Okay so i have this world that is inhabited by an alien race that outwardly is basically undistinguishable from an ordinary earth-human. Due to the specifics of this planet they live on they can breath just fine in air that would be way too rarefied for normal humans (So Heights are no problem at all for them) and due to their world having even more water than our earth also caused them due to evolutionary circumstances to be able to hold their breath for very, very long.

I kinda want them to have the ability to also breath in water (this ability has by the time my story takes place been neglected for so long most have forgotten that they actually have it, i plan on having it rediscovered due to the ability naturally kicking in when one of these humans is in danger of drowning due to not being able to surface before they can no longer hold their breath at wich point subconcious survival instinct basically kicks this ability into gear)

The thing is i basically want them to be able to breath both air and water just using their mouth and/or nose and lungs. Is this possible or would they need some strange biology like and extra set of lungs for waterbreathing?

Edit: If this is as impossible as it seems to be, could i perhaps get away with them having an extremely well developed/efficient diving reflex? I just need a way to get them to be able to be underwater a long time (compared to normal humans) without needing diving gear that does not entail me giving them gills or turning them into mer-people

  • $\begingroup$ Background reading: worldbuilding.stackexchange.com/questions/96017/… $\endgroup$
    – Willk
    Feb 22, 2020 at 13:35
  • $\begingroup$ Lungs do not work in water. They do not work mechanically -- water is too heavy and too viscuous, so that breathing water will rupture the delicate walls of the pulmonary alveolae. They do no work chemically -- there is too little oxygen in water to sustain the metabolism of a warm-blooded animal without dirty tricks which would be detrimental for living outside water (see how the tuna fish do it for an example of such dirty trick). $\endgroup$
    – AlexP
    Feb 22, 2020 at 15:41

2 Answers 2


It doesn’t sound plausible to me. In order to extract sufficient oxygen from water a large quantity of water needs to be processed and something like water flowing over gills rather than in and out of lungs would be so much better at doing this.

It would take some very unusual evolutionary trajectory to create an aquatic human that looked exactly like the homo sapiens that we know and love that did not have gills, webbed hands and feet and some degree of other under water adaption.


We have a problem here:the body shape you want is not suited for an amphibious lifestyle.

Can humans swim? Yes. Can they swim well? Kinda, with good training, though we still fall under many animals. Are we built for an amphibious lifestyle, spending a fair Share of our time in water? Not really. We adapted from tree dwellers to an upright walking lifestyle, so for us to adapt once again to a more amphibious life, we'd need to change.

A way to circumvent these issues would be for your humans to have internal modifications akin to seals and whales, allowing them to store oxygen in their bodies or to hold their breath for long periods without looking the part. If you're willing to do so: their anus could somehow have evolved in a similar way to a sea turtle's cloaca to allow breathing through there, though that'd by no means give them the oxygen they need to actively swim around. More like a safety emergency tool just so they can exchange gasses and remove the CO2 excess qs they passively float to the surface without drowning, most likely partially unconscious to require less oxygen and energy.

These excuses would be hard to evolve without other aquatic adaptations developing, but hey, evolution can be weird at times, so who knows?


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