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

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

Charles Darwin believed that evolution had created “endless forms most beautiful“. That’s a nice point, but it doesn’t explain why evolution keeps creating crabs.

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

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

Crabs belong to a group of crustaceans called decapod – literally “ten-legged” 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 movement of their abdomen, lobsters can launch themselves backwards and escape from predators.

Crabs, on the contrary, have a compressed abdomen, tucked under a flattened but expanded thorax and shell. This allows them to sneak into rock crevices for protection. Evolution has repeatedly stumbled upon this solution because it works well under similar circumstances.

Five groups of “crabs”

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

King crab (Lithodes longispina), which is an Anomura species. (Karen Gowlett-Holmes/CSIRO Marine Research)

However, at least four groups of Anomura – mushroom crabs, porcelain crabs, king crabs and the Australian hairy stone crab – have independently evolved into a crab-like form in much 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 true biological group. They are a collection of branches of the ten-legged tree that have evolved to look the same.

Brown hairy stone crab.
Hairy stone crab (Lomis Hirta); not true cancer either. (Tim Binns/Wikimedia Commons/CC BY-SA 2.0)

But crabs are no exception.

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

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

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

illustration of two microraptors
Microraptors had feathers and wings like modern birds (Durbed/Wikimedia Commons/CC-BY-SA)

Replay the bar 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 for us, when closely related lineages evolved and diversified on different continents. In many cases, these ancestral lineages have repeatedly offered the same or nearly identical solutions to problems.

One of the best examples is our own group, the 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 primarily in Australia and the Americas and placentals elsewhere.

This isolation led to two almost independent series of “experiments” 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 (thylacine, disappeared in 1936), whose skull and teeth match those of the placental wolf in astonishing detail.

The skulls and body shapes of a thylacine and a gray wolf, side by side.
Thylacine skull (top) and gray wolf (bottom). (Fagin and others. Genomic studies2019)

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

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

A translucent box jellyfish on a black background to visualize its internal structures, such as its small eyes at the base of its tentacles.
Box Jellyfish (Tripedalia cystophora). (Bielecki et al., PLUS ONE2014)

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

More recently, scientists have discovered that convergence also occurs at the molecular level. Opsin molecules in the eyes, which convert photons of light into chemical energy and allow people to see, have a very similar to those in the jellyfish boxand thus evolved in parallel.

It is even stranger that animals as different as whales and bats have a striking similarity in the genes that allow them to echolocate.

Are people really unique?

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

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

An octopus hides between two shells on a sandy seabed.
Octopus marginatus hiding between two shells. (Nick Hobgood/Wikimedia/CC BY-SA 3.0)

If things continue to develop in a similar way here on Earth, there is a possibility that they may also follow a similar course if life has evolved elsewhere in the universe. It could mean alien creatures seem less alien and more familiar than we expect.The conversation

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

This article was republished by The conversation under a Creative Commons license. Read on original article.

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