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Diva of the Devonian

  • Posted 02.01.02
  • NOVA

It's a classic chicken-and-egg conundrum: Did the distant ancestors of land animals come ashore and then evolve legs, or did they evolve legs and then come ashore? This is a daunting question considering the fish-to-four-legs transition took place some 370 million years ago. For decades, the former proposition held sway: land first, then legs. But Jenny Clack changed all that.

On an expedition to Greenland in 1987, this University of Cambridge paleontologist unearthed remains of a creature from the Devonian Period (408-360 million years ago) that skulked around swamps on four legs. Through careful study of the anatomy, Clack determined that this creature, known as Acanthostega, nevertheless didn't have a leg to stand on—that is, these rudimentary limbs could not support the animal's weight. But they do support the notion that legs came first. In this interview, Clack gives particulars of her field-shaking discovery and its impact.

Jenny Clack

Jenny Clack in her laboratory at the University Museum of Zoology, Cambridge.Enlarge Photo credit: WGBH/NOVA and BBC/Horizon

 

Expedition to Greenland

NOVA: How did you first become interested in going to Greenland?

Clack: It was my husband who really wanted to go. He thought it would be a really neat thing for me to do, because he wanted to. He hoped to find some more material of a thing called Ichthyostega, which is the Devonian tetrapod that most people have heard of if they've heard of Devonian tetrapods. It's the one often featured in children's books. I was very dubious about the possibility, but he kept nagging me.

You begin thinking, Wow, this is going to be the start of something big.

Eventually, through a long series of coincidences, we did get information about putting in a grant proposal for some expedition money. Along the way, I spoke to various experts who were familiar with getting to Greenland, some of them across the road in our own Department of Earth Sciences. One of them, the geologist Peter Friend, has had a series of students over the years who have been to Greenland, looking at the sedimentology. He gave me their notes and said, "Have a look at this. It might help you in your decision as to where to go and how to get there."

I was reading through these notes, and I found that back in 1970, one of his students had actually found material of Ichthyostega—or so it said in his notes. So I went back to Peter Friend, and I said, "You know those specimens that were found in 1970, have you still got them?" "Yes," he said, "I think I've got them in a drawer somewhere." In fact, he came back with several drawers full of Greenland material, some of which was tetrapod, some of which was Ichthyostega, but some of it was this other animal, Acanthostega, which had only been known from two specimens collected back in 1952.

The world's largest island holds secrets about the world's first tetrapods—secrets that Clack and her crew began uncovering in 1987. Enlarge Photo credit: WGBH/NOVA and BBC/Horizon

 

What was on the table in front of me was a block of material containing three different skulls and some other bits and pieces of Acanthostega. This suggested that, wherever the fossils came from, there was a big, rich vein of this animal waiting to be quarried out. To cut a long story short, we eventually did mount an expedition to go and find the spot.

NOVA: At the moment you saw that material, what were you thinking?

Clack: Well, very often you don't realize immediately what it is you're looking at. It will take several days, or even a week or two, for it to actually hit you that this is Acanthostega, this is a rich vein, go and get it! But at that point, a whole vista of things opens out. Because it's a crucial animal, it's a crucial period, and it's an area of study that nobody's looked at before. You begin thinking, Wow, this is going to be the start of something big.

NOVA: When you finally got to Greenland, how long was it before you found something?

Clack: We were there for a total of six weeks, but it must have been 10 days after we arrived before we hit the right spot. The landscape is vast. You have no sense of scale, because there are no trees. Something will look as though it will take you half an hour to reach, and it actually takes you several hours. Eventually we found the spot, 800 meters up on a hillside.

In a student's field notebook, Clack found a notation that helped inspire the Greenland expedition ("skull roof bones common"). Enlarge Photo credit: WGBH/NOVA and BBC/Horizon

 

It took a little while to find, because the student's notes said 820 meters. We began thinking, Are we on the right mountain? We checked, and we were, so we decided we'd start from lower down and walk up until we hit the right level. Then we began to find things out on the scree: a piece of skull, a piece of limb bone. We followed that up to where it was all coming from, and we found a little exposure, a little cliff. Then it was just a matter of quarrying it.

NOVA: And was it hard to dig the stuff out?

Clack: We simply had to hack bits off of the cliff and split it. That's not too difficult, except that you're on a 45-degree slope, and you have to be careful not to drop stuff down. Getting it up and down the mountain was quite hard, however.

NOVA: How long were you working on it?

Clack: We used to go every other day. We would go up in the morning, spend all day up there, have lunch, and then come down. Initially it was a four-hour climb, but by the time we finished, we got it down to two-and-a-half hours. We did that for four weeks.

NOVA: How much material did you come back with?

Clack: Well, all together the expedition brought back, I think, a metric ton of material. About two-thirds of that was tetrapod material.

Some tetrapods Enlarge Photo credit: WGBH/NOVA and BBC/Horizon

 

From fish to four legs

NOVA: What exactly is a tetrapod?

Clack: A tetrapod is an animal with four legs—you, me—or an animal whose ancestors had four legs. So I can talk about a frog, which jumps; a bird, which flies; a snake, which wriggles; a whale, which swims. They've all either got four legs now, or they had four legs earlier in their evolution. The word is an inclusive word to denote all those animals, and the fact that they are ultimately related to each other. They all go back to a common ancestor—in fact, the very kind of animal we're talking about here.

Basically, yes, it was a fish with fingers.

NOVA: And why is it important to study the first tetrapods?

Clack: Well, the origin of animals with legs marks one of the biggest transitions in evolution. The transition from water to land, from breathing primarily in the water to breathing primarily air, that transition allowed land animals like dogs and cats and dinosaurs and ourselves to evolve. If these early animals hadn't made the transition, we wouldn't be here today. It's important to understand how and when, and possibly where, that transition took place.

NOVA: What do we know about what this transition was like?

Clack: The transition from water to land, from creatures with fins swimming in the water to creatures with legs, with fingers and toes, we think took place perhaps around 370 million years ago. That's the first evidence we have of it. Now, at this time there were plants on the land—quite complex plants. There were creatures like centipedes and millipedes, old ancestors of those. There were all sorts of other "creepy-crawlies," if you like. But there were no creatures with bones.

Our closest living relatives among the fishes: the coelacanth (top) and the lungfish. Enlarge Photo credit: WGBH/NOVA and BBC/Horizon

 

What we think happened is that during that period, there was a group of fish that had fins, that had a particular pattern of bones in their makeup. For example, we have a single bone in our arm that joins to the shoulder girdle. That's quite characteristic of tetrapods, but also of the tetrapods' closest relatives. These are called lobe-finned fishes. There were lots of them around during this period of the Devonian. We have a few others today, like the coelacanth and the lungfishes. They are our closest relatives among the fishes. But in that period, they were a whole lot more diverse than they are now.

Some of them seem to have become waterside creatures feeding in shallows, finding their way through dense vegetation, pushing the vegetation aside, and hiding among weeds in swamps. So we're dealing with shallow-water forms that were ambush predators.

NOVA: And some of these evolved to walk on land?

Clack: Eventually, yes. First, you began in this period to get plants invading the water margins, forming dense swamps and forests around the water's edge. And some of the fishes living in the shallows would have found it advantageous perhaps to lose the fin webbing from their fingers and develop separate digits. Fish have bony supports for the fin web. If you lose those and make the digits separate, then you can begin to grasp things and push aside the vegetation. You can also grasp the vegetation to hold your position in the water. There are quite a lot of modern fish that do this. You find that they're lurking predators; they will sit motionless and then suddenly pounce. Many of these early vertebrates, the fishes and the early tetrapods, have got big deep tails to give them thrust through the water.

Gape-mouthed, many-toothed, and eight-fingered: Acanthostega as it may have looked back in the Devonian. Enlarge Photo credit: WGBH/NOVA and BBC/Horizon

 

What we think happened is that these creatures, which were developing this mode of life in the shallows, developed legs with digits before they ever started really to walk on the land at all. So they would have got their legs first, then gradually perhaps moved into shallower and shallower water—more and more vegetation and less and less water—and eventually emerged onto the land. But it took a very long time.

NOVA: Is this new? Did we used to have a different idea about this transition?

Clack: The story that you'll find in many of the old textbooks, and the pictures that you'll see in children's books and museum galleries, is a picture of a fish, usually it's a fish called Eusthenopteron, which is stranded in a drying pool trying with its fins to support itself out of water. It looks really odd if you look at it objectively, because this fish looks like a pike; it literally looks like a fish out of water. The old idea was that the fish came onshore first and then developed the legs. What we now think is that the tetrapods developed the fingers first and then left the water.

NOVA: Why did we used to think that?

Clack: Because there was very little evidence one way or the other. Among the first people to think about it was [vertebrate paleontologist] Al Romer in the States. He and his colleagues worked on what little evidence that there was. The fish, Eusthenopteron, was one of the best-known of the Devonian fishes. It has this lobed fin with the structure that some people thought would have given it an advantage in crawling over drying land to get to another pool. I don't really think that's plausible, partly because there's no evidence that they lived in this really arid climate that the model supposed.

Not five but eight fingers on each "hand" emerged from the rock encasing the remains of Acanthostega. Enlarge Photo credit: WGBH/NOVA and BBC/Horizon

 

Enter Acanthostega

NOVA: How did this new idea come about?

Clack: The new idea stems from the material we collected in East Greenland in 1987. That proved to be a really rich haul of material, though it took several years to develop it out of the rock and expose what was there.

One of the really surprising things was that there was a complete limb with its digits in place. That's rare enough among this age of fossil. But what was astonishing was that instead of the conventional five digits on the end of this limb, there were eight. The person who was preparing them was preparing them one by one, and when he got to five he thought he'd finished. But he hadn't! He went on to find three more. Of course we thought, Does this all belong to the same animal? Eventually we concluded that yes, it had to.

Dinosaurs always seemed to me a bit modern, not mysterious enough.

Then we began to look at the animal as a whole, and how it compared with other creatures of this sort of similar date, and how it related. There were several things about this specimen, this animal called Acanthostega, which suggested to us that it was a very primitive tetrapod, one of the very first with legs. If you look at the structure of the foot on the leg, it's clearly not designed for walking on. It had no proper ankle or wrist, so it wasn't a weight-supporting device. In fact, it looks more like a paddle. The animal also has a deep tailfin, and it has a number of other fish-like characters that are lost in all subsequent tetrapods.

NOVA: Did it have gills?

Clack: We believe it also had gills, yes, because we found some gill skeleton material with several of these specimens.

NOVA: So this was basically an aquatic creature with hands?

Clack: This was a swimming creature. We don't know whether it could ever have come out on the land, but it certainly wouldn't have walked in the conventional sense. It had gills as well as lungs. It had very short ribs, so we don't think it used its ribs for breathing. It had a deep tail with bony supports all along it. It also had features of the internal workings of the skull that were also fish-like. Basically, yes, it was a fish with fingers.

Humble to behold, Acanthostega shattered long-held preconceptions in the field of vertebrate paleontology. Enlarge Photo credit: WGBH/NOVA and BBC/Horizon

 

NOVA: What did this animal do on a day-to-day basis?

Clack: This beastie probably spent most of its time hanging around in the shallows, in the swamps, waiting for something to go by, and perhaps using its hands to steady itself while it waited. Then as something went by, it would swish its tail, which would give it a good thrust, and it would lunge, catch the prey in its open mouth, and then wriggle back into the shallows.

NOVA: So this was a big deal, wasn't it, this find about Acanthostega?

Clack: This was pretty revolutionary, yes. Partly it was because nobody had really looked at this transition in any detail. And there was very little evidence prior to this as to exactly what had gone on, so any new evidence was going to be quite startling. But it did turn on their heads all the preconceptions that had gone before. With respect to the origin of limbs in particular, it rewrote the textbooks.

Not only that, but because people are beginning to take an interest in the development of limbs from a developmental genetics point of view, and looking at the early embryology of limbs, it also fed into what they were doing. So paleontology and modern developmental genetics helped each other understand what exactly was happening very early on in the development of limbs.

NOVA: How so?

Clack: Well, up until about the mid-1980s, there was an idea or conception that the five-digited hand was the archetype, and that at the fish-tetrapod transition you went straight from the fin of something like Eusthenopteron to a five-digited limb. They tried to make parallels between the bone structure of one and the bone structure of the other. It was difficult if not impossible to do without having to make up a lot of intermediate forms.

When we found the limbs of Acanthostega, with eight digits, that completely knocked all the old arguments on the head. It was not something that had been conceived of under the old preconceptions. How do you get an eight-digited hand from a fin like Eusthenopteron's? Well, you can't.

Fortunately, it tied in with some work in embryology that suggested that all the old ideas of how you do it were wrong anyway. That work showed that the digits in modern animals do not arise in a regular pyramidal branching pattern but are formed around an arch that runs through the wrist or ankle, thumb or big toe last. In theory, all you'd need to do to produce eight digits is keep on going farther along the arch. The two discoveries very much came together, and one was able to explain the other.

It can take years of painstaking preparation for a single specimen from the Devonian to fully emerge from its sarcophagus of stone. Enlarge Photo credit: WGBH/NOVA and BBC/Horizon

 

NOVA: So what was the best thing that you found in Greenland?

Clack: After three or four years of preparation, it was clear that the very first block that we looked at was the most complete and articulated individual skeleton. That's the one with the limb preserved on it. Then from the same level are another two or three individuals, one of which has got a beautifully preserved head. It's in three dimensions, and it's rather cute. Those are the two best specimens. But putting them all together, we were able to reconstruct practically the complete bony skeleton of Acanthostega.

NOVA: Preparing those fossils is a major operation, right?

Clack: Absolutely. When you bring a specimen home from the field, you may have a hint that there's something inside the rock by a cross-section here, a piece of worn bone there. But you won't know what's in it until you've removed all the rock that sits on top of it. This is a special skill, and it requires a great deal of patience.

The overburden, the rock that's lying on the top of the bones, is called matrix, and there's usually some kind of color or texture difference between the matrix and the bone. You can often see a separation there. But you've got to do this under a powerful binocular microscope, and you have to do it very, very carefully—in the last stages, grain by grain with a mounted needle.

It can take literally years to dig out a good specimen from a lump of rock. Some of the ones we brought back from Greenland have taken three or four years.

All in a day's work: Clack commuting to Cambridge. Enlarge Photo credit: WGBH/NOVA and BBC/Horizon

 

A career paleontologist

NOVA: So what kind of a kid were you?

Clack: I was always interested in natural history, from as far back as I can remember. In England we have little "I Spy" books, in which you tick off what you see. I used to have a whole collection of those, and a whole collection of the little "Observer's" books. We used to go on holiday specifically so that I could either look at the local flora and fauna or look for fossils. One of my earliest holidays was to [the famous English fossil locality] Lyme Regis to see if I could find an ichthyosaur. Never did, of course.

NOVA: What about fossils did you find so interesting?

Clack: I was just fascinated by the idea of something that was so old that people could never have seen, that you really had to use your imagination to bring to life. And I was never particularly struck by dinosaurs. They always seemed to me a bit modern, not mysterious enough. So the further back you could go the better, for me.

There are people like me who would be the first in the time machine to go and see whether we were right or not.

NOVA: It's amazing to many people that we can know anything at all about something that happened so long ago.

Clack: You have to use your scientific imagination as well as your creative imagination. That's one of the things I really like. I mean, you have to impose some kind of scientific discipline on it in order to make what you come up with in some way testable, and that's a constraint. But nonetheless, it does require you to think about how these animals could have lived and worked.

During her daily commute by motorbike, Clack brings along a trusted friend—if only in spirit. Enlarge Photo credit: WGBH/NOVA and BBC/Horizon

 

NOVA: So paleontology was an obvious choice for career then.

Clack: Well, as far as paleontology is concerned, I was really struck by a particular book or set of books that introduced me to the geological periods. The idea of having something as mysterious as that, as old as that, that nobody had ever seen, but you could see and could infer from fossil material, was just amazing. The more unfamiliar the animals were, the better I liked them. Although I could only get an inkling of this from popular books, it really turned me on. Though I couldn't know it at the time, the thread of being a paleontologist has always been there, despite some diversions on the route.

NOVA: So how does it feel to have made such an important contribution?

Clack: I just feel immensely privileged. I still don't believe quite what I've found. When I first set out on my paleontological career, I was blessed with a very good series of finds, and I can't believe that that's continued in the way it has. You sometimes wonder, where do you go from here exactly? But I've been fortunate there too. You know, I see my name printed in the textbooks, and I think, "Yeah! That's good!"

NOVA: Do you owe your streak to tenacity, or do you feel like you have a particular eye?

Clack: I think my biggest talent is for interpreting crushed and distorted fossils, and working them into a 3-D shape from this mash. It's the imagination to try to piece things together into a coherent whole and to interpret something that perhaps nobody else has been able to interpret.

In the august precincts of Cambridge, Clack and her team are trying to paint a portrait of life on Earth as it appeared over 350 million years ago. Enlarge Photo credit: WGBH/NOVA and BBC/Horizon

 

The big picture

NOVA: So what are the big questions you're currently trying to answer?

Clack: Well, we want to understand the history of the planet, where it's been, and that might shed some light on where it's going. But you mustn't think of it just in terms of where humans fit into the picture, although that's one side of it—you know, how we came to be here. We also want to find out what life was like so many million years ago in order to put together a coherent story of what was going on then. There are several transitions for which you could put together a really neat, graded sequence of animals—for example, the origin of mammals. The origin of birds, too, is a story in which we're now beginning to fill in some of the gaps.

But even if there are gaps, you can still make sense of what you've got. That's really the challenge, not to think, Oh, I must have a whole lot more fossils before I can say anything about this—although the more you've got the better it is—but to try and derive from the information that you have available a scientifically testable idea about how the animals relate to each other, how evolution has proceeded.

NOVA: Why should the average Joe on the street care?

Clack: Well, why should you care about history at all? I ask this question of art historians, or historians of any sort. Why music? It's a cultural activity. It's a mistake to think of science as something that's just about producing things, making things, technology. Science is a cultural activity, and it expands our understanding and our appreciation of our place on the planet and in the universe.

Paleontology teaches you that human beings are very, very recent, and they perhaps need to be put in their place a little bit. Paleontology puts people in perspective. They've been around maybe half a million years, while the animals I'm talking about lived 370 million years ago. Life didn't begin on Earth until maybe four and a half billion years ago. It really makes you feel quite insignificant.

NOVA: What about the idea that we'll never know for sure, we'll never be positive about what happened, say, 370 million years ago?

Clack: That's the frustration. In my mind there are two sorts of paleontologists. There are those who really don't want to know the answer but are just content with the finding out of what they think went on. Then there are people like me who would be the first in the time machine to go and see whether we were right or not, or just to be there.

Interview conducted by Joel Olicker, Powderhouse Productions, and edited by Peter Tyson, editor in chief of NOVA Online

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