Black asked in Science & MathematicsBiology · 2 months ago

Why not re-evolve gills?

So we've seen many land animals over the course of millions of years re-evolve back into the seas, from extinct Ichthyosaurs, to extant Blue Whales. They've all re-evolved water dwalling bodies, from streamlined shapes to fins and tails designed only for motion in the water. But none of these animals have ever re-evolved gills so that they can breathe underwater without needing to surface. Everything else comes back, except the very important method of breathing in water??? In fact, we're even seeing some fish lose their gills and develop lungs too, while still living in the water! Why are gills such one-off evolutions that everything evolves away from, and can't evolve back to?

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  • Newton
    Lv 6
    2 months ago
    Favourite answer

    In biology, there is something called Dollo's principle or Dollo's law, and it is the result of Louis Dollo, one of the all time great biologists, studying lots of fossils and living animals anatomically. Dollo's principle states that,

    "an organism never returns exactly to a former state, even if it finds itself placed in conditions of existence identical to those in which it has previously lived ... it always keeps some trace of the intermediate stages through which it has passed."

    We can see this principle at work in fish gills. Fish develop gills in the embryonic stage using pharynegeal pouches. Land animals such as amphibians, reptiles, birds and mammals, however have lost their gills by turning those pharyngeal pouches into other vital body parts. To re-evolve gills, animals would need to change the developmental genes back to making gills out of the pouches again Since mutations are random, it would be difficult for the development genes to return to the way they were by chance. Also even if that is possible, the currently needed body parts would be disabled and be useless. When a body part is useless, then the genes that govern their development can quickly degenerate since harmful mutations will no longer be eliminated.

    As one can see, Dollo's principle describes how improbable it is for evolution to doubleback to the original condition, since it would take a lot of luck for random chance mutations to duplicate what once existed. Further every stage in the evolution of a new feature must be adaptive. And that means when animals evolve gills, they need to have every intermediate stage between what they have now and what the end product is to be adaptive. Organisms simply cannot evolve something that is not adaptive now, and hope that it will be adaptive in the future.

    That said, some animals have evolved new structures that can work similarly to gills. The hairy frog of Africa for example, have lots of hair like skin fibers that can absorb oxygen from water. They therefore function similar to gills. Turtles can breathe through the skin in their butts, so they do not have to come to the surface for air as often. These new ways to breathe underwater are easier than turning what once were gills back to being gills.

    • οικος
      Lv 7
      2 months agoReport

      It's not the skin in turtles' butts but the urinary bladder and it is only the soft-shelled turtles that do that. There is some evidence that snappers can do the same thing at the other end (pharynx) but this has not been substantiated other than by a master's thesis and was mostly a guess.

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  • 2 months ago

    OK, go for it! . . . . 

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  • 2 months ago

    I like the answer from user Newton, so instead of reiterating all that,

    These days it's because of Megalodons and people.

    Being warm-blooded works pretty well for cetaceans.  They have a blubber layer to insulate them against cool water.  This would make it unlikely for gas-exchange surfaces of capillary beds to develop near the skin surface.  But what about cetaceans that live in warm water?

    Up until a couple of million years ago, those would have been food for young Megalodons.  Yeah, those guys liked to grow up in warm water.

    Now, warm water cetaceans are being killed by people.  River dolphins?  All near extinction.  Vaquita?  That too.

    Your best candidate might be some of the sea snakes.  Please ask this question again in about five million years.

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  • Zirp
    Lv 7
    2 months ago

    what do you mean re-evolve? If the genes for gills are absent from your genome, where would they magically come from?

    Mutations happen RANDOMLY, and then can be passed on to offspring - or go extinct

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  • 2 months ago

    There is the original master from Revolver used to be from the USSR

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  • Jon
    Lv 6
    2 months ago

    There's nothing to prevent these animals from surfacing, so there's no selection for the ability to stay underwater indefinitely. If, say, something were to happen to their food supply and they were forced to seek nourishment at lower depths, then we might expect to see the eventual redevelopment of gills.

  • Kenny
    Lv 7
    2 months ago

    Apparently gills are not necessary to be successful at maintaining the bodies and successful reproduction . 

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    • Kenny
      Lv 7
      2 months agoReport

      Things only evolve abilities that allow the organism to live and reproduce . Not because that would be cool if .

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