From shark gills to human limbs? 'Sonic hedgehog' gene revives an old idea.
The evolution of human limbs originated in sharks鈥� gill arches, suggests Andrew Gillis, a biologist and research at the University of Cambridge in a paper .
Sharks, rays, and skates are cartilaginous fish, which means their gills are protected by arches made of cartilage rather than bone. These protective cartilage-arches have branchial rays: appendages that branch out like fingers or toes. Researchers have long suspected that gill arches were the evolutionary foundation for fins, which became the evolutionary foundation for paired limbs in mammals, but no fossil evidence has ever been found to confirm the idea. But now, Dr. Gillis says that genetic evidence can support this hypothesis.
Scientists have already confirmed that the development of fins and fingers is associated with the Sonic hedgehog gene, named for the Sega video game character. But Gillis and his colleague, Brian Hall, studied the same gene in a skate embryo, and by tweaking a protein at various stages of the embryo鈥檚 development, Gillis and Dr. Hall found that the Sonic hedgehog expression was indeed responsible for the development of the branchial rays as well.
鈥淭he shared role of Sonic hedgehog in patterning branchial rays and limbs may be due to a deep evolutionary relationship between the two,鈥� said Gillis in a press release from the Marine Biological Laboratory at Woods Hole, 鈥渙r it may simply be that two unrelated appendages independently use .鈥�
German anatomist Karl Gegenbaur first suspected that gill arches were the evolutionary foundation for first fins and then limbs back in 1878. But because Gegenbaur looked at the connection through an anatomical lens, and there were no fossil records to support his hypothesis, his idea was discounted.
鈥淕egenbaur speculated that gill arches and fins/limbs were evolutionary related because they appear to be built ,鈥� Gillis told Gizmodo. A conclusion that seems more than reasonable when simply comparing X-rays of gill arches and the human hand. But Gillis says he was finally able to back up Gengenbaur鈥檚 prediction by 鈥渋dentify[ing] a molecular feature that could be a key part of that ground plan.鈥澛�
But Gillis and Hall鈥檚 study doesn鈥檛 necessarily confirm Gegenbaur鈥檚 hypothesis. While it is still possible that the Sonic hedgehog gene鈥檚 development in gill arches led to its fin counterparts and then later human fingers, it is also possible, say Gillis and Hall, that branchial rays of gills and fins simply utilize the same gene independently of one another.
鈥淓ither way this is a fascinating discovery, because it provides evidence for ,鈥� said Gillis in a press release from the University of Cambridge. 鈥淲hile palaeontologists look for fossils to try to reconstruct the evolutionary history of anatomy, we are effectively trying to reconstruct the evolutionary history of genetic programmes that control the development of anatomy.鈥澛�
Even though the fossil record cannot yet confirm this hypothesis, Gillis says researchers should continue to study evolutionary connections through shark, rays and skates instead of 鈥渕utant mice of fruit flies.鈥�
鈥淚t is useful to study cartilaginous fishes, not only because they were the group that first inspired Gegenbaur鈥檚 theory, but also because they have a lot of unique features that other fishes don鈥檛,鈥� Gillis said in the Cambridge release, 鈥渁nd we are finding that we can learn a lot about evolution from these unique features.鈥澛�
