"There's a lot of literature on the evolution of limbs, but it doesn't really consider the neuronal side of things because it's much harder to study," New York University's Jeremy Dasen tells Smithsonian magazine. "There's no fossil record for neurons and nerves."
Instead, Dasen and his colleagues studied the genes of the little skate, Leucoraja erinacea, which walks on the seafloor using its fins in a manner similar to land animals. As they report in Cell, they found that the motor neurons of skate and mammals share a conserved genetic program that is controlled by a Hox-dependent regulatory program. This suggested to the researchers that gene networks needed for walking were present in the common ancestor of skates and mammals, which lived some 420 million years ago.
"These findings suggest [that] the genetic program that determines the ability of the nerves in the spinal cord to articulate muscles actually originated millions of years earlier than we have assumed," Dasen adds at Forbes.
However, Michael Coates from the University of Chicago advises caution at Nature News and says a wider range of animals needs to be analyzed.