalocarids) are slow to follow. By now there is a burgeoning plant ecology out there for grazing; a few million years later there is a burgeoning herbivore ecology. Life makes its way up the beach with increasing confidence. The land is conquered by a succession of variations on woodlice and horseshoe crabs and silverfish, derived trilobites experimenting with terrestrial locomotion.

What might not be evident is that many of the trilobite descendants have solved one problem that none who come after—in any timeline—will ever crack. They are immortal.