A new study by a University of Chicago researcher has revealed that fish possessed the genes associated with helping to grow hands and feet (autopods) much before animals with limbs (tetrapods) came onto the scene about 365 million years ago.
Lead author of the study in the Nature, Dr Marcus Davis said, the findings overturned a long held, but much debated theory that limb acquisition was a novel evolutionary event and "required the descendents of lobed-fin fish to dramatically alter their genes to adapt their bodies to their new environments of streams and swamps".
"An autopodial-like pattern of 'Hox gene' expression in the fins of a basal actinopterygian fish, shows that the genetic and developmental toolkit that builds limbs with fingers and toes was around long before the acquisition of limbs, and that this toolkit exists in some primitive form in a living primitive bony fish, the paddlefish. We found that the genetic capability seen in tetrapods to build limbs is present in even more primitive fish," said Dr. Davis.
For their study, Dr Davis and his colleagues used the paddlefish as a proxy for a more primitive ancestor, as these fish have an elaborate fin skeletal pattern similar to that seen in more primitive vertebrates such as sharks and many fossil fish.
The researchers studied the development of paddlefish fins to test whether the genes activated to make hands and feet in tetrapods were different from the genes activated to make fish fins.
They looked at Hox genes—which play a vital role in limb development—in the pectoral fins of paddlefish, and to track where the Hox genes are active in the fin, inserted molecular markers into them.
Findings revealed that the activity pattern in the pectoral fins was very much similar to the genes responsible for the development of tetrapod limbs.
Tetrapods have a second phase of Hox gene expression that happens later in development. During this second phase, hands and feet develop, and the scientists found this was present in the paddlefish.
In other words, the pattern of gene activity long thought to be unique to vertebrates with hands and feet is much more primitive, said Dr. Davis.