The branchial rays extend like a series of fingers down the side of a shark gill arch.
- Andrew Gillis
An idea first proposed 138 years ago that limbs evolved from gills, which has been widely discredited due to lack of supporting fossil evidence, may prove correct after all – and the clue is in a gene named for everyone’s favourite blue hedgehog.
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, influential German anatomist Karl Gegenbaur presented the theory that paired fins and eventually 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 have reinvestigated Gegenbaur’s ideas using the latest genetic techniques on embryos of the little skate – a fish from the very group that first inspired the controversial theory over a century ago – and found striking similarities between the genetic mechanism used in the development of its gill arches and those in human limbs.
Scientists say it comes down to a critical gene in limb development called ‘Sonic hedgehog’, named for the videogame character by a research team at Harvard Medical School.
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. The findings are published in the journal Development.
Dr Andrew Gillis, from the University of Cambridge’s Department of Zoology and the Marine Biological Laboratory, who led the research, says that 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.
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Image:Head skeletons of skate and shark showing gill arch appendages in red.
Credit: Andrew Gillis
Reproduced courtesy of the University of Cambridge
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