If we became crab-like it would be a slightly horrifying experience, an expert tells us. Crabs molt their entire outer exoskeleton, from their eyes to their gut.

Coconut crabs are massive. Up to three feet in diameter, they scuttle across remote tropical islands in the western Indo-Pacific Ocean, cracking open coconuts like they’re made of butter, eating anything edible, and hiding in burrows. This past spring, Harvard University evolutionary biologist Joanna Wolfe, Ph.D., was in Okinawa, Japan, gathering information for her research on carcinization, or the study of how animals take on crablike characteristics when she got up close to these monsters. Despite their name and their intimidating bulk, they are not at all crabby in personality—they’re slow and “super chill,” she says.

The thing is, coconut crabs are not crabs; they’re just posers. Over millions of years, evolution has “crabified” five different types of animals—including the coconut crab, the largest “land crab”—to adapt to marine or marine-adjacent environments. True crabs have been around for about 300 million years, since before the dinosaurs, and other crustaceans have been evolutionary followers for almost as long.

For carcinization to occur, an animal must lose its functional tail. It needs to develop a flat, rounded body with a bent, segmented abdomen and multiple legs. So, could humans ever undergo the same type of carcinization? Will your great-great-great-great-great-grandson or whatever be crab-like?

“Do you know how arthropods grow? They molt their entire outer exoskeleton. Part of their exoskeleton includes the outside of their eyes and the inside first half or so of their gut. Do you want to do that?”

UNFORTUNATELY FOR THE SOUTH PARK FANS AMONG US who have dreamt of transforming into humanoid Crab People, there’s no way you could turn into a crab. At the very least, you would first need to be some sort of arthropod, a group that contains crustaceans, spiders, and insects, Wolfe says. She is the lead author of a March 2021 paper that discusses various cases and characteristics of carcinization. Even though scientists have known that carcinization has existed as an evolutionary advantage in crustacean environments for nearly 150 years, the paper was widely covered in mainstream publications, which got people excited.

In several interviews, Wolfe has broached the question, “Could humans ever become crabs?” It makes her laugh, but it’s also a bewildering idea.

“Our body is not jointed and segmented like that. So we just can’t do it, because you have to be jointed in order to fold up the way that a crab does,” Wolfe explains. “So it’s kind of baked in from the earliest time of animal evolution that we are on a different path from them.”

The most recent common ancestor of humans and true crabs, genus Decapoda (meaning “ten legs”), was likely a worm-like animal, called a bilaterian, that lived hundreds of millions of years ago. Vertebrates and invertebrates marched down very different evolutionary paths on the tree of life. Mammals don’t have the developmental and genomic flexibility to become carcinized, just as crabs don’t have the ability to turn into mammals. The problem is that groups of genes in living creatures act collectively to perform biological functions, so “it is increasingly difficult to make a change in one place without deleterious effects in another,” says Matthew Wills, Ph.D., an evolutionary paleobiologist at the University of Bath in England. Compare long necks in birds and mammals. While birds retained the plasticity to develop different numbers of neck vertebrae, mammals all have seven neck vertebrae—including you and the giraffe. With very few exceptions, we’re stuck with this number, Wills says.

We do see significantly different animal groups evolving the same characteristics at times, a phenomenon called convergent evolution. The most striking example of this may be in the development of the eye, says Wills. “Convergence becomes more remarkable the longer the separation between lineages is,” he says.

You, a land mammal with a full internal skeleton complete with a spine, have eyes. But so does a squid, a squishy marine animal with no bones. Somewhere along your ancestry, you both independently evolved to see the world with camera eyes, a complex mechanism with stunning similarities. These include the structure of the eyeball itself, the way the surrounding muscles move the eyeball, the pupil controlling the light penetration, and the light-focusing lens. Other animals in wildly different parts of the Tree of Life evolved eyes, too. Snails, fish, and even the cubozoan jellyfish all developed visual organs that share similarities with human eyes—long after their predecessors had branched off from any ancestor in common with us.

“Crabification” is another ideal model of convergent evolution, and it has great advantages. It involves becoming quite flat and wider than you are long. Because you can fold your abdomen under your body, you can protect it. This shape is great for crawling under rocks to escape predators, and for safeguarding your vulnerable soft body under a layer of thick chitin and calcium carbonate. But gaining these traits includes a trade-off: you lose the ability to escape quickly, Wills says. For example, lobsters and shrimp can fold almost in half and snap open fast to shoot away from danger. So that king crab you like to eat actually might have been able to escape the net if it hadn’t carcinized from its earlier, lobster-like shape. (Sorry, the delicious king crab is not a true crab).

WHAT PROMPTS CARCINIZATION IS THE CENTRAL MYSTERY Wolfe and her colleagues are trying to solve. She’s now investigating the gene pathways involved in carcinization, and she’s hoping to learn which gene groups are common among carcinized animals, true crabs included.

While she thinks crabs are cool, Wolfe is sure she wouldn’t want to be crabified, herself. “Do you know how arthropods grow? They molt their entire outer exoskeleton. Part of their exoskeleton includes the outside of their eyes and the inside first half or so of their gut. Do you want to do that?”

Humans are still evolving, though. In one case, modern descendants of people who settled in the high Tibetan plateau more than 10,000 years ago can do what most can’t: live their lives higher than 11,480 feet above sea level but experience no hypoxia, which is oxygen starvation in the body’s tissues. That’s because they’ve developed traits that help their blood deliver oxygen more efficiently throughout their bodies, according to a recently published study of Tibetan women. Blood flows into their lungs at an increased rate, and their hemoglobin—the protein in red blood cells responsible for delivering oxygen to tissues—can carry super-saturated levels of oxygen. Their hearts are bigger than most people’s, too, with a wider-than-average left ventricle, the chamber of the heart responsible for pumping oxygenated blood into the body.

Given enough time, humans can and do evolve. Just not into crabs.

https://www.popularmechanics.com/science/animals/a62696580/could-humans-turn-into-crabs/