Evolutionarily, the chicken follows the more primitive shark, skate and sturgeon. However, a genetic technique for manipulating the avian egg has now provided clues about fin development in these earlier animals (see page 311).

When postdoc Randall Dahn joined Neil Shubin's evolution and development lab at the University of Chicago in Illinois, he brought with him a molecular-biology tool kit for manipulating chicken eggs. Dahn investigated how genetic tweaks affected the birds' development. “The goal was to link that to palaeontological and evolutionary standpoints,” Shubin says.

They decided to study limb and fin development in more primitive organisms, following reports that sharks develop fins without Sonic hedgehog (Shh), a gene thought to be essential for the process in many organisms. Dahn used his tool kit on shark and skate eggs. “Randy treating shark like chicken was brilliant,” says Shubin. Postdoc Marcus Davis did something similar to the eggs of sturgeon and other primitive fish.

They found that Shh does have a role in fin development in the organisms they studied. This shows that some Shh-related mechanisms are deeply conserved in vertebrates. And, they suggest, variations in Shh signalling among species could give insight into appendage evolution.