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The origin of lungs, the swim bladder as an oxygen-enriched diving system
The illustration of the swim bladder in fishes ... shows us clearly the highly important fact that an organ originally constructed for one purpose, namely, flotation, may be converted into one for a widely different purpose, namely, respiration. The swim bladder has, also, been worked in as an accessory to the auditory organs of certain fishes. All physiologists admit that the swim-bladder is homologous, or ideally similar in position and structure with the lungs of the higher vertebrate animals: hence there is no reason to doubt that the swim bladder has actually been converted into lungs, or an organ used exclusively for respiration. According to this view it may be inferred that all vertebrate animals with true lungs are descended by ordinary generation from an ancient and unknown prototype, which was furnished with a floating apparatus or swim bladder.
- Charles Darwin, On the Origin of Species by Means of Natural Selection, 1853
The origin of the lung is the story of the origin of the swim bladder. So, what is the origin of the swim bladder? Historically, it has been assumed to be an adaptation for buoyancy. While this is true, there is a second function of the swim bladder that is not as widely recognized. The swim bladder is able to adjust the blood gas ratios (Hall, 1924), an adaptation that allows for increased vertical range of movement.
The ocean is in an equilibrium with the atmosphere, and the dissolved gas is primarily nitrogen. The ocean gases are then in equilibrium with the blood and tissue of the organism. The organism consumes oxygen in cellular respiration, but not nitrogen, making nitrogen more prone to accumulate in tissue. During ascending vertical movement, the pressure exerted on the organism from the ocean decreases. This reduction in the pressure on the nitrogen that is dissolved in solution within tissues of the body, will cause bubble formation. These bubbles can damage tissues.
The evolution of the swim-bladder as an oxygen-enriched diving system, allowed organisms to reduce nitrogen in their tissues. The swim bladder does this in a way that is analogous to human divers using technology for oxygen-enriched air when diving, and it does it for the same reasons, to prevent decompression sickness.
This function of the swim bladder, an oxygen-enriched diving system, is not widely explored, and it might even be the primary selective pressure for the evolution of swim bladders, and therefore, for lungs.
References
HALL, F. G. (1924). THE FUNCTIONS OF THE SWIMBLADDER OF FISHES. The Biological Bulletin, 47(2), 79-[126]-1. https://doi.org/10.2307/1536532