Researchers explore evolutionary link between shark gills, limbs

Is new research from the University of Cambridge suggesting the terrifying idea of sharks with limbs?

Well no, not really. But the research suggests our limbs share evolutionary history with shark gills. CBC Radio science columnist Torah Kachur explains.

What does the research say about sharks and limbs?

Fortunately, not that sharks will be walking — or worse, running — anytime soon.

The new research, which was recently published in the journal Development, looks at the evolutionary development of limbs in humans, and all jawed vertebrates.

Andrew Gillis, a Canadian now working at Cambridge, was one of the lead researchers. And he said it all started with a theory about how all limbs and fins developed that goes back more than a century.

"One idea in particular, which was a very old idea, that perhaps paired fins evolved through modification of a gill arch structure, which is the skeleton that supports the gills of fishes," said Gillis.

"And this was an idea that was proposed about 150 years ago by a German comparative anatomist named Carl Gegenbaur, and it was based on the anatomy of this group of animals that I'm studying now — the cartilaginous fishes."

So Gegenbaur's theory was essentially that fins, and eventually limbs, evolved from a structure resembling the gill arch found in these fish. The problem was that nothing in the fossil record supported this theory.

But now, the researchers from Cambridge are revisiting Gegenbaur's theory with new evidence related to genetics and how embryos develop.

How are gills and limbs related?

Sharks, skates and rays all have a flap of skin that covers their gills, which is unique compared with the other fish. 

Those flaps of skin have to be propped up a bit so they don't lay flat across the gills, and so they are supported by arches of cartilage.

It's this cartilage that has finger-like appendages called branchial rays — and they look a lot like the pattern we have in our hands.

These branchial rays attracted the attention of researchers, who decided to study the underlying genetic mechanism that forms them in a shark relative — the skate (if you aren't familiar with them, they look a bit like a ray, but are generally smaller and not quite as round).

What were the researchers looking for in the skate?

Ultimately, they were interested in the genetics of how branchial rays and arches form.

As an embryo develops, when we are forming any complicated structure in the body, we first lay out a pattern — it's just making sure we know where the head is going to go, what's left and what's right.

There's a really interesting set of genes that lay down this pattern.

This patterning also happens within smaller structures like our hand — it starts out looking like a mitten when we're in the womb, then genes decide where the pinky will be, and the thumb. After that, the hand starts to form from the pattern.

The Cambridge researchers found that when skates, rays and sharks pattern their brachial rays across their gill flap, it happens in pretty much the same way.

And more importantly, it uses the same genes that other jawed vertebrates use — a gene called sonic hedgehog, as well as several others.

There's a gene called 'sonic hedgehog'?

Yes — surprisingly, geneticists have a sense of humour.

It all started when a Nobel Prize-winning geneticist discovered a gene that made fly embryos look spiky, so she called the gene "hedgehog."

Then, a related gene was discovered and characterised and named "sonic hedgehog" — though it should be noted this was before the Sonic the Hedgehog video game became hugely popular. The researcher who named it "sonic" took the name from what was, at the time, an obscure British comic book featuring the character. But the name stuck.

And despite the name, it's really not a goofy gene at all. It not only plays a role in refining the digits on our hand, but also makes sure that our limbs grow at a length proportional to our body. So the fact that it is also used to pattern and cut out shark gills is a very intriguing result indeed.

Is this research saying that shark gills grew into limbs?

No, it's saying something more subtle — that the genetic machine that created branchial arches is similar to the one that made limbs and fins. 

"The fins of fishes and the limbs of land animals have basically the same sets of genes acting to do the same sorts of things," said Andrew Gillis.

"And what we found in this recent research is that if you look back a step further, you can see that actually, [that] developmental pathway in particular seems to be doing the same thing in the gill arches of cartilaginous fishes as it's doing in the fins and limbs of other vertebrate animals."

• Fossils fill evolutionary gap between fish and 4-legged beasts

It looks like the ancestral structure that gave rise to a gill arch eventually allowed the body to develop more sophisticated body plans. Using the same toolbox, you can create different structures (gills, fins, legs) depending on what organism you are in and where in the organism you are.

Now it's up to fossil hunters to hopefully find something that represents this in the fossil record.