Rare shark-like fish reveals evolutionary quirk

The elephant fish is an unusual shark relative that lives in the waters around Australia and New Zealand. Divers recently brought back rare examples of its embryos, which reveal the shocking evolutionary similarities between elephant fish gills and human hands.

The embryos revealed which genes governed the formation of the fish's skeletal gill covers, and they turned out to be exactly the same one that reptiles and mammals use to form their fingers and toes. Considering the elephant fish went its separate ways from these animals over a hundred million years ago, this is a remarkable example of how adaptable genes can really be, and how good evolution is at finding new purposes for the same basic mechanisms.

Cambridge researcher Andrew Gillis explains:

"The research highlights how evolution is extremely efficient, taking advantage of preexisting mechanisms, rather than inventing new ones. By simply tinkering with the timing of when or where a gene is expressed in an embryo, you can get very different anatomical outcomes in adults."

Indeed, that timing of expression means dramatic differences in the embryonic development of the elephant fish and one of its closest relatives, the dogfish. Because the gene is expressed in a slightly different place and at a slightly different time in the elephant fish's developments, its gill covers end up looking different from those of the dogfish. And yet, from a much broader perspective, the elephant fish has essentially the same structures as lizards, dogs, and even humans.

Fellow researcher Neil Shubin of the University of Chicago uses a metaphor to explain these differences:

"You have a common nail that's used for many different pieces of furniture. This esoteric fish with this esoteric anatomical system is showing us something very fundamental about the evolutionary tree: that there's a common process at work among disparate types of organisms. It's basically showing that the limb story is part of a much more general narrative, which is the story of outgrowths. There's a common development toolkit for all the outgrowths that we know in the body; they're all versions of one another in a developmental sense."

The story of evolution is a vast and complicated one, but it's also got a lot of parallel storytelling and built-in redundancies. And, as the elephant fish reveals, once the evolutionary process happens upon a good genetic mechanism, it doesn't give it up easily no matter where the organisms go, even after more than a hundred million years.

[Proceedings of the National Academy of Sciences]