Although the century-old theory that human limbs evolved from gills has been widely discounted, a new University of Cambridge study provides new support for it through an analysis that reveals a shared genetic program between human limbs and the gills of cartilaginous fishes such as sharks and skates. The original theory was proposed in 1878 by influential German anatomist Karl Gegenbaur, who claimed that paired fins and limbs evolved from a structure similar to the gill arch of these fishes.
Using genetic techniques, the team examined the embryos of the little skate, a fish that belongs to the group of fishes that inspired Gegenbaur's initial theory on the origins of human limbs, revealing numerous similarities in the genetic mechanisms that develop its gill arches to those seen in the development of human limbs.
The most crucial gene in limb development, named the "Sonic hedgehog" gene after the video game character, determines the identity of each finger during human limb development and also maintains the growth of the human skeleton. The new study shows that the functions of this gene are mirrored in the development of the branchial rays in skate embryos.
"Gegenbaur looked at the way that these branchial rays connect to the gill arches and noticed that it looks very similar to the way that the fin and limb skeleton articulates with the shoulder," said Andrew Gillis, a member of the University of Cambridge's Department of Zoology and the Marine Biological Laboratory and first author of the study. "The branchial rays extend like a series of fingers down the side of a shark gill arch."
"The fact that the Sonic hedgehog gene performs the same two functions in the development of gill arches and branchial rays in skate embryos as it does in the development of limbs in mammal embryos may help explain how Gegenbaur arrived at his controversial theory on the origin of fins and limbs," he added.
The team found that when they interrupted the Sonic hedgehog gene early in the developmental process of skate embryos, the branchial rays formed on the wrong side of the gill arch, whereas when it was interrupted later in the process, the branchial rays that did form were on the correct side of the gill arch. This process is extremely similar to the way that the gene works in the development of human limbs.
"Taken to the extreme, these experiments could be interpreted as evidence that limbs share a genetic program with gill arches because fins and limbs evolved by transformation of a gill arch in an ancestral vertebrate, as proposed by Gegenbaur," Gillis said.
"However, it could also be that these structures evolved separately, but re-used the same pre-existing genetic program," he added. "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."
The findings were published in the April 19 issue of the journal Development.