This is a good example. A fish grew a foot millions of years before it needed it on land. Thus, as generations of schoolchildren have learned, did these marine creatures give rise to frogs, birds, dinosaurs and all the rest. For one thing, the first creatures to waddle ashore were arthropods with well- developed legs and pincers. Moreover, in shape and function, Acanthostega's fully jointed toes bear no resemblance to the spiky, fanlike fins of a fish.
Scientists believe they understand how a fish's gills evolved into an amphibian's lungs.
The answer may be in the genes. That's the tantalizing conclusion of a team of researchers from the University of Geneva in Switzerland. They have discovered that genes associated with the formation of fins in fish are the same ones that orchestrate the development of paws in mice. What a mouse does is take a fin and put something extra on top of it. That something extra, Duboule and his colleagues suggest in the journal - Nature, is provided by a special set of genes that act as master architects in a surprisingly broad range of animals, from rodents to roundworms.