It’s already been well established that as mammal embryos develop, including our own, they form gill slits early on in the process of development. These then go on to form different parts of the fetus's head, with the last gill arch creating the structure of the lower jaw. But a more radical theory, first suggested over a century ago, proposed that deeper in our evolutionary past, the gill arches were also the progenitor of arms. 
Widely dismissed due to a lack of supporting evidence, a new study might have given a bit of credibility to this notion. Researchers from the University of Cambridge have found that the same gene that controls digit growth and patterning in all tetrapods is also involved in the development of the gill arches in sharks, skates, and rays. While this could suggest a shared developmental origin, it could equally be that the two separate functions simply use the same underlying genes to control their development. Either way, it provides a fascinating insight into the evolution of animals.
It was in 1878 that the link between gill arches and tetrapod forelimbs was made by German anatomist Karl Gegenbaur. In sharks and rays, the gill arches are supported by finger-like appendages called branchial rays. He based his conclusions solely on how these branchial rays look and fit with the gill arches.
“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,” explains Dr. Andrew Gillis, who co-authored the study published in the journal Development, in a statement. “The branchial rays extend like a series of fingers down the side of a shark gill arch.”
The new research found that the gene dictating this “finger-like” pattern in the branchial rays was the same that plays a vital role in the development of digits in tetrapods, the somewhat interestingly named Sonic hedgehog gene. In the limb, it is this gene that determines which side will form the thumb and which the little finger, as well as how long each finger grows.
By producing skate embryos in which the Sonic hedgehog gene was programmed to turn off at different points in development, they were able to determine that it is responsible for not only making sure that the brachial ray formed on the right side of the gill arch, but that it was involved in determining the number of “finger-like” appendages. This has a striking similarity to how it is used in tetrapod limb development.
“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,” says Dr. Gillis. “However, it could also be that these structures evolved separately, but re-used the same pre-existing genetic programme.”
Without the fossils of an intermediate organism to show a link between the gill arches and the limbs, it will be very difficult to prove either way whether Gegenbaur was in fact correct all along.
Image in text: The researchers conducted their experiments on skate embryos. Andrew Gillis/University of Cambridge
2