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.
Why, after all, do we die? Yes, there are technological remedies. There is the virtual life everlasting that is “uploading,” and we’ll see more of that; there is the bloody-minded persistence of medical science without limits, procuring a bespoke longevity for those who can afford or mandate it. But these Cambrians simply evolved out of death by natural causes.
So again, why do we die? Because each time our cells divide, we eat into the chemical caps at the ends of our chromosomes. And as we wear away these telomeres, we accelerate towards an end point where the grabby hands of our cells’ biochemical mechanisms can’t get purchase on them. They fumble and drop them until, giving up in disgust, they refuse to replicate any more. Even where such degradation fails to happen, we reach another sticky end: cancers, cells out-dividing their neighbours, hostile and uncompromising.
The trilobite descendants, or at least one lineage, practically sweat telomerase—rebuilding their battered chromosomes every time their cells divide. They still die like flies from predation—sooner or later something will get you—but built into their design is this truth: they don’t have to die. The philosophers that they will one day produce will never face the existential question that haunts us. The question of why, no matter how good the show, the curtain must always come down.
Their biggest challenge—the one that limits their size and kills them despite their genetic chicanery—is moulting their exoskeleton, when all that hard support turns soft. Grow large enough and the effort of shrugging out of their entire skin is eventually fatal. And that, surely, is the end of the road for the Cambrians.
Except for one lineage of trilobite descendants, still passably trilobite-like despite the intervening millions of years. These Cambrians moult their shells away in flakes, shedding pieces heavy with corralled bacteria and parasites, growing constantly without ever needing to discard their entire hard wardrobe in one go. They grow big; they grow old. Give them a few million years and they’re as big as whales in the sea, as big as elephants on land.
Big doesn’t mean smart, of course; ask any sauropod. But they achieve a kind of sentience around the time our own distant ancestors are exiting the sea. Their sentience is a strange thing, a personal thing. As they master their world, so they learn to avoid the countless deaths by misfortune that would otherwise cut short their lives. Communication between individuals arises from intellectual desire, not social need. Their technology, as it develops, focuses on self-modification and improvement, further taking the reins from mere brute evolution. They become masters of their own growth and shapes, changing themselves for new environments and purposes, growing their exoskeletons into the tools they need to unpack the universe.
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.
Until one day, in some far orbit, a discovery is made by one such arthropod descendant… philosopher? Scientist? God? A discovery of such dire import that it breaks a million years of silence and burns a planet’s worth of energy in its haste to communicate the news to its vastly distributed kin.
It is coming to an end. It is all coming to an end. And both their mastery and immortality are finally revealed as a cruel sham.