By: B. Mizne
My first encounter with carcinization was, unsurprisingly, on the Internet, during the Great Internet Surf of the 2020 pandemic. I found myself bored all the time, but one late afternoon in October, I came across a meme referring to scientists coining a term for nature's tendency to evolve crustaceans into crabs. Granted, it was exaggerated for effect, and came across as quite bleak, stating that humans and all life on earth would eventually transform into crabs. In my mind, this would replace cockroaches as the superior species on the planet. Alas, when I searched up the phenomenon of carcinization, I was both relieved and a bit disappointed that my descendants wouldn't become crustaceans.
I was, however, surprised to find a surprisingly high value of truth to the claim. After conducting some research, I found that carcinization is, indeed, a biological term used to describe convergent evolution. Most of us learn from Biology class or through independent means, be it books or television, that evolution is divergent, meaning that, as a lengthy natural process, generates species different (or divergent) from their predecessors. Take the classic example of Darwin's finches: during his travels on the HMS Beagle, he found and collected various specimens of different finches in the Galapagos Islands. When conducting research, he theorized that these finches had a common ancestor found in mainland South America, suggesting that these birds originally colonized the islands. But, considering the island finches' variations in sizes, beaks and claws, he hypothesized that the cause of this divergent evolution was a difference in diet. One species had long beaks and sharp claws to catch insects on the ground and trees, while another had large and powerful beaks to crack open nuts. They evolved to suit the conditions found on each island and were categorized as different species.
Phew! All this preamble just to explain the opposite process: convergent evolution, i.e., carcinization. Around 10 million years ago, near the coast of New Zealand, lived a round creature with a spiky carapace and legs, roughly the size of a baseball. Its scientific name is Paralomis debodeorum (quite flattering) and it is widely considered to be the ancestor of king crabs. Yes, the same crabs we now eat. Animals of the Paralomis genus belong to the infraorder of Anomura, also known as "fake crabs". Notable "crabs" that belong to this category include the Porcelain crab and the Coconut crab. Each of the Anomura crabs independently evolved into crab-like beings.
Crab-like organisms in general date back to the Paleozoic period, around 355 million years ago, to the fossil named Palaeopalaemon. A few years later, roughly 260 million years ago, they split into "fake crabs" and "true crabs", called brachyurans. Just because these Latin names aren't enough, two more contenders for the oldest of their families are Eoprosopon, the first undisputed brachyuran, dated back to around 185 million years ago in the late Jurassic period; and Platykotta of the anomura in the late Triassic period.
In 1916, English zoologist Lancelot Alexander Borradaile coined the term "carcinization" as Nature's tendency to become crab-like, which is a ludicrous assumption in itself. He theorized that the long tail of a lobster, called a pleon, grows shorter over time and tucks into the body to avoid the grasp of predators. At the same time, the front part of the animal, the carapace, grows wider and flatter until it winds up looking like what one would call a crab. The rounder shape allowed greater mobility, letting them walk, swim and burrow more efficiently (and some even climb trees!). A study in the 1980s analyzed fossil crustaceans throughout the Mesozoic era and concluded that they were really prominent then, more specifically in the Cretaceous period. It was dubbed the "Cretaceous Crab Revolution"or the "Mesozoic Decapod Revolution", if you prefer. Decapods are a general term for crabs, crayfish, lobsters and shrimp, as they all have ten (deca) feet (pods).
To make a long story short, scientists are still uncertain about the reasons for carcinization in crustaceans. So, while we as humans won't eventually turn into crabs over the course of millennia, I urge you not to forget the frightening nature of crabs.