As XEC, the latest COVID variant takes hold, we are watching viral evolution play out on a time scale short enough to follow, with different strains of the SARS-CoV-2 virus independently acquiring similar or functionally similar mutations that improve its ability to infect us or to evade existing vaccines.

This is the same process that occurs over tens of thousands, even millions of years in living creatures, from slugs to dogs to you and me, producing the incredible diversity we see in the tree of life along with startling replays of the same idea, known formally as convergent evolution. This is simply when nature finds similar solutions to similar problems in evolutionarily distant groups — think about how dolphins and bats each evolved echolocation, despite being unrelated.

One of the most prominent — and pinchy — ways this manifests is known as carcinization, the idea that nature keeps evolving crabs. Indeed, a crab-like body shape, or morphology, has evolved numerous times independently throughout evolutionary history. From an outsider’s view, it seems like crabs appear so often because Mother Nature “loves” crabs. In the immortal words of English zoologist Lancelot Alexander Borradaile, who coined the term, carcinization is “one of the many attempts of Nature to evolve a crab.”

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The concept is so intriguing and delightful, it has spawned the crab meme, which swept some little nerdy part of the internet a few years ago and with it, a wacky speculation that we are all going to evolve into crabs one day. But all bizarre fantasies aside, what’s really happening here is far more interesting.

It goes without saying, nature is not consciously trying to evolve anything. Even human intelligence arose through the randomness of natural selection. With all due respect to Borradaile and his fans, that’s not how this works. Rather, if the same sort of thing is evolving over and over, it’s probably because that sort of thing is a trait that offers a survival advantage to species existing in similar situations.

Convergent evolution is what we see when we observe that bats and birds have similar development of their arms into gliding wings, which initially allowed them to glide, and then to fly. Or when we notice that the extinct ichthyosaurs, prehistoric fish, have a very similar body outline, down to the bottlenose shape and tiny teeth, as the modern dolphin — which is not a fish at all, but a mammal. In either case, the hydrodynamic body shape lets them both swim rapidly over long distances.

Carcinization is “one of the many attempts of Nature to evolve a crab."

Another example can be seen among the marsupial mammals of Australia, an island where animal evolution diverged from the rest of the world far back in evolutionary time, we see creatures with eerie parallels to mammals from other continents, creatures that occupy the same ecological niche or role and have evolved similar body shapes or abilities to cope.

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