Look at your hands – probably something you don’t stop to consider much, if at all. They’re such an intrinsic part of ourselves, and the that way we live day-to-day, that we often overlook them. But our hands are incredible feats of limb evolution. Our manual dexterity, our ability to articulate our limbs in ways that allow us to grasp, climb and run, have been acquired gradually through evolution. But could it be that our limbs have an evolutionary legacy stretching back to before limbs had even first appeared?
Cue the little skate (Leucoraja erinacea) – a member of the cartilaginous fish (all skates, rays and sharks – with cartilage skeletons), found in abundance in the North Atlantic. It doesn’t possess any limbs, but it does have structures called gill arches – unique to cartilaginous fish. Gill arches and limbs do have some immediate resemblances – located in roughly the same body region, both being paired appendages. They also have ‘branchial rays’ – finger-like extensions, which join to the gill arch in a manner similar to the way the arms joins to the shoulder. Researchers from the University of Cambridge, led by Dr. Andrew Gillis, may just have found the connection between these gill arches and limbs.
In 1878, a German anatomist called Karl Gegenbaur theorised that fins and limbs evolved from gill arches, also pointing to the little skate as his primary example. “Gegenbaur looked at the way that these branchial rays connect to the gill arches and noticed that it looks very similar to the way that the fin and limb skeleton articulates with the shoulder,” said Gillis. “The branchial rays extend like a series of fingers down the side of a shark gill arch.”
However, since Gegenbaur’s proposal, there hasn’t been any fossil evidence showing this transition. The Cambridge researchers used modern genetic approaches to find the connection, finding surprising amounts of common ground in the genetic mechanisms used in both the embryonic development of the limb and gill arch.
One of the most crucial genes in limb development is a gene called ‘Sonic Hedgehog’ (Shh). The gene was given its cartoonish name after researchers saw fruit flies with mutations involving Shh develop ‘spikey’ features on their back. Shh has many roles in the developing embryo, but its key function in forming the limb is to direct its orientation – determining which side eventually forms the ‘thumb-side’ and which forms the ‘pinky-side’. It is also required to continue the outgrowth of the budding limb, so that it extends to full length. The researchers, already knowing from earlier studies that Shh is present in the developing gill arches, wanted to understand its function in gill arch development.
One of the best ways to understand what it is a gene does, is to disrupt it somehow, and see how it affects the organism. Doing exactly that, knocking Shh out at different stages of development, they found that its function was very similar to that in the limb. Knock Shh out early on, the branchial rays formed on the wrong side of the gill arch – a bit later, and only a few of the rays formed, but on the correct side.
This shows that at least some aspects of Gegenbaur’s theory was correct – though the results do not conclusively point to limbs evolving from gill arches, it demonstrates the similarity of the genetic pathways underlying their development. It could be that the two structures evolved separately, but that the existing genetic pathway for gill arches was co-opted for the development of the limb. No sense in re-inventing the wheel, after all.
To further study the relationship between limbs and gill arches, the research team plan to look at other important genes in limb development, to see what roles they may play in the gill arches. The evolution of paired appendages, like fins and limbs, is one of the major landmark events of the history of life on Earth. It enabled many of the key steps in our own evolution, from the first emergence of life onto land, to the opposable thumbs in our primate ancestors. But more research is turning up evidence that our the origins of our hands may stretch much further back in time than you might expect.
Gillis, J. A., & Hall, B. K. (2016). A shared role for sonic hedgehog signalling in patterning chondrichthyan gill arch appendages and tetrapod limbs. doi: 10.1242/dev.133884
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