Can evolution go ‘backwards’?

Michael Dhar

Evolution has created surprisingly complex features, from octopus arms laced with neurons to mammalian ears. However, can evolution go ‘backwards’, returning complex creatures to older and simpler forms?

In this so-called ‘regressive’ evolution, living things may lose their complex features and therefore appear to have changed into ‘backward’ and simpler forms. However, experts point out that evolution does not actually go backwards in the sense of following its own evolutionary steps. “The possibility of reversing evolutionary changes in the same way is extremely improbable,” William R. Jeffery, a biologist at the University of Maryland in the US, told Live Science.

Losing Complexity

Beth Okamura, a life sciences researcher at the Natural History Museum in London, told Live Science that regressive evolution involves the loss of complex forms that arose earlier. An extreme example comes from myxozoans, parasites with very simple anatomy—and no mouths, nervous systems, or guts—and very small genomes. Okamura says the simplest type is “essentially single cells.”

Okamura reports that myxozoans, long classified as ‘unicellular protozoans’, eventually emerged as highly regressed animals. They evolved from cnidarians, a group that includes jellyfish, and lost many traits they no longer needed in a parasitic lifestyle. That being the case, Okamura notes, myxozoans may appear to have returned to an earlier evolutionary stage, at least morphologically. “They converge on single-celled organisms,” he says.

However, Jeffery says, the process in ‘regressive’ evolution has not followed in its footsteps. Similarly, creatures living in caves frequently undergo regressive evolution and lose complex features such as eyes that are not needed in a dark environment. On the other hand, Jeffery emphasizes that the loss of eyes in cave fish, for example, does not necessarily mean a return to a primitive ancestor that did not have these organs. Rather, the processes that previously formed the eye partially stop and a vestigial eye is formed that grows with the skin. “The process may seem to work in reverse,” says Jeffery. “However, the eye does not evolve in the opposite direction. It just stops moving forward.”

In addition, Okamura says, “Losses in complexity can accompany less perceptible increases in complexity, such as the biochemistry that parasites use to infiltrate hosts,” says Okamura. However, there are many other features that we cannot see at the physiological and biochemical level.”

Similarly, eye loss in cavefish may mask alternative complexity. These fish develop large amounts of vibration-sensitive organs, giving them a way to sense their surroundings in dark environments. Jeffery reports that these organs find a suitable place for themselves in the empty eye sockets of the fish, in the already overfilled head.


Brian Golding, a biologist at McMaster University in Ontario, Canada, told Live Science that one of the reasons evolution doesn’t follow its own pace is because adaptations lead to other kinds of changes. This makes it extremely complex to reverse a particular change.

“If you’ve made a change … you’re going to fine-tune this adaptation, and that adaptation will interact with other genes,” Golding says.

In cavefish, for example, the original development of an eye may have been accompanied not only by the proteins required for the formation of the eyes, but also by changes in the skull structures that make up the eye socket. A mutation affecting any eye protein would not result in an organism emerging without a nest yet.

As a result, experts warn that the term “reverse evolution” can be misleadingly understood to mean that “evolution had a goal of creating more complex forms.” In addition, Okamura cites that evolution only favors traits that make an organism better suited to a particular environment.

So ‘regressive evolution’ is actually just evolution as usual. Jeffery argues that the loss of complexity can better adapt a parasite or cave creature to its new environment; For example, he states that he can do this by eliminating the energy-based cost of producing a complex organ. Okamura said, “Evolution is [çeşitlenmenin] He is always pro-progress, because the individuals to whom he is expressed choose traits that increase their suitability for the environment.”

*The Devonian is the geological time period in which Devonian rock systems were formed as the fourth subdivision of Paleozoic time.

Original article Live Science taken from the website. (Translated by Tarkan Tufan)