Feathers and the Appendix

To take your example of fishes developing lungs: Fishes have developed a swim bladders to adjust their buoyancy. Having a connection between the swim bladder and the outside of the body is an advantage to fishes which change fast from deep to shallow water, as it allows them to adapt to decreasing water pressure rapidly by expelling excess gas to relieve the pressure as they swim towards the surface, as the gas pressure could potentially rupture the swim bladder. For Fishes living in shallow, warm water with low oxygen content, the ability to gulp air into their swim bladders, where the blood vessels can take up oxygen from this air to supplement the limited amount of oxygen taken up by the gills is an advantage. Thus the swim bladders develop involutions to increase the surface area, developing into primitive lungs. Thus the lung fish can survive in habitats where other fishes cannot - e.g. migrating over dry land from one pond drying up to a pond that still contain water. As the fish gradually develops into an amphibian, gills become less important for survival, while the lungs become more important. Amphibians usually only have gills in their juvenile stages, which they loose then they become adult. In mammals, there is only a trace of gill development visible during embryonal development, and these embryonal structures, which in fishes would become gills, differentiate into yet another organ, the tiny bones in the inner ear.Thus we first see the introduction of redundancy - a fish with both gills and lungs for oxygen uptake - coupled with the swich to a different habitat - shallow costal regions rather than deep water - and only after the new organ (lung) has been refined its function, the old organ (the gills), now redundant, looses functionality and eventually develops into yet another organ (the inner ear)

Hello, Nadine.It is my understanding that it is still a matter of controversey whether lungs developed from swim bladders, or the other way around. In fact, I am under the impression (perhaps incorrect) that the lungs-first scenario is more widely accepted; I will accept correction on this.Just as your scenario does make sense and is plausible, so is the other. Fish living in stagnant water would supplement their oxygen intake by gulping air at the surface; those with more blood vessels near the surface of their throats would be able to absorb slightly more oxygen from the air as it moved toward the lungs; even more oxygen would be absorbed if this region of the throat were to become "bowl-shaped" (more surface area); then further division into chambers, leading to air-sacs, would not only increase the surface area, but it would also separate (and protect) the lungs from the throat, allowing for thinner walls in the air sacs.Then when a population of this ancestral teleost moved back into open water, acquiring oxygen from the air would be less important, but buoyancy in the larger volume of water would be very useful, and so the lungs would adapt to becoming strictly a buoyancy device.Now, I don't know which scenario (if either) is correct; for some reason, though, I am partial to the lungs-first scenario.

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