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 (opens in new tab), 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 (opens in new tab), 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 (opens in new tab), 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, Space.com, The Fix, Earth.com and others and has edited for the American Medical Association and other organizations.

  • rahullak
    This is fascinating. Of course, from evolution 101 we know that organisms evolve to be best suited to their specific environment, so there's no real "backwards" and "forwards". The dinosaurs evolved and roamed the Earth for a long time until the impact event 65 million years ago, as theorized. Dinosaurs did not re-evolve after that, which proves that there was no real environmental incentive or impulsion for the recreation of dinosaurs.
    Reply
  • xolhid
    Evolution isn't a mind or guided by one. There are no goals. It is a process guided only by selection pressures such as environmental factors. Therefore living things can't be more or less evolved. There is no forward or backward.
    Reply
  • Giovani
    This is a fascinating theory. I have pondered the possibility of humankind having a "shelf life" or "outdate" attached to our "evolution". In the past three years I have noticed something quite different from what is being espoused by leading pundits concerning the current plight of humans.
    I now believe there is an evolution of human endeavors culminating in what we see currently and that is the unsustainable goals of this present administration and their ilk. This debacle began approx. six decades ago and matured into what we are experiencing now.
    It may be a degrading of humankind's genome left unchecked for too long. Now the toxicity of a progressed Marxist entity reveals what we have allowed to grow and rule America.
    Much deeper than we are able to remedy. The cause lies beyond our ability to correctly recognize what we are struggling against. In this fashion of resistance, I see there is no way out. Our unnatural state will not allow the natural response of purging the proven destructive pathogen now threatening societies.
    Reverse evolution is a rational theory but is in effect substantiated by an observation of ancient civilizations having gone through what we are experiencing. In every major society there comes the same generation completely irrational in their unsustainable views and bold in confrontation. America and the world are in this state mainly coming from a young generation basically lost and estranged from reality and predictable in weakening any populace allowing their degeneration to the point of endangering the entire sustainable society.
    Reply
  • Frank F
    Giovani said:
    This is a fascinating theory. I have pondered the possibility of humankind having a "shelf life" or "outdate" attached to our "evolution". In the past three years I have noticed something quite different from what is being espoused by leading pundits concerning the current plight of humans.
    I now believe there is an evolution of human endeavors culminating in what we see currently and that is the unsustainable goals of this present administration and their ilk. This debacle began approx. six decades ago and matured into what we are experiencing now.
    It may be a degrading of humankind's genome left unchecked for too long. Now the toxicity of a progressed Marxist entity reveals what we have allowed to grow and rule America.
    Much deeper than we are able to remedy. The cause lies beyond our ability to correctly recognize what we are struggling against. In this fashion of resistance, I see there is no way out. Our unnatural state will not allow the natural response of purging the proven destructive pathogen now threatening societies.
    Reverse evolution is a rational theory but is in effect substantiated by an observation of ancient civilizations having gone through what we are experiencing. In every major society there comes the same generation completely irrational in their unsustainable views and bold in confrontation. America and the world are in this state mainly coming from a young generation basically lost and estranged from reality and predictable in weakening any populace allowing their degeneration to the point of endangering the entire sustainable society.
    Most folks don't associate mental or psychological devolution. So, bravo to you! I grew up and lived in Calif and witnessed such decline over 5 decades. Staggering decline in cognition.
    Reply
  • Hawken
    Since evolution does not exist, the answer would be No. There are no transitory species, humans did not evolve from apes, and life did not ooze onto the ground from some primordial puddle of lightning stricken goop.
    God created the heavens and the earth and His proof is displayed every moment by looking upon His creation. Adam and Eve sinned and as a result the fallen world became what it is. Through Christ it will be restored. The Bible is the true Word of God.
    Reply
  • Giovani
    Frank F said:
    Most folks don't associate mental or psychological devolution. So, bravo to you! I grew up and lived in Calif and witnessed such decline over 5 decades. Staggering decline in cognition.
    Not many people even of PHD status would consider my observations. I realize my views border on a metaphysical nature due to the complexity involved in a degenerating genome.
    You, Frank F. are a rare individual whose mind can except a wildly rational posit. Fascinating to examine whether this current world debacle, which appears to have evolved to toxic proportions over six decades, has appeared in history and records, letters, among many sources, and how they experienced true downfall from exactly what we are going through now in their own fashion.
    The weakening always originates from the generation who didn't have the responsibilities and toil as the former did. The same deterioration took place as now by substandard individuals conducting themselves and saying things much as currently, only in a different way.
    Reply
  • CyrusNYC
    Purpose of evolution is survival, different species evolve in different ways because it serves their survival interests, for example animals that become household pets end up losing many abilities like hunting skills, become dumber in many ways, yet adapt and learn emotional intelligence from humans as well as speech recognition and other qualities. Animals farmed for food becomes less intelligent than the ones in wilderness, because they are fed and sheltered and only need to grow faster to genetically survive and used to produce the next generation. Many rodents and insects on the other hand became more advanced trying to survive alongside humans that seek to exterminate them. In other words, evolution can go both ways, depending on the species and the circumstances, that's how we have such diversity
    Reply
  • bolide2
    Giovani said:
    This is a fascinating theory. I have pondered the possibility of humankind having a "shelf life" or "outdate" attached to our "evolution". In the past three years I have noticed something quite different from what is being espoused by leading pundits concerning the current plight of humans.
    I now believe there is an evolution of human endeavors culminating in what we see currently and that is the unsustainable goals of this present administration and their ilk. This debacle began approx. six decades ago and matured into what we are experiencing now.
    It may be a degrading of humankind's genome left unchecked for too long. Now the toxicity of a progressed Marxist entity reveals what we have allowed to grow and rule America.
    Much deeper than we are able to remedy. The cause lies beyond our ability to correctly recognize what we are struggling against. In this fashion of resistance, I see there is no way out. Our unnatural state will not allow the natural response of purging the proven destructive pathogen now threatening societies.
    Reverse evolution is a rational theory but is in effect substantiated by an observation of ancient civilizations having gone through what we are experiencing. In every major society there comes the same generation completely irrational in their unsustainable views and bold in confrontation. America and the world are in this state mainly coming from a young generation basically lost and estranged from reality and predictable in weakening any populace allowing their degeneration to the point of endangering the entire sustainable society.
    Still deploring the hippies, are we?
    Reply
  • Giovani
    bolide2 said:
    Still deploring the hippies, are we?
    Interesting you should state your comment in this fashion. All of the scourges currently plaguing America matured from the 60's. The degeneration began prior to that decade but has matured to toxic levels created by a race who has seen their zenith having already transpired.
    There is no remedy for the world's plight. It's not a matter of politics, perversion, or even the deep state's diabolical designs. It is something we have no control over, and worldwide tumult is only an indicator of degeneration snowballing.
    Our technology is outpacing us. As we experience this perhaps twilight of humankind, the things we have created will be our destroyers. Technology is rising while humans are declining. Even a glimpse of human character seals the posit.
    This is not going away, in fact perplexity is setting in and we are finally cornered in tribulation which was recoverable in the past. As I mentioned, the maturation of seeded ills sixty years ago has provided the current and perpetual dead end to humankind.
    Reply