Scientists have long wondered whether there is a limit to what evolution can do or whether Darwin’s ideas are correct. The truth may lie somewhere between the two.
Because while there seems to be no limit to the number of species that can evolve, there may be a limit to the number of basic forms a species can evolve into. The evolution of crab-like organisms may be one of the best examples of this, as it has happened not once but at least five times.
If you don’t know, crabs belong to a group of crustaceans called decapods. The word literally means “ten legs,” as they have five pairs of legs for walking. Some species of decapods, such as lobsters and shrimp, have thick and muscular bellies. With just a quick snap from the abdomen, lobsters can dash backwards and escape predators.
In contrast, crabs have a compressed belly, tucked under the thorax, and a flattened but broad shell. This allows them to burrow into rock crevices for protection. Evolution has shown that this solution seems to be the most viable.
Five groups of “crabs”
Now, let’s get acquainted with a new concept. “Crabs” are not a true biological group. They are a collection of branches in a decapod tree, which have evolved to look similar.
The largest group of crabs is Brachyura (also known as true crab) which includes edible crabs and Atlantic blue crabs. They have an ancestor that also took the form of a crab. Some species have evolved “backwards” and have their bellies straightened out.
And another large group is called the Anomura (or fake crab), whose ancestors looked more like lobsters.
However, there are at least four groups in Anomura, sponge crabs, porcelain crabs, king crabs and Australian rock crabs, which have evolved independently of each other, but have crab-like appearance. real.
Like real crabs, their compact body is more defensive and can move horizontally faster.
Crab is not the only exception
Something similar happened in the evolution of birds, from the feathered dinosaurs. Feathers may have evolved from the very beginning to provide effective insulation, attract mates, protect eggs, and also form “nets” to catch prey. Millions of years later, feathers evolved and streamlined to aid in flight.
Paleontologists disagree on the details, but all modern birds (clade Neoaves) evolved from terrestrial ancestors shortly after the mass extinction event that wiped out the dinosaurs long ago .
However, feathered wings and the ability to fly also evolved earlier in other dinosaur groups, including Troodontidae and Dromaeosaurs. Some of these, like Microraptor, have up to four wings.
Rewind the tape of life
Unfortunately, we cannot conduct evolutionary experiments to see if the same things continue to happen. Because that would take hundreds of millions of years. But the history of life has done something similar to us, as closely related lineages have evolved and diversified on different continents. In many cases, these ancestral lines consistently offer the same or nearly identical evolutionary solutions to problems.
One of the best examples is the group of mammals.
There are two main groups of mammals. Subclasses of placental mammals (including humans) and marsupials (mammals that give birth to small children). Both groups evolved from a common ancestor more than 100 million years ago, marsupials mainly in Australasia and the Americas and placental mammals elsewhere.
This isolation has led to nature twice conducting almost independent “experiments” to see what it can do with the mammalian body map. And the result are marsupial and placental versions of moles, mice, anteaters, Australian flying squirrels and cats. There’s even a marsupial wolf (the kangaroo, which became extinct in 1936), whose skull and teeth match in astonishing detail.
Not only do body forms evolve independently, but so do other organs and structures. Humans have eyes designed as a complex camera with the lens, iris, and retina. Squid and octopus, which are molluscs and more closely related to snails and mussels, also evolved eyes with similar components.
Overall, the eye may have evolved up to 40 times independently in different groups of animals. Even box jellyfish, which have no brain, have eyes with lenses at the base of their four tentacles.
The more we look at nature, the more similar cases we see. Structures such as jaws, teeth, ears, fins, legs and wings all continue to develop independently on the animal’s tree of life.
More recently, scientists have found that convergence also occurs at the molecular level. Opsin molecules in the eye convert light photons into chemical energy and allow humans to see. And it closely resembles the molecules in the eye in box jellyfish. Even more strangely, animals as diverse as whales and bats have a striking convergence of genes that allow them to echolocate.
Are people really unique?
Many of the things we would like to see as what makes humans special have turned out to be reinvented by evolution in other species. Some crows have the intelligence to solve many problems and, along with owls, can use simple tools.
Whales and dolphins have complex social structures and their large brains allow them to develop language. Dolphins use tools such as sponges to cover their noses as they feed on the rocky seabed. Octopuses also use tools and know how to learn from observing what happens to other octopuses.
If things continue to evolve in similar ways here on Earth, it is likely that they will follow a similar process if life has evolved elsewhere in the universe. That could mean extraterrestrial beings will look less alien and familiar than we expected.
Consult the conversation
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