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Crabs aren’t the only things that evolution keeps making. An expert explains. : Science alert

Crabs aren't the only things that evolution keeps making.  An expert explains.  : Science alert
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Charles Darwin believed that evolution was created”endless forms most beautiful“. It’s a nice sentiment, but it doesn’t explain why evolution keeps making crabs.

Scientists have long wondered whether such exist Limits of what evolution can accomplish or whether Darwin had the right idea. The truth may lie somewhere in between.

While there appears to be no upper limit to the number of species that can evolve, there may be limitations on how many basic forms these species can evolve into. The evolution of crab-like creatures is perhaps one of the best examples of this, as they didn’t just evolve once, they evolved at least five times.

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

Some decapods, like lobster and shrimp, have a thick, muscular belly, which makes up most of the animal we eat. With a quick belly movement, lobsters can launch backwards and escape predators.

crabs, on the other hand have a compressed abdomen, hidden under a flattened but expanded thorax and shell. This allows them to crawl into rock crevices for protection. Evolution has encountered this solution repeatedly because it works well under similar circumstances.

Five groups of “crabs”

The largest group of crabs are the brachyura (real crabs) including the edible crab and the Atlantic blue crab. They had an ancestor that was also crab-shaped. Some species have evolved “backwards” and straightened their abdomens again. The other large group are the anomura (false crabs) with an ancestor that looked more like a lobster.

A red colored king crab on a dark navy blue background.
A king crab (Lithodes longispina), which is a type of Anomura. (Karen Gowlett-Holmes/CSIRO Marine Research)

At least four groups of Anomura – sponge crabs, Porcelain crabs, king crab and the Australian hairy stone crab – have independently evolved into a crab-like shape, much like the true crabs. Like true crabs, their compact bodies are more defensive and can move sideways more quickly.

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

A brown colored hairy stone crab.
Hairy Stone Crab (Lomis hirta); not a real crab either. (Tim Binns/Wikimedia Commons/CC BY-SA 2.0)

But crabs are not the exception.

Something similar happened in the evolution of feathered birds dinosaur. Feathers may have evolved first for insulation, to attract mates, to protect eggs, and also possibly as “webs” for catch prey. Millions of years later, feathers were lengthened and streamlined for flight.

Paleontologists disagree on the details, but all modern birds (Neoaves) evolved from soil-dwelling ancestors shortly after the mass extinction that wiped out the other dinosaurs.

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

Illustration of two microraptors
Microraptors had feathers and wings like modern birds (Durbed/Wikimedia Commons/CC-BY-SA 3.0)

Let the tape of life run again

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

But the history of life has already done something similar for us as closely related lineages evolve and diversify on different continents. In many cases, these ancestral lines always came up with the same or almost identical solutions to the problem.

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

There are two main groups of living mammals. The placental animals (including us) and the marsupials (deprived mammals that give birth to tiny young). Both groups evolved from the same common ancestor 100 million years agothe marsupials mostly in Australasia and America, and the 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 Thylacine (the Thylacine, Extinct in 1936), whose skull and teeth match those of the placental wolf down to the smallest detail.

The skulls and body shapes of a thylacine and a gray wolf side by side.
Thylacine (top) and gray wolf (bottom) skulls. (Feigin et al. genomic research2019)

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

Eyes can generally have evolved up to independently of each other 40 times in different animal groups. Even box jellyfish, which don’t have a brain, have eyes with lenses at the base of their four tentacles.

A translucent box jellyfish on a black background to reveal its internal structures, such as  B. Their small eyes at the base of their tentacles.
A box jellyfish (Tripedalia Cystophora). (Bielecki et al., FLAT2014)

The more we seek, the more we find. Structures such as jaws, teeth, ears, fins, legs, and wings all develop independently throughout the animal tree of life.

Recently, scientists discovered that convergence also occurs at the molecular level. The opsin molecules in the eyes, which convert light photons into chemical energy and allow humans to see, have a strong resemblance to those in box jellyfishand developed in parallel.

Even more bizarre is that such disparate animals as whales and bats have a remarkable convergence in genes enable them to echolocate.

Are people really unique?

Many of the things that we think make people special have been reinvented elsewhere by evolution. Corvids like 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 do this cover their noses while foraging over rocky seabeds. Octopuses also use tools and learn from watching what happens to other squid.

Octopus hides between two seashells on sandy seabed.
Octopus marginalatus hiding between two shells. (Nick Hobgood/Wikimedia/CC BY-SA 3.0)

If things develop in a similar way here on Earth, there is a chance that they will develop in a similar way when life has developed elsewhere in the universe. It could mean extraterrestrial beings look 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 is republished by The conversation under a Creative Commons license. read this original article.

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