Fish out of water: fins to feet

Graihagh Jackson has been exploring how, over 350 millions years ago, our ancestors first evolved to walk on land...

Graihagh - Once upon a time, there was an era known as Romer's gap. It spanned 25 million years and, much to paleontologists' dismay, it's a complete fossil blackout. Nobody has been able to find any and it's a bit of blow because what happened in that period is pretty important when it comes to understanding how we evolved to live on land.

Before Romer's gap there were only fish, and afterwards, we have an abundance of animals walking around with limbs and fingers - what scientists call tetrapods.

So how did this happen? How did our ancestors get from sea to soil? It's all a bit of a mystery but, a little over 25 years ago, Professor Jenny Clack found something in the basement of Cambridge University's Sedgwick Museum that changed everything...

Jenny - Well, at the beginning of my career, I had the opportunity to examine material in the Sedgwick Museum and, low and behold, it turns out that a student had picked up some tetrapod fossils.

Graihagh - It was called Acanthostega...

Jenny - If you count up the number of features, it's about two thirds of a tetrapod and one third of a fish.

Graihagh - And that's not all...

Jenny - He wasn't particularly interested in them so we put them back in the drawer. And I found out about this and went to have a look, and what the student had found was what would appear to be been a quarry full of these things.

One of the very first things we found out about it was, as we prepared the best specimen we had, was that actually it had eight fingers.

Graihagh - Eight fingers! That's a lot of fingers.

Jenny - Yes. And so that profoundly changed about how we thought about the origin of tetrapod limbs, and uses, and how they developed.

Graihagh - On previous fossils, they'd only found five finger and toes, and scientist thought this mean that fish had slowly come onto land, lumbered around, and then developed limbs so they could eventually walk. But eight fingers, along with some other evidence, suggested otherwise...

Jenny - We thought that digits arose for use in water and that walking on land came later. And having digits in the water gives you a certain amount of advantages for crawling through reed beds and that kind of thing.

Graihagh - A lot was happening during this time. Not only was there a mass extinction, but also climate changed dramatically and gave rise to swampy conditions with lots of vegetation, which meant fish needed digits rather than fins to push these leaves aside and swim on through. It was quite a revolutionary theory and so naturally I wanted to see the evidence in the flesh, or you might say - bone...

Jenny - I'll need to some help with this.

Graihagh - Yes.

Jenny - There should be a wedge...

Graihagh - A door wedge?

Jenny - I hesitate to say this but this is the animal we call Boris.

Graihagh - Why do you call it Boris?

Jenny - I don't know but it got that name soon after we collected it. The overall proportions look a little bit like a small crocodile or a small alligator.

Anyway, this is the forearm, this is the humerus (the upper arm bone), the radius and ulna (the lower arm bones), and here is the hand. Now my colleague was preparing it bit by bit - he found a finger, and then he found another one, and another one, and another one, and so on so that, in the end, he'd got eight.

Graihagh - I imagine him just sort of slowly moving away bits of rock and thinking - ah, one, two, three, four, five - ah, there's five fingers, there you go, I'm done.

Jenny - He nearly did exactly that. But there was a bit more matrix to remove so he thought we might as well make it a proper job and take the rest of the matrix and, of course, he found the other three.

Graihagh - I mean thank goodness he did because, I mean, removing rock is painstaking work, isn't it?

Jenny - Yes it is, yes.

Graihagh - It's 25 years since you published that paper, so what have you been doing in between?

Jenny - About 10 year ago my colleague, Tim Smithson, who's in the next door office, decided that what he wanted to do was to explore to see whether he could find anything from Romer's gap and, after many years, he actually found them.

Graihagh - So does that mean Romer's gap is a fallacy - Romer's gap doesn't exist anymore?

Jenny - That's correct! Because subsequently, over the past four years, we've found a whole lot more. Not only tetrapods, but the fossil record of other vertebrates as well: Lungfishes in particular, and chondrichthyans (shark-like animals), and we've increased the number of known species from one in the case of the Lungfish to seven, and from two in the case of the chondrichthyans to ten new species. One of the things we haven't found in the Devonian so far is small animals. And that's one of the things we're finding most of in Romer's gap - lots of small animals. It's partly preservation, but it's also because we think that some aspects of becoming terrestrial required these animals to become smaller so that they could walk about more easily.