Will humans evolve into crabs? That’s a real question being posed around the internet these days. It’s a silly thing to ask, right? To set the record straight: No, humans probably won’t evolve into crabs. But we see where the internet theorists are coming from. After all, Mother Nature and Father Evolution certainly play tricks on the timelines of their species. And, let’s face it, evolution seems to love the crab.
In this article, we’re going to explore why humanity’s potential evolution into the Brachyura family is in question and the potential paths by which this would naturally be accomplished. If it were to happen at all. Which it probably won’t.
A meme set off the query but for good reason
If you’re around on the internet, you’ve likely been exposed to this trend. A meme stating that everything turns into crabs is all it took. Thousands more followed suit. “Let’s go be crabs,” “all things become crabs,” etc. Even the Monterey Bay Aquarium got in on the fun, and they’re a respected marine biology institution. Turning into crabs is the craze these days, but don’t worry. It’s an evolutionarily backed one.
— Monterey Bay Aquarium (@MontereyAq) December 8, 2020
This trend sprang up from evolution’s love of the crab. Nature has such an affection for the crab that it’s evolved it through at least five independent evolutionary tracks. Granted, not all of these crabs count as crabs. However, in all these cases, the basic body plan is remarkably similar. As they say, “If it walks like a crab, talks like a crab, scuttles like a crab, and pinches like a crab, people will wrongfully classify it as a crab.” At least colloquially. There’s a reason why this type of wanna-be-crab charade happens though. It’s a morphologically fashionable mechanism known as convergent evolution.
Convergent evolution makes crabs out of everything
Okay, convergent evolution doesn’t make crabs out of literally everything, but it has certainly remade the crab from several non-crab species. The idea is that when a body plan, or part of one, is advantageous to survival, it evolves down independent lines. This causes separate species to resemble each other in physical form or in the way they function. For example, bats and birds both have wings and both can fly, but they’re along totally different evolutionary tracts. The same goes for crabs and – well – their imposters.
The family of false crabs, Anomura, are more closely related to lobsters and shrimp than Brachyura, the true crabs. The last common ancestor of false crabs and the lobster family was several million years more recent than the one they shared with true crabs, as an article from Integrative and Comparative Biology explains. Regardless, if you look at a king crab or a coconut crab, you’d have a hard time calling them anything else. That’s because these decapods have undergone a process known as carcinization, a term coined in 1916 to mean “turns into a crab.” Literally.
In this process, the long tails of hermit crab and lobster-like animals shorten, thin, and fold under their now-thickening carapace. Their bodies flatten and harden, and their limbs become more centralized. Boom! Instant crab! And all it took was a few million years.
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Became a crab and back again
Some closely related decapod species have not only converged into a crab-like body plan but have reverted to their previous forms later down the line. This resembles the normal divergent evolution that we’re used to seeing. It’s a little odd though since evolution decided to flip-flop on their creation, but long-lasting evolution often takes a lot of time to secure.
This back-and-forth has posed questions on the origins of the carcinization process. If it were simply becoming more crab-like for evolutionary advantages, then Mother Nature would likely make the crab state permanent. More research is happening in this field daily. Though we don’t currently understand much about this phenomenon, it’s possible that carcinization and decarcinization are tied to genes passed down from a common decapod ancestor. Only time and further research will tell.
These are the benefits of the crab body plan
Looking at the body plan of the Brachyura family shows clear advantages over other decapods. Whereas lobsters and shrimp are better suited to swimming, that’s about all they have going for them. Minus the pistol shrimp, of course, but that’s an entirely different story. As far as the others go, the long, muscular tail provides great propulsion, but it’s also a liability. One chomp from the right fish and that dangling bit of flesh is done for.
Not only does tucking the tail into the underside of the body help prevent it from being eaten, but the thickness of a crab’s carapace protects it from all but the most powerful predators. The other decapods tend to have relatively tender “shells.” The shape of the carapace could likewise make a difference in survival. The rounder body of most decapod families is easier to grab between a hunter’s teeth. The slim, often sloped and rigid, carapace of Brachyura is harder for mouths to find purchase. Then there is movement.
The directional capabilities afforded by the crab body shape are light years beyond most decapods. Crabs can escape in any direction, climb diverse terrain, and maneuver in ways that outperform their predators. The movement of their legs is simply less constricted in the crab form. That same form also makes the crab more flexible. Yes, you heard that right.
Crab bodies obviously aren’t flexible in the traditional ways, but they’re more flexible when it comes to future evolution than most of their relatives. The crab can, with enough time, turn into virtually whatever it needs to thrive, which is shown in the sheer number of diverse Brachyura. Out of roughly 15,000 decapod species, crabs are responsible for 7,000 of them.
Will humans evolve into crabs? Not likely
Let’s get down to answer the question of “will humans evolve into crabs?”…
To begin with, humans probably won’t evolve into crabs simply because we’re too far separated from the Brachyura family on the evolutionary tree. We’d have to de-evolve hundreds of millions of years and start over. Humans won’t be evolving into the Anomura family of false crabs for the same reason. Now, that’s not to say there isn’t a chance to form a similar body plan millions of years in the future.
We certainly would have a hard time gaining all of the crabby characteristics that Brachyura is known for. For instance, our vestigial tail is already so small and internal that it’s unlikely to fold along the bottom of a carapace. Speaking of, for us to develop a carapace, we’d have to pick up traits that diverged from our lineage approximately 310 million years ago, which, as Science Daily notes, was the last time we shared a common ancestor with turtles and tortoises. Even then, the reptile carapace greatly differs from the crab’s. To gain a true crab carapace, we have to roll back more than 500 million years to when the first vertebrates sprang into existence and redo our entire evolutionary line from scratch.
But is it possible that humans humans resemble crabs at all?
Parts of the crab body plan may still be possible, but a serious mutation would be necessary to bring them about. Sure, we might get flatter and rounder as time goes on. It already happens to some of us anyway (like our editor). But the chance of growing the proper limbs in the correct quantity would take a miracle. According to Dr. Yingzi Yang and Dr. Scott H. Kozin in a paper titled “Cell Signaling Regulation of Vertebrate Limb Growth and Patterning,” the shape and functions of our limbs are decided through the limb buds on our developing embryos and their interaction with signaling chemicals that tell them to grow. Since we only inherited four limb buds from our tetrapod ancestor long before vertebrates crawled onto land, the only direction we can easily go is down. More limbs would take an extremely random mutation.
Here’s the thing about evolution though: Where there’s a random, chaotic collection of mutated DNA deciding to do whatever it wants with a species, there’s a way. Unfortunately, though, there is still a more depressing reason why humans probably won’t evolve into crabs.
Humans don't have time to evolve into crabs
Here’s the thing about evolution: it takes a long time. According to Oregon State University, any form of evolutionary trait that will truly last takes around 1 million years to secure its hold. But even then, a single trait is minor in terms of evolution. Think one species becoming a closely related, barely indistinguishable species – that sort of thing. Most of the speedier evolutionary branching has required isolation or a bottleneck event to occur so rapidly. The kind of divergence necessary to grow extra limbs and completely change our basic form so we can spend our time scuttling in the sand would take hundreds of millions of years at the earliest.
Decapods came into existence 450 million years ago, the American Natural History Museum explains. It took another 200 million years for the lines of crabs and false crabs to split, according to ABC. From there, it would take millions more years for false crabs to resemble the classic crab body plan. Since we’re so far separated from the decapod order, it would likely take us exponentially longer to adapt a similar shape. And, sadly, humans don’t have that sort of time.
A human extinction event is looming
It’s terrifying to face, but humans probably won’t survive long enough to make any major evolutions, let alone to recreate the crab. Many scientists agree we’ve overstayed our welcome, as Scientific American explains. Mammals don’t usually last the 300 million or so years that humans have already been around, and our lack of genetic diversity, likely caused by globalization, puts us in a poor position to handle major extinction events.
Other factors, such as our effect on our habitat and environment have put us in what’s known as an “extinction debt.” This type of situation, caused by species that greatly alter their environment, has historically led to inevitable extinction. It didn’t matter if the species in question appeared to be on the thriving end of the spectrum, as we like to think we are. The toll has always been collected, and humanity’s seems to be due. It all depends on when the scales decide to balance, and the historical trend of extinction debt doesn’t seem to be tipping them in our favor. But, look on the bright side, at least we’ll never be on the wrong end of Cajun Jack’s Seafood Boil.
