The genetic toolkit that controls the development of gills in sharks, is the same as that used by animals to build fins and limbs, according to research published in Proceedings of the National Academy of Sciences.

Andrew Gillis, lead author of the paper, says “This new finding is consistent with an old theory, often discounted in science textbooks, that fins and (later) limbs evolved from the gills of an extinct vertebrate. A dearth of fossils prevents us from definitely concluding that fins evolved from gills. Nevertheless, this research shows that the genetic architecture of gills, fins and limbs is the same.”

The research builds on the breakthrough discovery of the fossil Tiktaalik, a “fish with legs,” by Neil Shubin and his colleagues in 2006.

According to the researchers, the genetic circuitry that patterns paired appendages (arms, legs and fins) has a deep evolutionary origin that actually predates the origin of paired appendages themselves.

They found that treating a skate embryo (a relative of the shark) with Vitamin A’s retinoic acid, affected the gill arch skeleton in the same way as an arm, leg or fin skeleton.

The deep structural, functional and regulatory similarities between paired appendages and developing gill rays, as well as the antiquity of gills relative to paired appendages, suggest that the signalling network that is induced by retinoic acid had a patterning function in gills before the origin of vertebrate arms, legs or fins, the research concludes. And this function has been retained in the gill rays of living cartilaginous fishes like skates, rays and sharks.

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