Gills are fragile, exposed continuously to parasites and bacteria. Having gills is like going around the streets with "eat my respiratory system please" written on your back.

Plus animals with lungs breathe better and thus grow larger and stronger.

What situation could create a world where gills never evolved?

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    $\begingroup$ gills aren't actually that big of a risk most of time, and also lungs aren't nearly as efficient, plus the problem of gills can work amazingly well in water due to being both passive respiration (body doesn't need to constantly remind the respiratory system to pull air unlike with lungs which have to actively move air) $\endgroup$ – zackit Apr 16 at 15:01
  • $\begingroup$ Dunkleosteus could grow heavier than am African elephant and could slice chunks off of sharks with a bite so fast you could barely perceive, Megalodon still holds the record for most powerful bite of any animal topping even the T-Rex, which had one of the most efficient respiratory systems that ever existed. Both used gills to breathe. $\endgroup$ – ProjectApex Apr 16 at 15:22
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    $\begingroup$ Not a bad start, I've not seen this subject broached before. Welcome to worldbuilding Thera, We invite you to take our tour and when you have a bit of spare time, read-up in the help center about how we work. Enjoy our site. $\endgroup$ – A Rogue Ant. Apr 16 at 16:02
  • $\begingroup$ @ProjectApex Sharks actually have relatively weak bites, not that it is related to their breathing in any way. $\endgroup$ – John Apr 16 at 18:09
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    $\begingroup$ if lungs are breathing water they are even more vulnerable to parasites and bacteria, the problem is not with the respiratory system it is with the media being breathed. water is dense enough to much more easily carry bacteria and parasites. $\endgroup$ – John Apr 16 at 18:13

If multi celled animals on the fictional planet evolved in very shallow water, or could only live very close to the surface of water, they might evolve lungs instead of or along with gills.

Most boney and ray-finned fish have organs called gas bladders or swim bladders which are quite similar to lungs.

Which came first, the lung or the swim bladder?

On first thought most people might assume that lungs evolved from swim bladders.

As lobe finned fish were adapting to live in partial water or on land, 420 million years ago during the Devonian, they seem to have split off into multiple groups. Two such branches are known to survive to the present day, the coelacanths and the lungfish.

It's worth note that, despite the name "lungfish", fish evolved lungs before lungfish, or even lobe-finned fish. The common ancestor of lobe-finned and ray-finned fish had lungs, but in most surviving branches of ray-finned fish these evolved into swim bladders used for floatation, instead of breathing. Some, like the bichirs, do retain their lungs, and several other traits that appear to have been common to lobe-finned and ray-finned fish.[12]

While the coelacanth shares many traits with reptiles, the lungfish shares specific other traits with amphibians that the coelacanth does not have. Both coelacanths and lungfishes share the category sarcopterygian with the tetrapods, which includes land animals like reptiles, amphibians, birds, and mammals, e.g. humans. Evidence suggests that the tetrapods are related more closely to lungfish than to coelacanths.[13]


Swim bladders are evolutionarily closely related (i.e., homologous) to lungs. Traditional wisdom has long held that the first lungs, simple sacs connected to the gut that allowed the organism to gulp air under oxygen-poor conditions, evolved into the lungs of today's terrestrial vertebrates and some fish (e.g., lungfish, gar, and bichir) and into the swim bladders of the ray-finned fish. In 1997, Farmer proposed that lungs evolved to supply the heart with oxygen. In fish, blood circulates from the gills to the skeletal muscle, and only then to the heart. During intense exercise, the oxygen in the blood gets used by the skeletal muscle before the blood reaches the heart. Primitive lungs gave an advantage by supplying the heart with oxygenated blood via the cardiac shunt. This theory is robustly supported by the fossil record, the ecology of extant air-breathing fishes, and the physiology of extant fishes.[12] In embryonal development, both lung and swim bladder originate as an outpocketing from the gut; in the case of swim bladders, this connection to the gut continues to exist as the pneumatic duct in the more "primitive" ray-finned fish, and is lost in some of the more derived teleost orders. There are no animals which have both lungs and a swim bladder.

The cartilaginous fish (e.g., sharks and rays) split from the other fishes about 420 million years ago, and lack both lungs and swim bladders, suggesting that these structures evolved after that split.[12] Correspondingly, these fish also have both heterocercal and stiff, wing-like pectoral fins which provide the necessary lift needed due to the lack of swim bladders. Teleost fish with swim bladders have neutral buoyancy, and have no need for this lift.[13]


So lungs and swim bladders seem to have evolved from the same source, or possibly swim bladders evolved from lungs and most earlier fish had lungs. There was an era when most fish species had both gills and lungs.

Fish that evolved lungs would obviously live near the surface of fresh or salt water, where they could reach the surface to gulp air easily.

The deeper the water is, the greater is its pressure. Deep water organisms evolved with internal pressure equal to the external water pressure, to avoid being crushed. So when deep water organisms are caught and raised toward the surface, the lower water pressure causes them to swell up and die.

Lifeforms which live too deep cannot survive reaching the surface, so lungs enabling them to breath surface air would be useless to them. So fish with swim bladders instead of lungs must have had ancestors which evolved swim bladders in deep water. The function of swim bladders is to regulate the bouyancy of fish, to make it easier to stay in their proper depths, and so avoid going to deep and being crushed or going to high and and swelling up.

In a diffferent world where all bodies of water are shallow, fish equivalents with both gills and lungs would probably not evolve to transform their lungs into swim bladders.

Crustaceans, molluscs, and some aquatic insects have tufted gills or plate-like structures on the surfaces of their bodies. Gills of various types and designs, simple or more elaborate, have evolved independently in the past, even among the same class of animals. The segments of polychaete worms bear parapodia many of which carry gills.3 Sponges lack specialised respiratory structures, and the whole of the animal acts as a gill as water is drawn through its spongy structure.[11]


So either various gropus of animals evolved gills independently at different times, or else the ancestors of fish had gills for many millions of years before fish evolved lungs.

So a planet where lungs evolved first would have to be one where all the multicelled aquatic lifeforms lived very close to the surface of the water, where reaching the surface to breath would be very easy, and there would be no advantage to having gills.

This would proably have to happen in very thin sheets of water, especially if tides or waves often left the organisms on land and they had to struggle back to the water.

Posssibly the shallow bodies of water contained substances which were highly opaque to the light frequencies necessary for photosynthesis, so the one celled photosynthisizing organism which were at the base of the food chain, and the organisms which fed on them, and the organisms feeding on them, and so on, were all concentrated at the surface of the water.

When life first evolved on Earth the Moon was much closer and tides were much larger, though multicelled organisms with lungs and/or gills didn't evolve until photosynthesis produced large amounts of oxygen billions of years later.

And possibly such organisms might spread to deeper water, but most would stay near the surface and not evolve gills or lose their lungs.

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    $\begingroup$ Swim bladders actually are lungs. Lungs came first and then later fish re-purposed them into swim bladders. $\endgroup$ – user2352714 Apr 18 at 3:59

Life formed in water, therefore gills evolved before lungs.

If you want lungs without gills, you need life to evolve out of water, which is rather difficult.

Moreover, consider that gills are rather simple: a tube in contact with the water and exchanging gas with it. Lungs, with their very high specific surface, are way more complex which can hardly form first time right. It's more likely that an hypothetical land evolved life form would develop something like the leaves of a tree, which do not look much better than gills with respect to the problems you listed.

  • $\begingroup$ Not necessarily. Lungs & Gills both evolved after the evolution of life which filled the atmosphere with oxygen. What needs to happen is for life to evolve and move onto land before life that produces oxygen evolves. $\endgroup$ – Shufflepants Apr 17 at 7:31

Life begins in water. You need a wet environment around your cells until you develop the rather specialized structure called a "Skin". Until this time, an air-dwelling creature is simply not viable.

In water gills are better than lungs. The closed/one-entry breathing system of lungs simply cannot move enough volume of water to satisfy the oxygen needs of a large creature..

Even "cheating" the system as Dolphins do, is very problematic. To breathe, they need access to the surface, which requires being able to find the surface. A non-trivial problem in deep water.
They need to actively guide their motion to expose the air intakes to open air, which requires a functional mind. An awake mind. Which means a dolphin can never fully sleep. They literally sleep half-a-brain at a time.

All of this micromanagement to allow breathing in the water requires a pretty advanced, specialized mind and body. Which a newly-evolving primitive waterdweller most certainly will not have.

And P.S. Lungs are more delicate than gills! They are just conveniently placed in an immensely more protected location. There is a reason why your lungs are inside your ribcage!

  • $\begingroup$ Well, I'd imagine having ribs in your lungs would be a lot less productive than the other way around! :p $\endgroup$ – Redwolf Programs Apr 17 at 2:46
  • $\begingroup$ "You need a wet environment around your cells until you develop the rather specialized structure called a "Skin". Until this time, an air-dwelling creature is simply not viable." There are many types of body-covering organs that aren't skin. Bark, mucus membranes, chitinous exoskeletons, etc. $\endgroup$ – nick012000 Apr 17 at 13:34
  • $\begingroup$ @nick012000 Those things you mentioned are all skin. The word "skin" does not restrict itself to the organ you have covering your body. It is a much wider word. For example, it is even used in nonliving, inorganic conditions. Paint forms a "skin" on its surface when it starts drying. The point is that a primitive organism is water-based, and if exposed to air, it needs protection from that air, especially the dehydrating aspect but also the free oxygen. This barrier mechanism is not a trivial thing to grow, and is an absolute requirement for a water-based being to function in air. $\endgroup$ – PcMan Apr 17 at 13:43

Gills evolved before lungs, but oxygen production evolved before gills. There was a point before macroscopic life when there was no significant O2 in our atmosphere, and then all of a sudden, a few micro-organisms started producing O2 and flooding the atmosphere with it.

What you need to happen is for life to evolve and move onto land before the evolution of life that produces oxygen. If the land is already covered in life before the atmosphere is flooded with oxygen, you could get the first macroscopic life to make use of it being creatures with lungs. Though, to make it more plausible, you'd need to posit some kind of other organ that these land creatures had which could become lungs that they were using for some other purpose than respiration.


Gills are good if you are moving around. Less good if you are a sessile animal. Some animals which don't move too much use

Anal respiration.

A number of organisms from different phyla use the cloaca or rectum to perform oxygen exchange aka respiration. These include sea cucumbers, aquatic dragonfly nymphs, and certain turtles.

Dragonflies can go both ways and use their "rectal gill" to breathe air or water. Turtles have lungs as well as the cloacal system of course. I think a good prospect for your lung before gill system is the sea cucumber - echinoderms do not have lungs but I can imagine this rectal system could evolve into an air/water system and then finally an air only lung without the organism ever having external gills in the fish mode.

Is it ok to use the term "bung lung"? Asking for a friend.


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