Origins of The Vertebrate Lung
The lungs of today's terrestrial vertebrates and the gas bladders of today's fish are believed to have evolved from simple sacs (outpocketings) of the esophagus that allowed early fish to gulp air under oxygen-poor conditions. These outpocketings first arose in the bony fish; in some of the ray-finned fish the sacs evolved into gas bladders, while in other ray-finned fish (such as the gar, bichir and amia) as well as the lobe-finned fish they evolved into lungs. The lobe-finned fish gave rise to the land-based tetrapods. Thus, the lungs of vertebrates are homologous to the gas bladders of fish (but not to their gills). This is reflected by the fact that the lungs of a fetus also develop from an outpocketing of the esophagus and in the case of gas bladders, this connection to the gut continues to exist as the pneumatic duct in more "primitive" teleosts, and is lost in the higher orders. (This is an instance of correlation between ontogeny and phylogeny.) No known animals have both a gas bladder and lungs.
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