Life's Little Mysteries

Does evolution ever go backward?

Tetrapod evolution, illustration. We see a fish in the water, followed by a greenish amphibian crawling onto the sandy shore. Next, we see a lizard-like creature walking away.
In the Devonian period, fish evolved into amphibians, which, after millions of years, evolved into four-legged terrestrial animals. But can evolution ever go backward? (Image credit: MARK GARLICK/SCIENCE PHOTO LIBRARY via Getty Images)

Evolution has produced stunningly complex features, from neuron-studded octopus arms to the mammalian ear. Can evolution ever go "backward," though, reverting complex creatures to previous, simpler forms?

In so-called regressive evolution, organisms can lose complex features and thus appear to have evolved "back" into simpler forms. But evolution doesn't really go backward in the sense of retracing evolutionary steps, experts say.

"The chances that the same tape [of evolutionary changes] would be … reversed in the same way is highly improbable," William R. Jeffery, a biologist at the University of Maryland, told Live Science.

Losing complexity

Regressive evolution involves the loss of previously evolved forms of complexity, Beth Okamura, a life sciences researcher at the Natural History Museum in London, told Live Science. An extreme example comes from the myxozoans, parasites with very simple anatomies — no mouths, nervous systems or guts — and very small genomes. The simplest type "are essentially single cells," Okamura said. 

Long classified as single-celled protozoans, myxozoans eventually revealed themselves to be highly regressed animals, Okamura said. They evolved from cnidarians, a group that includes jellyfish, losing many features no longer needed in a parasitic lifestyle. 

Thus, myxozoans may seem, at least morphologically, to have returned to a previous evolutionary stage, Okamura said. "They're sort of converging on single-celled organisms," she said.

Related: How long do new species take to evolve?

Blind cave fish across the world have lost their eyes, as they have no need to see in the cave's dark waters. (Image credit: Reinhard Dirscherl via Getty Images)

Still, the evolutionary process doesn't retrace its steps in regressive evolution, Jeffery said. Cave-dwelling creatures also frequently undergo regressive evolution, losing complex features, like eyes, that are not needed in dark environments. But eye loss in cave fish, for example, doesn't mean an exact return to a primordial ancestor without these organs, Jeffery said. Instead, processes that previously produced the eye stop partway through, leaving a vestigial eye overgrown with skin. 

"Things can look like they're going into reverse," Jeffery said. "But the eye didn't go in reverse. It just stopped going forward."

Additionally, losses in complexity may accompany less-obvious increases in complexity, such as the biochemistries parasites use to get inside hosts, Okamura said. "It's very easy for people … to think of evolution in terms of what you see … what the morphological features are," she said. "But there are also lots of other features that we don't see at the physiological and the biochemical level."

In cave fish, lost eyes may similarly obscure alternative complexity. Organs responsive to vibrations appear in great quantities in these fish, providing a way to sense in dark environments. And in the already-overstuffed head, these organs found available real estate in the fish's empty eye sockets, Jeffery said.

Backtracking through complexity

Part of the reason evolution doesn't retrace its steps is that adaptations lead to other changes, Brian Golding, a biologist at McMaster University in Ontario, told Live Science. That makes simply dialing back a specific change extremely complicated. 

"If you've made a change … you're going to fine-tune that adaptation, and that adaptation will interact with other genes," Golding said. "Now, if you reverse that one change, all of the other genes are still going to have to be changed" to reverse evolution.

In cave fish, for example, the original development of an eye may have come with changes not only to proteins needed for eyes but also to skull structures of an eye socket. A mutation affecting an eye protein wouldn't cause an organism to revert to one without the socket.

Finally, experts cautioned that the term "backward evolution" may imply, misleadingly, that evolution has a goal of creating more complex forms. However, evolution merely favors features that make an organism more fit for a particular environment, Okamura said.

In this way, regressive evolution is just evolution as usual. Losing complexity may make a parasite or cave dweller better adapted to its new environment — for example, by eliminating the energy costs of making a complex organ, Jeffery said. 

"Evolution is always progressive in that it's selecting for features that improve the fitness of the individuals in which that variation is being expressed," Okamura said.

Michael Dhar
Live Science Contributor

Michael Dhar is a science editor and writer based in Chicago. He has an MS in bioinformatics from NYU Tandon School of Engineering, an MA in English literature from Columbia University and a BA in English from the University of Iowa. He has written about health and science for Live Science, Scientific American,, The Fix, and others and has edited for the American Medical Association and other organizations.