Maybe human beings limbs evolved from shark gills

Head skeletons of skate and shark showing gill arch appendages in red (Image Andrew Gillis)
Head skeletons of skate and shark showing gill arch appendages in red (Image Andrew Gillis)

An article published in the journal “Development” describes a genetic study which suggests that the limbs of tetrapods, and consequently those of humans, might have evolved from the gills of cartilaginous fishes such as sharks and skates. A new analysis shows that a genetic program is shared between those gills and human limbs finding possible evidence to a theory that was discarded over a century ago.

Karl Gegenbaur (1826-1903) was a famous German anatomist whose comparative anatomy studies were important among other things for the developments in research on evolution. In 1878 he proposed the hypothesis of the evolution of the limbs of tetrapods from a structure similar to the gill arch of cartilaginous fishes. No one could find evidence in the known fossils and that idea was abandoned but perhaps Gegenbaur’s intuition  was right.

A team led by Dr. Andrew Gillis from the Cambridge University’s Department of Zoology and Marine Biological Laboratory carried out a research on embryos of a species called little skate, scientifical name Leucoraja erinacea. Skates are part of the group of cartilaginous fishes like sharks and, unlike other fishes, have skin flaps supported by arches of cartilage that protect their gills. These flaps are supported by arches called gill rays that have a finger-like look.

The result of this research was the discovery of similarities at genetic level between the mechanism of development of the gill arches in those skates and in that of human limbs. In particular, scientists identified a gene crucial for the development of limbs that was called “Sonic hedgehog” after a series of video games. This gene determines the identity of each finger and the growth of the limb skeleton in humans and, in a similar manner, of gill rays in skate embryos.

Andrew Gillis’ team conducted a number of experiments to test the influence of the Sonic hedgehog gene in Leucoraja erinacea’s embryos. If this gene is inhibited in the first stage of embryonic development the gill rays formed but on the wrong side of the gill arch. If this gene is inhibited during a later stage of development fewer gill rays formed but on the right side of the arch.

These experiments can be interpreted as evidence that limbs share a genetic program with gill arches because fins and limbs evolved from the transformation of a gill arch in a primitive vertebrate, as suggested by Karl Gegenbaur. However, Andrew Gillis himself offers the alternative hypothesis that those structures evolved separately reusing the same genetic program.

The problem is the same that led to abandon Karl Gegenbaur’s hypothesis: the fossil record is incomplete with a gap between species without fins and species with fins. Andrew Gillis and his collaborators are carrying out further research on the function of other genes involved in the development of skates gills and of mammals limbs.

Modern genetic techniques could compensate for the lack of fossils that limits paleontological research on the development of limbs in tetrapods, an event that deeply affected their physical appearance. Perhaps Karl Gegenbaur’s intuition was correct, anyway research such as the one conducted by Andrew Gillis’ team is providing new information on an important phase in the evolution of a large amount of animals and human beings too.

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