By the end of the Permian period they are the inheritors of a culture with a million years of unbroken sophistication—ossified and brittle, with an assurance of its own endlessness. The end of the Permian is when Earth devours its children, though, a time of colossal environmental upheaval and extinction on a global scale. The Cambrians are not ready. Individuals ten thousand years old expire in the long heat-death of a mass extinction. Vast carcasses float and decay in the waves. On land, assemblages of fallen, hollowed segments mark the deaths of these lords of creation.

The surviving Cambrians regard the world with bitter betrayal. Something new has come to them: an understanding of what it means to be complacent in an uncaring universe. From their sheltered refugia they expand out again, forced into an unwonted but necessary proximity with each other. They are finally becoming a civilization, where before they were only ever distantly polite individuals. They fear, now, and because they do not die they do not mythologize that fear or forget its causes. Instead, they improve their senses until they can see forever into the night sky. With their naked eyes they view and measure the minute disruptions that alien planets make to distant stars.

Around the time dinosaurs rule our Earths, the Cambrians are walking with their many legs upon the moon. They taste the harsh atmosphere of Mars and amend their biology to suit. They grow themselves photon drives and solar sails. And in space, they grow really big. Gravity has always been an inconvenience to them.

Long before we are even a dream in the eye of an ape, they have spread out through the galaxy, truly the greatest of all the Earths’ children. But then, they had a head start, after all. Half a billion years is a long time.

And if you saw a really big one…just imagine. Through the dust clouds and debris of some far star system, there is an articulated body a hundred kilometres from blunt prow to trailing segmented tail. Its stony carapace is pocked with impact craters and dust abrasion, in places grown into outlandish horns and curving spines longer than skyscrapers according to alien necessity or sartorial preference. It cruises past planets under its own power, devouring asteroids and converting their mass into pure energy to power its eternal flight. And within the vaulted chambers and organs of its body, within the hundred-metre-thick shell, there is a mind vaster than you can imagine. It regards the universe with a patient curiosity, a being that knows it has all the time there ever will be to discover everything there is to know. Because they are realistic about their accomplishments, they consider themselves the lords of all creation. They are now proof against any mere global catastrophe, immortal as long as the universe itself persists.