In an interesting twist of fate, new genetic evidence shows that the evolutionary origins of human limbs actually lie in the gill arches of sharks, skates, and rays.
Gill arches is actually the name of the collection of looped bones that support gills and are the place where gill arch appendages sprout from. Biologists have suggested that gill arches were the evolutionary foundation for paired fins in early fish. This later paved the way for paired limbs in many mammals, including humans.
Now, we have more genetic evidence than ever to back up this theory.
When looking at the embryonic development of these gill arches and limbs, scientists found a gene that has been called the "Sonic the Hedgehog" because it plays a similar role in governing the formation of protruding spaces. It is a gene that produces a signaling protein that plays a key role in determining the axis of development. Later on in development, it also controls limb skeleton growth.
"The shared role of Sonic hedgehog in patterning branchial rays and limbs may be due to a deep evolutionary relationship between the two, or it may simply be that two unrelated appendages independently use the same gene for the same function," Andrew Gillis, a biologist and researcher at the University of Cambridge, said.
Gillis, appropriately named, is working on research at the Marine Biological Laboratory in Woods Hole, Massachusetts. He will continue to analyze other genes that are present in the embryonic development of gill arch appendages and limbs for the next few months.
"Branchial rays will figure prominently in the story of the evolutionary origin of vertebrate animal appendages, either by shedding light on the evolutionary antecedent of paired fins, limbs, or by teaching us about the genetic mechanisms that animals can use to invent new appendages," Gillis added.
His latest research was published this week in the journal Development.
The lab produced this video to help explain:
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