Way back in 1878, German biologist Karl Gegenbaur proposed a theory that human limbs evolved from the gill arches of sharks, skates, and rays. It was criticized and rejected by the scientific community due to lack of evidence. More than 130 years later, new genetic evidence suggests that Gegenbaur was right all along! A study conducted by Andrew Gillis of the University of Cambridge has found a genetic link between human limbs and shark gills: the Sonic hedgehog gene.
Sonic hedgehog plays the same role in all three features
Findings of the study were published in the latest issue of the journal Development. Gegenbaur had said that gill arches laid the foundation for paired fins in ancient fish, which later paved the way for limbs in mammals. While studying the embryonic development of gill arches, fins, and limbs, researchers identified a gene called “Sonic hedgehog” that plays a similar role in the formation of all three features.
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The gene produces a vital signalling protein that helps determine the axis of development and later controls the growth of limb skeleton. It means the Sonic hedgehog gene dictates which fingers go where, and then ensures that the limb skeleton grows to its full size. In cartilaginous fish, the arches of cartilage have finger-like appendages called branchial rays. The Sonic hedgehog gene has the same function in the development of branchial rays.
A deep evolutionary relationship
Lead author Dr. Andrew Gillis said in a statement that the “shared role of Sonic hedgehog in patterning branchial rays and limbs may be due to a deep evolutionary relationship between the two.” In 1878, Gegenbaur studied the way branchial rays connect to the gill arches, and noticed that it was pretty similar to the way the limb and fin skeleton articulates with the shoulder.
To test whether Sonic hedgehog works the same way in skate embryos, scientists blocked the gene at different stages during their development. When the gene was blocked early, the branchial rays formed on the wrong side of the gill arch. When researchers blocked it at a later stage, only a few branchial rays appeared, but they grew on the correct side.