5 Creatures That Evolved Into Crabs - Has Mother Nature Run Out of Shape Ideas?

Charles Darwin believed that evolution had created 'endless forms most beautiful'. However, his theory of evolution and his feelings do not explain why nature continues to create crabs.

Scientists have long wondered whether there are limits to what evolution can do, or whether Darwin's ideas were correct. The truth may lie somewhere in between.

Because while there seems to be no limit to the number of species that can evolve, there may be limits to the number of basic forms that species can evolve into . The evolution of crab-like creatures may be one of the best examples of this, as it happened not just once, but at least five times.

Crabs are a classic example of the anti-evolution theory.

If you didn't know, crabs belong to a group of crustaceans called decapods . The word literally means 'ten-legged', because they have five pairs of legs to help them walk. Some decapods, such as lobsters and shrimp, have thick, muscular abdomens. With a quick flick of their abdomens, lobsters can dart backwards and escape predators.

In contrast, crabs have a compressed abdomen, tucked under their thorax, and a flat but wide shell. This allows them to squeeze into rocky crevices for protection. Evolution suggests that this solution is the most likely.

Five groups of 'crabs'

King crabs evolved from lobster-like ancestors in Anomura.

Red-eyed sponge crabs like to carry sponges for camouflage.

Now, let's introduce a new concept. "Crabs" are not an actual biological group. They are a collection of branches in a decapod tree that have evolved to look like each other .

The largest group of crabs is the Brachyura (also known as true crabs), which includes edible crabs and the Atlantic blue crab. They have a crab-shaped ancestor. Some species have evolved 'backwards' and straightened their bellies.

The appearance of porcelain crabs is very similar to species of the true crab group.

And another large group called the Anomura (or false crabs) , whose ancestors looked more like lobsters.

However, there are at least four groups within Anomura, the sponge crabs, porcelain crabs, king crabs, and Australian stone crabs, which have evolved independently of each other, but together into a crab-like form in a similar way to true crabs.

Like real crabs, their compact bodies are more defensive and can move horizontally faster.

The hairy stone crab (Lomis hirta) is also a crab-like crustacean.

Crabs are not the only exception.

Something similar happened during the evolution of birds, from feathered dinosaurs. Feathers may have first evolved to provide insulation, to attract mates, to protect eggs, and possibly to form 'nets' to catch prey. Millions of years later, feathers grew longer and were streamlined to aid in flight.

Paleontologists disagree about the details, but all modern birds (the Neoaves clade) evolved from ground-dwelling ancestors shortly after the mass extinction event that wiped out the dinosaurs.

Microraptor had two pairs of wings.

However, feathered wings and the ability to fly also evolved earlier in other groups of dinosaurs, including the Troodontidae and Dromaeosaurs. Some of these, like Microraptor, had as many as four wings.

Rewind the tape of life

Unfortunately, we can't conduct evolutionary experiments to see if the same things keep happening. That would take hundreds of millions of years. But the history of life has done something similar to ours, as closely related lineages evolved and diversified on different continents. In many cases, these ancestral lines repeatedly came up with the same or nearly identical evolutionary solutions to problems.

One of the best examples is the group of mammals.

There are two main groups of mammals. The subclass placental mammals (which includes humans) and marsupials (mammals with pouches that give birth to young). Both groups evolved from a common ancestor more than 100 million years ago, with marsupials mainly in Australasia and the Americas and placental mammals elsewhere.

The skulls of the thylacine or thylacine (left) and the placental gray wolf (right) show striking phenotypic convergence, despite their separate evolution on different continents.

This isolation led to nature running two almost independent 'experiments' to see what could be done with the mammalian body plan. The result was marsupial and placental versions of moles, mice, anteaters, sugar gliders and cats. There was even a thylacine (the thylacine, which went extinct in 1936), whose skull and teeth matched in astonishing detail.

Not only did body types evolve independently, but so did other organs and structures. Humans have eyes designed like complex cameras with a lens, iris, and retina. Squid and octopus, which are mollusks and more closely related to snails and clams, also evolved eyes with similar components.

In all, eyes may have evolved independently up to 40 times in different animal groups. Even the box jellyfish, which has no brain, has eyes with lenses at the base of its four tentacles.

The eyes of the box jellyfish. These invertebrates near the base of the animal evolutionary tree have complex eyes.

The more we look at nature, the more we see similar examples. Structures like jaws, teeth, ears, fins, legs, and wings all evolved independently on the animal tree of life.

More recently, scientists have discovered that convergence also occurs at the molecular level. Opsin molecules in the eye convert photons of light into chemical energy, allowing humans to see. And they closely resemble the molecules in the eyes of box jellyfish. Even more bizarrely, animals as diverse as whales and bats have striking convergence in the genes that allow them to echolocate.

Are humans really unique?

Many of the things we like to think make humans special turn out to have been reinvented by evolution in other species. Some crows have the intelligence to solve many problems, and along with owls, they can use simple tools.

The banded octopus (Amphioctopus marginatus) knows how to use everything around it to camouflage itself.

Whales and dolphins have complex social structures and their large brains have allowed them to develop language. Dolphins use tools such as sponges to cover their noses as they search for food on the rocky seafloor. Octopuses also use tools and have learned to learn by watching what happens to other octopuses.

If things continue to evolve in similar ways here on Earth, it's likely they'll follow a similar course if life has evolved elsewhere in the universe. That could mean extraterrestrials will look less alien and more familiar than we expect.