Human limbs may have evolved from sharks' gills

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London, April 20 (IANS) Human limbs share a genetic programme with the respiratory organs of fishes such as sharks and skates, reveals a new research, providing evidence to support a century-old, widely discounted theory on the origin of limbs.

German anatomist Karl Gegenbaur, in 1878, first proposed the theory that human limbs were evolved from gills -- respiratory organs of sharks and skates. 

These fishes have a series of skin flaps -- that protect their gills -- and are supported by arches of cartilage -- a connective tissue found joints between bones -- with finger-like appendages called branchial rays attached to them.Investigation into the embryos of the little skates -- cartilaginous fishes -- revealed striking similarities between the genetic mechanisms used in the development of its gill arches and those in human limbs.

The researchers found 'sonic hedgehog' -- a critical gene in limb development -- performing the same two functions in the development of gill arches and branchial rays in skate embryos as it does in the development of limbs in mammal embryos.In mammal embryos, the Sonic hedgehog gene sets up the axis of the limb in the early stages of development. 

In human limb development, it dictates the identity of each finger and maintains the growth of the limb skeleton.In the later stages of development, Sonic hedgehog maintains outgrowth so that the limb grows to its full size."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 Andrew Gillis from the University of Cambridge in Britain. 

The study, detailed in the journal Development, provides greater understanding of the origin of jawed vertebrates -- the group of animals that includes humans.

"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," Gillis explained.​

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