In the process of regressive evolution, organisms may lose complex traits and can evolve “backward.” However, experts state that evolution does not revert to its original steps.
Evolving into Simpler Forms
Cave fish evolving by gradually losing their eyes.
Evolution has produced astonishing complex traits, from the octopus’s tentacles with neural networks to the ears of mammals. However, can the evolutionary process go backward, returning complex organisms to simpler forms?
In what is known as regressive evolution, organisms can lose complex traits, seemingly evolving backward into simpler forms. Nonetheless, experts argue that evolution does not truly revert in the sense of returning to the original state.
William R. Jeffery, a biologist at the University of Maryland (USA), states: “The ability to rewind evolutionary changes… being reversed in a similar way is highly unlikely.”
Meanwhile, Beth Okamura, a life sciences researcher at the Natural History Museum in London, notes that regressive evolution involves the loss of previously evolved complex forms.
An extreme example comes from myxozoans—parasites with very simple anatomical structures. These organisms have no mouth, nervous system, or gut. Additionally, they possess a very small gene set. According to researcher Okamura, this represents the simplest form of “single cells.”
For a long time, myxozoans have been classified as unicellular protists. Ultimately, this organism has revealed itself to be highly regressive. They evolved from cnidarians—a group that includes jellyfish—and gradually lost many traits no longer necessary for their parasitic lifestyle.
Thus, at least morphologically, myxozoans seem to have reverted to an earlier stage of evolution. “They are converging towards single-celled organisms,” Okamura states.
Adaptation Leading to Change
Scientists say regressive evolution is completely normal.
Thomas Smith, a professor of ecology at the University of California, Los Angeles (USA), states: Earth today is estimated to host around one trillion species. Depending on each species and environmental conditions, the evolutionary process can range from observable timescales to tens of millions of years. Genetic changes, reproductive rates, or an organism’s generation time will limit the speed of new species formation.
However, according to biologist Jeffery, the evolutionary process does not return to its previous steps in regressive evolution. Cave-dwelling organisms often undergo regressive evolution, losing complex traits such as eyes—features that are unnecessary in dark environments.
Jeffery notes that the evolutionary loss of eyes in cave fish does not mean a precise return to their primitive ancestors. Instead, previous processes that created eyes cease, leaving remnants of eyes beneath the skin. “Everything seems to be changing in the opposite direction. However, the eyes have not completely disappeared. They have merely stopped developing further,” researcher Jeffery explains.
Moreover, according to Okamura, the loss of complexity can be accompanied by an increase in less obvious complexity. A specific example is the biochemical substances that parasites use to penetrate host organisms.
“People tend to think of evolution in terms of what you can see in morphological traits. However, there are also many other features that we do not see at physiological and biochemical levels,” the researcher shares.
In cave fish, the lost eyes may obscure similar replacement complexities. Organs that respond to vibrations are abundant in these fish species.
This helps them sense their environment in darkness. “After numerous experiments, researchers discovered these vibration-sensing organs in the empty eye sockets of the fish,” Jeffery states.
Brian Golding, a biologist at McMaster University in Ontario (Canada), suggests that one reason the evolutionary process does not revert to its original steps is that adaptation leads to other changes. This makes reverting a specific change exceedingly complex.
“If you have made a change… you will refine that adaptation. That adaptation will interact with other genes. Now, if you reverse one of those changes, all the other genes will still need to be altered to reverse the evolutionary process,” Golding shares.
For instance, in cave fish, the initial development of eyes might be accompanied by changes not only to the proteins necessary for eyes but also to the skull structure of the eye sockets. Mutations affecting eye proteins will not revert the organism to a state without eye sockets.
Experts warn that the term “regressive evolution” can mistakenly imply that evolution aims to create more complex forms. However, according to researcher Okamura, evolution simply favors traits that make an organism more suited to a specific environment.
In this way, regressive evolution is merely a form of normal evolution. Jeffery notes that the loss of complexity can help a parasitic species or cave-dwelling resident adapt better to its new environment, such as by eliminating the energy costs of developing a complex organ.
“Evolution always progresses in that it selects features that help improve the fitness of individuals in which that variant is expressed,” researcher Okamura states.
