Crabs aren’t the only things evolution keeps doing. An expert explains. : ScienceAlert

Charles Darwin believed that evolution created “infinite more beautiful formsIt’s a nice sentiment, but it doesn’t explain why evolution keeps making crabs.

Scientists have long wondered if there is one limits to what evolution can do or whether Darwin had the right idea. The truth may lie between the two.

Although there does not appear to be a ceiling on the number of species that could evolve, there may be restrictions on how many fundamental forms those species can evolve. The evolution of crab-like creatures may be one of the best examples of this, as they have evolved not just once but at least five times.

Crabs belong to a group called crustaceans decapods – literally “ten feet”, as they have five pairs of walking legs.

Some decapods, such as lobsters and shrimps, have a thick, muscular abdomen, which is most of the animal we eat. With a quick flick of their abdomen, lobsters can shoot backwards and escape from predators.

Crabs, on the other hand, have a compressed abdomen, hidden under a flattened but widened thorax and shell. This allows them to burrow into rock crevices for protection. Evolution repeatedly hit on this solution because it works well in similar sets of circumstances.

Five groups of “crabs”

The largest group of crabs are the Brachyura (true crabs) including the edible crab and the Atlantic blue crab. They had an ancestor who was also in the form of a crab. Some species have evolved “backwards” and stretched their abdomens again. The other large group is the Anomura (false crabs), with an ancestor that looked more like a lobster.

However, at least four groups of Anomura: spongy crabs, porcelain crabs, king crabs and the Australian hairy stone crab – have independently evolved into a crab-like form in the same way as true crabs. Like true crabs, their compact bodies are more defensive and can move sideways faster.

This means that “crabs” are not a real biological group. They are a collection of branches of the decapod tree that evolved to look alike.

But crabs are no exception.

Something similar happened in the evolution of birds from feathers dinosaurs. Feathers may have evolved first for isolation, to attract mates, to protect eggs, and possibly also as “nets” for capturing prey. Millions of years later, feathers lengthened and aligned for flight.

Paleontologists disagree on the details, but all modern birds (Neoaves) evolved from ancestors of the earth right after the mass extinction that wiped out the other dinosaurs.

However, feathered wings and flight also evolved earlier in other groups of dinosaurs, including troodontids and dromaeosaurs. Some of these, like Microraptor, had four wings.

Re-running the tape of life

Unfortunately, we can’t do evolutionary experiments to see if the same things keep happening because that would take hundreds of millions of years.

But the history of life has already done something similar to ours, when closely related lineages evolve and diversify on different continents. In many cases, these ancestral lines repeatedly found the same or nearly identical solutions to problems.

One of the best examples is our own group, mammals.

There are two major groups of living mammals. Placentals (including us) and marsupials (mammals with pouches that give birth to tiny young). Both groups evolved from the same common ancestor 100 million years agomarsupials mainly in Australasia and the Americas and placentals elsewhere.

This isolation led to two almost independent runs of the “experiment” to see what could be done with the mammalian body plan. There are marsupial and placental versions of moles, mice, anteaters, gliders and cats. There was even a marsupial wolf (the thylacine, extinguished in 1936), whose skull and teeth match those of the placental wolf in striking detail.

Not only do body shapes evolve independently, but so do organs and other structures. Humans have complex camera eyes with a lens, iris and retina. Squids and octopuses, which are molluscs and more closely related to snails and clams, also evolved camera eyes with the same components.

In general, eyes may have evolved independently until 40 times in different groups of animals. Even box jellyfish, which have no brain, have lensed eyes at the base of their four tentacles.

The more we look, the more we find. Structures such as jaws, teeth, ears, flippers, legs, and wings continue to evolve independently through the animal tree of life.

More recently, scientists discovered that convergence also happens at the molecular level. The opsin molecules in the eyes that convert light photons into chemical energy and allow humans to see have a closely resembling those of box jellyfishand thus evolved in parallel.

Even stranger, animals as different as whales and bats have a surprising convergence in genes that allow them to echolocate.

Are humans really unique?

Many of the things we like to think make humans special have been reinvented by evolution elsewhere. Corvids such as crows and crows have problem-solving intelligence and, along with owls, can use simple tools.

Whales and dolphins they have complex social structures, and their large brains allowed them to develop language. Dolphins use tools like sponges cover your nose while feeding on stony seabeds. Octopuses also use tools and learn from watching what about the other pops.

If things continue to evolve in a similar way here on Earth, there is a possibility that they will also follow a related course if life has evolved elsewhere in the Universe. It could mean extraterrestrial beings seem less alien and more familiar than we expect.

Matthew WillsProfessor of Evolutionary Paleobiology at the Milner Center for Evolution, University of Bath

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