United Kingdom, University of Cambridge – A gene known as Sonic Hedgehog has been determined to play an important role in the development of both human and skate fish limbs, allowing scientists to pose the question of whether human limbs may have evolved from sharks’ gills.
Scientists analyzed embryos of skate fish, a variety of fish that was initially thought to have a close relationship to the development of human limbs. This theory was first presented in 1878 and it has now surfaced yet again as a real possibility.
The study on the development of limbs
The study was carried on by the University of Cambridge, and it is based on the theories of German anatomist Karl Gegenbaur, who suggested that paired limbs and fins, such as human arms and legs or shark fins, happen to evolve from a structure that is closely related to the gills of skate fish.
Cartilaginous fishes, such as it is the case of skate fish, rays and sharks, have folds of skin that protect their gills. They are supported by arches made of cartilage, these appendages are known as branchial rays.
The gene responsible for the correct development of each finger of the human body is known as “Sonic The Hedgehog,” named after the video game character. The functions that this gene carries on are also found in the evolution of the branchial rays of skate fish.
To test this, the lead author of the study Dr. Andrew Gillis and his team inhibited the gene at different points during the development of embryos of skate fish. The research team noted that if they stopped the action of the Sonic The Hedgehog gene, the branchial rays would form in the wrong side of the gill arch, a process that is enormously similar to how human limbs develop.
“In a hand, for instance, Sonic hedgehog tells the limb which side will be the thumb and which side will be the pinky finger” says Gillis.
To date, there has not been any fossil evidence to back up this theory, but the research efforts do provide a better understanding of the anatomical origin of jawed vertebrates, the group where humans are included. Gegenbaur’s theory states that fins and limbs evolved through a transformation of a gill arch in a common ancestral vertebrate.
Dr. Gillis added that “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.” Paired limbs are one of the features that set jawed vertebrates apart from other groups, so it is of great interest for scientists and researchers to find out more about the origins of paired appendages such as fins and limbs.