It has long been believed that the lungs of land vertebrates like us humans evolved from "swim bladders" -- gas-filled sacs in bony fish that help them adjust their depth. However, newly published research by a team including scientists at Jikei University School of Medicine in Tokyo shows that swim bladders and land vertebrate lungs both evolved from the primitive lungs of a common ancestor.
Most fish have a swim bladder, which they can expand and contract to help them rise and descend in the water. In his revolutionary work "On the Origin of Species" (1859), British naturalist Charles Darwin wrote that the lungs of land vertebrates evolved from fish swim bladders.
Jikei University medical school professor Masataka Okabe and other members of the research team believe that the prehistoric fish species polypterus -- thought to be the most ancient ancestor of modern bony fish and which can still be found in the wild today -- may be the origin point of both swim bladders and land vertebrate lungs.
Polypterus have lungs, not a swim bladder, and the team found that these lungs grow and develop in much the same way as those of land vertebrates. Furthermore, three DNA sections indispensable for growing lungs behave the same way in both polypterus and land vertebrates, leading the research team to conclude that both types of animals develop lungs using the same genetic mechanism.
Based on these observations it appears that land vertebrate lungs did not evolve when fish first began venturing onto land, but that these fish had primitive lungs already -- lungs predating swim bladders, the team concluded.
"You can say that human lungs and the swim bladders in regular fish actually evolved from the primitive lungs of a common ancestor," Okabe commented.