Human limbs may have evolved from shark gills

Human limbs may have evolved from the gills of fishes such as sharks and skates, according to new study that provides evidence to support the century-old but widely discounted theory on the origin of limbs.

Unlike other fishes, cartilaginous fishes such as sharks, skates and rays have a series of skin flaps that protect their gills. These flaps are supported by arches of cartilage, with finger-like appendages called branchial rays attached.

In 1878, German anatomist Karl Gegenbaur proposed that paired fins and limbs evolved from a structure resembling the gill arch of cartilaginous fishes. However, nothing in the fossil record has ever been discovered to support this.

Now, researchers at the University of Cambridge have reinvestigated Gegenbaur’s ideas using the latest genetic techniques on embryos of the little skate and found striking similarities between the genetic mechanism used in the development of its gill arches and those in human limbs.

Scientists said it comes down to a critical gene in limb development called ‘Sonic hedgehog’, named for the videogame character. The new research shows that the functions of the Sonic hedgehog gene in human limb development, dictating the identity of each finger and maintaining growth of the limb skeleton, are mirrored in the development of the branchial rays in skate embryos.

It shows aspects of Gegenbaur’s theory may in fact be correct, and provides greater understanding of the origin of jawed vertebrates — the group of animals that includes humans, said Andrew Gillis, from the University of Cambridge.