February 27, 20254 min readCrab Memes Amplify Mistaken Ideas about EvolutionMemes about repeated evolution of crabs have been co-opted to joke about technology and “ultimate forms.” They’re hilarious, but they oversimplify natural variation, giving bad arguments a scientific veneerBy Joanna Wolfe edited by Dan VerganoLee Foong Lee/Alamy Stock PhotoYou’ve seen the memes—“everything will eventually evolve into a crab” or “crab is the ideal form”—while scrolling online. These are parodies of a real scientific concept, carcinization. In biology, this term describes an evolutionary process where forms that look like a crab have appeared at least five times within crustaceans. But in popular culture, safe to say, the word has evolved an entirely new meaning.Since its meme explosion starting in 2019, the word “carcinization” has become shorthand for “a thing that happens multiple times.” A new wave of memes came up with “intellectual carcinization,” describing the situation where multiple people have converged on the same idea. It’s delightful to reference crabs when lamenting how multiple car companies independently shift towards SUVs, or how multiple social media influencers weirdly converge on similar styles.Unfortunately, these memes contain some faulty assumptions that lead to public misunderstanding of science. As a biologist who studies carcinization, I worry this confusion carries the seeds of past misuses of evolutionary theory that harmed people.On supporting science journalismIf you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.In evolutionary carcinization, multiple groups within decapod crustaceans (shrimps, lobsters and crabs themselves) developed a crablike form with a flat and rounded carapace, and an abdomen folded under the body. King crabs are an entirely different group from the true crabs, separated by nearly 300 million years of evolution, starting within the separate hermit crab group. Although all the lineages that developed the crab form were still within crustaceans (sorry, you won’t personally be evolving into a crab), their common ancestors were elongated and perhaps lobsterlike, changing repeatedly over time.Carcinization is just one example of a biological/evolutionary phenomenon called convergent evolution, where unrelated species evolve similar traits in response to similar environmental challenges. It’s how wings evolved separately in birds and bats, and camera eyes evolved in vertebrates and octopuses. Convergence isn’t limited to form; even the caffeine in your morning coffee or tea evolved separately in those plants (and in at least three other plant lineages)!Rear view of a Red frog spanner crab (Ranina ranina), a crab species which walks with forward motion, an example of decarcinization, from Kochi, Japan.Tony Wu/Nature Picture Library/Alamy Stock PhotoThat’s the Darwinian version of evolution. But there is a kernel of truth to the notion of “intellectual carcinization,” because culture also evolves. People’s behaviors are passed down through the generations by teaching instead of DNA. We can use the same study methods as evolutionary biologists to look at the pattern of how different languages have changed and spread over time, or the adjustments to technologies over “generations” (a new phone model every year, anyone?). The trends and maybe even some of the processes of cultural evolution resemble Darwinian evolution. But the cause is not the same. The things we believe and make are a consequence of decisions that human beings have made.Why should we care about this distinction? Well, the ancestors of crabs didn’t decide to fold up their bodies. As in any organism, mutations produced natural variation in genes. These small genetic changes, some causing visible changes, and some without obvious effect, were passed on through each generation, and filtered by natural selection. This wasn’t always a completely random process, because crablike forms might have been selected for under similar circumstances (although in this case, we aren’t quite sure yet if there is a specific problem the crab body solves that a lobster, for instance, can’t). Nevertheless, at least five times over the last 300 million years, crabs proved themselves to be the ultimate form, or it wouldn’t have happened, right?Still nope. Some species are on their own adventure, and evolve unique traits shared with no others. For example, the aye aye has shockingly long fingers that no other lemur has; it apparently uses them to pick its nose. If the same adaptations were always optimal, strange ones like this would not exist (or they would be widespread). Even among crabs themselves, the signature round and flat body isn’t universal. At least seven lineages that evolved to look crabby some time ago have re-evolved elongated bodies with the abdomen no longer tucked away, a situation nicknamed “decarcinization.” Some types of decarcinized crabs, like the frog crabs, burrow in the sediment, where it might be beneficial to have a body shaped like a torpedo. So it turns out that the crab form isn’t superior.The big problem lies in the metaphor implied by the memes. Some carcinization memes explicitly state that evolution has a goal of turning other species into crabs. That’s wrong. When we use words that imply some organisms are “ultimate forms,” it implies they are genetically better than others. Not so, because there are millions of species already well adapted to their own environments that haven’t turned into crabs. The world is always changing, so some species will inevitably go extinct. But extinction isn’t a value judgement. If we assume that Darwinian evolution is directed by goals, then we are applying our own biases where they don’t belong. It’s a slippery slope from “Nature loves a crab” to “If crabs are better than others, then some people might be too.” Unfortunately, that’s also the basis of eugenics, which used natural selection as a false justification for mass sterilization of people considered “undesirable” in the last century. To be clear, there is no evidence from the prevalence of convergent evolution to support discrimination or eugenics.We, as humans, can certainly learn from convergent evolution. COVID viruses repeatedly evolve the same mutations in different variants, because they must evade our immune response in order to replicate. We can use this information to help design new drugs and vaccines. Only humans know how to harness the variation in nature to build something of our own choosing. But there’s still something important we can learn from crabs (and meme about): an appreciation for the astonishing variation of lifestyles and forms on our planet.This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.

Memes about repeated evolution of crabs have been co-opted to joke about technology and “ultimate forms.” They’re hilarious, but they oversimplify natural variation, giving bad arguments a scientific veneer