Charles Darwin believed that evolution createdThe most beautiful forms are endlessIt’s a nice feeling, but it doesn’t explain why evolution continues to produce crabs.
Scientists have long wondered if it was there Limits to what evolution can do Or if Darwin had the right idea. The truth may lie somewhere between the two.
While there does not appear to be an upper limit to the number of species that may evolve, there may be restrictions on the number of basic forms that a species can evolve into. The evolution of crab-like creatures may be one of the best examples of this, as they evolved not just once but five times at least.
Crabs belong to a group of crustaceans called decapod – literally “ten feet”, because they have five pairs of walking legs.
Some decapods, such as lobsters and shrimp, have a thick, muscular abdomen, which is the largest part of the animal that we eat. With a quick flick of the lobster’s abdomen it can launch backwards and escape predators.
crabs, by contrast, You have a compressed stomach, tucked away under a flattened but widened breast and shell. This allows them to move into crevices of rocks for protection. Evolution has hit upon this solution again and again because it works so well under similar sets of circumstances.
Five groups of “crabs”
The largest group is crabs brachyura (true lobster) including edible crab and Atlantic blue crab. They had an ancestor that was also lobster-shaped. Some species have evolved “backwards” and straightened their stomachs again. The other large group is the anomura (false crabs), with an ancestor that looks a lot like a crab.
However, at least four groups of Anomura – Sponge crabAnd the Porcelain crabAnd the King of crabs and aussie Hairy stone crab They independently evolved into a crab-like shape in the same way as true crabs. Like true crabs, their compact bodies are more defensive, and they can move sideways faster.
This means that “crabs” are not a true biological group. It is a group of branches in the decapod tree that have evolved to look alike.
But crabs are no exception.
Something similar happened in the evolution of birds from birds dinosaurs. Feathers may have evolved initially to be insulating, to attract mates, to protect eggs, and perhaps also to act as “nets” for catching prey. After millions of years, the feathers stretched out and streamlined for flight.
Paleontologists disagree about the details, but all modern birds (Neoaves) evolved from Earth ancestors 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, like Microraptor, have four wings.
Restart the life bar
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 done something similar to that for us, when closely related lineages evolve and diversify on different continents. In many cases, these ancestral lineages have time and again come up with the same or nearly identical solutions to problems.
One of the best examples is our group, the Mammals.
There are two major groups of living mammals. placentals (us included) and marsupials (embryonic mammals that give birth to tiny live young). Both groups evolved from the same common ancestor 100 million years agolargely marsupials in Australia and the Americas and placentals elsewhere.
This isolation led to two almost independent processes of ‘experimenting’ 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 (Tasmanian tiger, It became extinct in 1936), whose skull and teeth match a coyote’s placenta in amazing detail.
Not only the forms of the body develop independently, but also other organs and structures. Humans have it Complex camera eyes With lens, iris and retina. Squids and octopuses, which are mollusks and closely related to snails and clams, have also evolved camera eyes with the same components.
The eyes in general may have even evolved independently 40 times in different groups of animals. Even box jellyfish, which have no brain, have eyes with lenses at the bases of their four tentacles.
The more we look, the more we find. Structures such as jaws, teeth, ears, fins, legs, and wings continue to evolve independently across the animal tree of life.
Recently, scientists have discovered that convergence also occurs at the molecular level. Opsin molecules in the eye that convert photons of light into chemical energy and enable humans to see Close resemblance to the box jellyfishand evolved in this way in parallel.
Even more surprisingly, animals as diverse as whales and bats have an amazing convergence in genes that Enabling them to find echolocation.
Are humans really unique?
Many of the things we like to think make humans special have been reinvented by evolution elsewhere. Corvids such as ravens and crows possess problem-solving intelligence, as well as owls, Simple tools can be used.
Whales and dolphins It has complex social structuresTheir large brains allowed them to develop language. Dolphins use tools like sponges cover their noses While they feed on rocky sea bottoms. Octopuses also use tools and Learn from watching What happens to other octopuses?
If things continue to evolve in similar ways here on Earth, there is a possibility that they may also follow a related path if life evolved elsewhere in the universe. may mean it Extraterrestrials look less exotic And more knowledgeable than we expect.
Matthew WellsProfessor of Evolutionary Paleobiology at the Milner Center for Evolution. Bath University
This article has been republished from Conversation Under Creative Commons Licence. Read the The original article.
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