Charles Darwin believed that evolution created “the most beautiful infinite forms”. It’s a nice feeling, but it doesn’t explain why evolution keeps making crabs.
Scientists have long wondered if there are limits to what evolution can do or if Darwin had the right idea. The truth may lie somewhere in between.
While there doesn’t appear to be a cap on the number of species that can evolve, there may be restrictions on the number of fundamental forms these species can evolve into. The evolution of crab-like creatures is perhaps one of the best examples of this, since they evolved not just once, but at least five times.
Crabs belong to a group of crustaceans called decapods – literally “ten-legged”, since they have five pairs of walking legs.
Some decapods, like lobsters and shrimps, have thick, muscular abdomens, which make up most of the animal we eat. With a simple movement of the abdomen, lobsters can fly backwards and escape predators.
Crabs, on the other hand, have a compressed abdomen, hidden under a flattened but enlarged thorax and carapace. This allows them to squeeze into rock crevices for protection. Evolution has repeatedly hit on this solution because it works well under similar 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 also had the shape of a crab. Some species have evolved “upside down” and straightened their abdomens. The other large group are the Anomura (false crabs), with an ancestor that looked more like a lobster.
However, at least four groups of Anomura – sponge 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 laterally faster.
This means that “crabs” are not a true biological group. They are a collection of branches from the decapod tree that have evolved to resemble each other.
But crabs are no exception.
Something similar happened in the evolution of birds from feathered dinosaurs. Feathers may have first evolved for insulation, to attract mates, to protect eggs, and possibly also as “nets” to catch prey. Millions of years later, feathers have elongated and streamlined to fly.
Paleontologists disagree on the specifics, but all modern (neoave) birds evolved from terrestrial ancestors right after the mass extinction that wiped out the other dinosaurs.
However, feathered wings and flight also evolved earlier in other dinosaur groups, including troodontids and dromaeosaurs. Some of them, like Microraptor, had four wings.
Re-run the band of life
Unfortunately, we can’t conduct 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 evolve and diversify across different continents. In many cases, these ancestral lines have repeatedly offered identical 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 (pouched mammals that give birth to tiny little ones). Both groups evolved from the same common ancestor over 100 million years ago, marsupials primarily in Australasia and the Americas and placentals elsewhere.
This isolation led to two nearly independent rounds of “experimenting” to see what could be done with the mammal’s body plan. There are marsupial and placental versions of moles, mice, anteaters, gliders, and cats. There was even a marsupial wolf (the thylacine, extinct in 1936), whose skull and teeth match those of the placental wolf in astonishing detail.
It is not only bodily forms that evolve independently, but also 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, have also evolved camera eyes with the same components.
Eyes more generally may have evolved independently up to 40 times in different groups of animals. Even box jellyfish, which have no brains, have eyes with lenses at the base of their four tentacles.
The more we seek, the more we find. Structures such as jaws, teeth, ears, fins, legs, and wings continue to evolve independently through the animal tree of life.
More recently, scientists have discovered that convergence also occurs at the molecular level. The opsin molecules in the eyes that convert photons of light into chemical energy and allow humans to see are very similar to those of box jellyfish and have evolved that way in parallel.
Even more bizarre, animals as different as whales and bats have a striking convergence in the genes that allow them to echolocate.
Are humans really unique?
Many of the things we like to think of that make humans special have been reinvented by evolution elsewhere. Corvids like ravens 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 have allowed them to develop language. Dolphins use tools like sponges to cover their noses while feeding on the rocky seabed. Octopuses also use tools and learn by watching what happens to other octopuses.
If things continue to evolve in a similar way here on Earth, it is possible that they will follow a related course as well if life has evolved elsewhere in the Universe. This could mean that extraterrestrial beings seem less alien and more familiar than expected.
Matthew Wills, Professor of Evolutionary Paleobiology at the Milner Center for Evolution, University of Bath
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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