Genetic Evidence Confirms That Swim Bladders In Fish Are Homologous To Lungs In Tetrapods

A swim bladder

A diagram of a heterocercal tail. The
dorsal lobe would evolve into the tail

Darwin himself, in his world famous book The Origin Of Species, said that swim bladders in fish are evolutionarily homologous to lungs in tetrapods. Swim bladders are only possessed by cartilaginous fish which only evolved when they split from their ancestors, the bony fish. This has long suggested that swim bladders are the same in both animal groups, but are expressed genetically in different ways. Further evidence comes from the tail shape in bony fish.

The heterocercal tail present in bony fish has a longer upper tail lobe into which the vertebrae extend and would eventually evolve into the tail in amphibians. This has been a long-standing and popular theory. However such a theory cannot be confirmed by fossil evidence as there would not be enough morphological distinction in the structure between the early forms of the two animal groups after they split from the common ancestor. Therefore the only way of proving if the theory was correct was to find indisputable genetic evidence.

Of course it is impossible to analyse the genes of fossils. DNA is a very fragile molecule and can easily be damaged or disintegrate if disturbed. No traces ever preserve during fossilisation. The only way to solve this puzzle would be to analyse the modern descendants of each group. No evidence was found...until now. A team of geneticists from the Auburn University, America studied the development of embryos of the cartilaginous zebra-fish and the embryos of mice and humans.

They found that in the early stages of development, the transcription genes hoxc4a, hoxc6a, hoxc8a and foxf1 were especially prominent in the development of the swim bladder organ. Yet they were expressed in rather different ways. This difference gave rise to lungs in bony fish which would eventually allow the first amphibians to gulp air and rise from the water onto the dry, air filled and arthropod infested terrestrial world. Gene analysis of modern descendants is becoming a far more prominent study when trying to piece together ancient evolutionary puzzles.