| MadSci Network: Evolution |
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The problem of which evolved first, the lung or the swim bladder, is complicated because their developmental origins are similar. Both arise from an outgrowth of the gastrointestinal tract.
You might think that the ancestor of all fish would need a swim bladder, an organ whose function is to control bouncy. You might also think that fish, since they live in water, would have no need for lungs, which are used to breathe air. |
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Ah, if only the world were so simple! However, there are fish with lungs and fish without swim bladders! There is evidence, based on the phylogenetic distribution of the two organs, that lungs are the more primitive of the two. But perhaps this is an overly simplistic view. You can easily see how a lung could act as a swim bladder (you float better when your lungs are inflated) and a swim bladder as a lung (oxygen in a inflated sack could be recovered and used for respiration). Fish with jaws and fish capable of living in fresh water first arose in the Silurian period. The current consensus (as far as I can make out) is that earliest lungs were sacks used to hold air gulped in response to low levels of oxygen in stagnant pools. It is thought that drought (and hence fish stranded in stagnant pools) was common in the early Devonian period. The development of a "back-up' breathing system would therefore have been quite advantageous, even if it worked rather poorly. Once developed, the protolung/gas sack could be easily modified to improve its efficiency as a gas exchanger. This would also make it possible to use regulator of density, i.e. to function as an swim bladder. As Romer and Parsons (1977) state, "The swim bladders is found only in one subdivision of the bony fishes, the actinopterygians, whereas the most primitive member of that group, and members of the Sarcopterygii as well, have a lung.& quot; |
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