NOVA Online | The Missing Link | Diva of the Devonian (2)

Some tetrapods

Diva of the Devonian
Part 2 | Back to Part 1

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.

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.

Our closest living relatives among the fishes: the coelacanth (top) and the lungfish.

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.

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.

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.

Continue: Enter Acanthostega

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