Science News is reporting on a study involving genes of modern fish that code for tetrapod limbs. Elizabeth Pennisi writes:
Further sequencing of lungfish showed something additionally startling:
In the trio of studies published last week in Cell, genes in living fish took the place of fossils as a way to peer back in time. One set of clues came from studies of mutagenized zebrafish, a favorite model for studying development. M. Brent Hawkins, then a Harvard University graduate student and now a postdoc, was shocked to discover zebrafish mutants with two bones resembling the forelimb bones of land animals in their front fins, complete with muscles, joints, and blood vessels. The finding is “quite spectacular,” says Marie-Andrée Akimenko, a developmental biologist at the University of Ottawa.
The genome of the lungfish offers a glimpse of later adaptations along the path to terrestrial life. It includes additional pulmonary surfactant genes that the ray-finned fishes lack, as well as DNA for specifying five toes, connecting nerves to limb muscles, and for sensitizing the brain to react fast. All those genes were previously thought to be unique to tetrapods.The authors conclude that, since modern teleost fish do not possess these genes, that they must have been present in the common ancestors of tetrapods and modern fish. The authors further note that all of these studies suggest that “...the origin of tetrapods was something that was waiting to happen.”