A group of geneticists from the University of Cambridge and the Marine Biological Laboratory have shown that human limbs may have evolved from the gills of cartilaginous fish such as sharks or rays. It turned out that the same gene, called Sonic hedgehog, is responsible for the development of the branchial arches and human arms and legs. The research results are published in the Development journal.
For the first time, the hypothesis of the origin of fish fins, and then of the limbs of land animals from the gill structures of cartilaginous fish, was proposed in 1878 by the German anatomist Karl Gegenbauer. Cartilaginous fish, in contrast to bony fish, have gill septa, which are attached to the arches of the cartilaginous arch and protect the gills from damage. Finger-shaped branchial petals extend from the arches themselves. However, none of the fossils found supported the anatomist's assumption.
Scientists have tested Gegenbauer's idea using the latest genetic techniques in studies of stingray embryos and found similarities in the mechanisms involved in the formation of gill arches in cartilaginous fish and limbs in humans. According to biologists, the Sonic hedgehog gene, named after the hero of computer games Sonic the Hedgehog, plays a key role.
Sonic hedgehog, which is involved in the development of human limbs and determines the formation of each toe, was also important for the formation of branchial rays in the ray embryos. In mammals, the gene indicates to the body how the developmental axes of the limbs will be located, for example, which side will be the thumb, and which side will the little finger. Later, he begins to support the growth of the limb. To test whether a gene performs the same function in fish, scientists suppressed its activity at various stages of embryonic growth.
If the Sonic hedgehog is inhibited early in development, the branchial lobes will form on the wrong side of the arch. In the later stages, interruption of gene activity led to the fact that the gill lobes were underdeveloped. The gene works in a similar way with human limbs.
Geneticists emphasize that the work does not prove the origin of human hands and feet from gills, but in any case, their development is controlled by the same genetic program, which means that they have a fundamental evolutionary relationship. Scientists hope that the new fossils will help answer the question of whether the limbs of animals really originated from gills or whether they were formed separately, but borrowing the same mechanisms for themselves.