Charles Darwin believed that evolution created "an endless succession of the most beautiful forms." That's a good point, but it doesn't explain why evolution keeps producing crabs.
Scientists have long wondered whether evolution has limited capabilities, or whether Darwin's idea of was correct. The truth probably lies somewhere in between.
While there appears to be no upper limit to the number of species that may evolve, there may be limits to how many basic forms these species can evolve into. The evolution of crab-like creatures is probably one of the best examples, as they evolved not just once, but at least five times.
Crabs belong to a group of crustaceans called Decapoda—literally meaning "decapods" because they have five pairs of walking legs. Some decapods, such as lobsters and shrimps, have a thick, muscular abdomen, which is what makes up the majority of the animals we eat. With a quick flick of the lobster's belly, it can shoot back and hide from predators.
In contrast, crabs have a compressed abdomen hidden under a flattened but broadened thorax and shell. This allows them to burrow into rock crevices for protection. Evolution keeps finding this solution because it works well in similar circumstances.
Five groups of "crabs"
King crabs evolved from lobster-like ancestors within the Anomura
The largest crab species is the Brachyura (true crab), which includes the table crab and the Atlantic blue crab. They have an ancestor that was also crab-shaped. Some species have evolved "backwards" and straightened their bellies again. Another large group is the Anomura (false crab), whose ancestors look more like lobsters.
However, at least four groups of Anomura - sponge crabs , porcelain crabs, king crabs and Australian hairy crabs - independently evolved crab-like forms in much the same way as true crabs. Like real crabs, their compact bodies are more defensive and can move sideways faster.
This means "crabs" are not a real group of organisms. They are a collection of branches in the decapod tree that evolved to look identical.
stone crab
But crabs are no exception.
Something similar happened when birds evolved from feathered dinosaurs. Feathers may have originally evolved for insulation, attracting mates, protecting eggs, or as a "net" to capture prey. Over millions of years, the feathers elongated and streamlined for flight.
Paleontologists disagree on the details, but all modern birds (Neornithes) evolved from ground-dwelling ancestors after the mass extinction that wiped out other dinosaurs. However, other groups of dinosaurs, including troodontids and dromaeosaurids, also evolved feathered wings and flight earlier. Some of them, like Microraptor , had four wings.
Microraptor had two pairs of wings
Replaying the tape of life
Unfortunately, we can't do evolutionary experiments to see if the same thing keeps happening, because it would take hundreds of millions of years. But the history of life has done something similar for us, when closely related lineages evolved and diversified on different continents. In many cases, these ancestry repeatedly presented the same or nearly identical solutions to problems. One of the best examples of
is within our own group, the mammals.
There are two major groups of living mammals. placental animals (including us) and marsupials (marsupial mammals that give birth to babies). The two groups evolved from a common ancestor more than 100 million years ago, with marsupials found mainly in Australasia and the Americas, while placental animals are found elsewhere.
This isolation led to two nearly independent runs of "experiments" to see what could be done with a mammalian body plan. Marsupials and placentals include moles , mice, anteaters , gliders and cats. There's even a Thylacine (Thylacine, extinct in 1936) whose skull and teeth match those of a placental wolf in stunning detail.
The skulls of the marsupial thylacine (left) and the placental wolf (right) show striking convergence, even though they evolved differently on different continents
Not only body shape, but also organs and other structures evolved independently. Humans have complex camera eyes with a lens, iris, and retina. Squids and octopuses are mollusks more closely related to snails and clams, and they have also evolved camera eyes with the same components.
More generally, eyes may have evolved independently as many as 40 times in different groups of animals. Even the brainless box jellyfish has eyes with lenses at the base of its four tentacles.
The more we look, the more we discover. Structures such as jaws, teeth, ears, fins, legs, and wings evolved independently throughout the animal tree of life.
Recently, scientists have discovered that convergence also occurs at the molecular level. Opsin molecules in the eye convert photons into chemical energy, allowing humans to see, much like the opsin molecules in box jellyfish, and have evolved in parallel in this way. Even stranger, animals as disparate as whales and bats share surprising convergence in genes that enable them to echolocate.
Are humans really unique?
Much of what we like to think makes humans unique has been reshaped by evolution elsewhere. Corvids like crows and crows possess problem-solving intelligence and can use simple tools like owls.
Whales and Dolphins have complex social structures, and their large brains enable them to develop language. When dolphins are looking for food on the rocky seabed, they will use tools such as sponges to cover their noses. Octopuses also use tools and learn by observing other octopuses.
Octopus marginatus hidden between two seashells in East Timor
If things continue to evolve in a similar way on Earth, it's possible that if life evolved elsewhere in the universe, they would also follow related processes. This could mean aliens look less alien and more familiar than we thought.
