London | 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. 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, said Gillis.
In mammal embryos, the Sonic hedgehog gene sets up the axis of the limb in the early stages of development. In a hand, for instance, Sonic hedgehog tells the limb which side will be the thumb and which side will be the pinky finger, said Gillis.
In the later stages of development, Sonic hedgehog maintains outgrowth so that the limb grows to its full size. To test whether the gene functions in the same way in skate embryos, researchers inhibited Sonic hedgehog at different points during their development.
They found that if Sonic hedgehog was interrupted early in development, the branchial rays formed on the wrong side of the gill arch. If Sonic hedgehog was interrupted later in development, then fewer branchial rays formed but the ones that did grow, grew on the correct side of the gill arch – showing that the gene works in a remarkably similar way here as in the development of limbs.