In 1878, German anatomist Karl Gegenbaur proposed a theory that fish fins and human limbs evolved from a structure that resembles gill arches, a collection of bony "loops" in fish that support the gills.
Unfortunately, the idea was widely discounted because no fossil record was discovered to support the theory.
More than a century later though, Gegenbaur's idea is getting support. New genetic evidence has linked the evolutionary origins of the paired fins in early fish and eventually the paired limbs in mammals to the gill arches of cartilaginous fishes such as sharks, skates and rays.
In a research published in the journal Development, Andrew Gillis, from the University of Cambridge, and colleagues used the latest genetic techniques to study the embryos of the little skate and found a striking resemblance between the genetic mechanism behind the development of the gill arches and that in human limbs.
The researchers said it comes down to a key gene in limb development known as "Sonic hedgehog." The role of this gene in dictating the identity of fingers and maintaining the growth of the limb skeleton in human limbs was found to resemble that of the gill arches, or branchial rays, in skate embryos.
For the study, Gillis and colleagues inhibited the Sonic hedgehog gene at different points during the development of skate embryos and found that once the gene was interrupted early, the branchial rays formed on the wrong side.
If the interruption occurred later in development, fewer branchial rays formed. These grew on the correct side of the gill arch, showing that the gene works in a similar manner as the development of limbs.
"The shared role of Sonic hedgehog in patterning branchial rays and limbs may be due to a deep evolutionary relationship between the two, or it may simply be that two unrelated appendages independently use the same gene for the same function," Gillis said.
The researcher said that their findings show that aspects of the century-old Gegenbaur's theory may be correct and shed light on the origin of jawed vertebrate, the animal group that includes humans.
"These findings highlight striking parallels in the axial patterning mechanisms employed by chondrichthyan branchial rays and paired fins/limbs, and provide mechanistic insight into the anatomical foundation of Gegenbaur's gill arch hypothesis," Gillis and colleagues wrote.
