ABSTRACT
The onset of air breathing in vertebrate evolution is believed to have taken place more than 350 million years ago in the Silurian and Devonian geological periods. The prevailing climatic conditions then led to a marked reduction in the oxygen concentration of large freshwater basins. To survive, the aquatic animals have followed various strategies for utilizing atmospheric oxygen. Gas-exchange areas have appeared in various corporal cavities and the skin. One of these structures, the air bladder (or swim bladder) of fish, was destined for a great evolutive fate, since it was at the origin of the lungs of tetrapodia. The air bladder was probably firstly used as an accessory respiratory organ; this function has been lost in most modern fish, but has been retained in some archaic forms which it probably prevented from extinction. In other phylla of vertebrates, this endodermal outgrowth from the gut evolved into more and more finely subdivided and vascularized pulmonary structures.
