Put down that shark steak, you cannibal. A new study has found an evolutionary link between sharks and humans. It involves shark gills, human limbs and Sonic the Hedgehog. Wait, what?
The study, published in Development, found that human limbs have a genetic connection with the gills and gill flaps of sharks, skates, rays and other cartilaginous fish (members of the Chondrichthyes species having paired fins and cartilage instead of bones). If that sounds familiar, you’re up on your discounted evolutionary theories because it was first proposed in 1878 by German anatomist Karl Gegenbaur. He believed that human limbs evolved from something that looked like a shark’s gill arch – the cartilage in gill skin flaps whose branchial rays (appendages) look like fingers. Nice idea, but Gegenbaur had no photos or fossils to prove it.
Enter zoologist Dr. Andrew Gillis from the University of Cambridge’s Department of Zoology and the Marine Biological Laboratory. He looked at embryos from skates using something Gegenbaur didn’t have – genetic analysis. He found that the gene that triggers gill arches in skate embryos is the same one that triggers limb development in human embryos – the Sonic hedgehog gene.
In a hand, for instance, Sonic hedgehog tells the limb which side will be the thumb and which side will be the pinky finger.
Gillis and his researchers suppressed the Sonic hedgehog gene at various stages in skate embryo growth. They found that the deformations in branchial ray development caused by the genetic disruption paralleled the same deformations which occur when the gene is interrupted in human embryo development.
Gillis is probably tired of being asked if this means that humans evolved from sharks.
Taken to the extreme, these experiments could be interpreted as evidence that limbs share a genetic programme with gill arches because fins and limbs evolved by transformation of a gill arch in an ancestral vertebrate, as proposed by Gegenbaur. However, it could also be that these structures evolved separately, but re-used the same pre-existing genetic programme. Without fossil evidence this remains a bit of a mystery – there is a gap in the fossil record between species with no fins and then suddenly species with paired fins – so we can’t really be sure yet how paired appendages evolved.
In evolution and in subways, it’s important to mind the gap.