NOVA | Transcripts | The Missing Link

Publish date: 2024-08-20

PBS Airdate: February 26, 2002
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NARRATOR: In an obscure museum in eastern Europe, a fossil hunter has made a startling discovery. While randomly sifting through a set of drawers he found a collection that hadn't been examined for 30 years.

PER AHLBERG (Natural History Museum, London): I was going through these drawers, finding drawer after drawer of very much the sort of fossil I would expect to find, really nothing of any particular excitement. And then, pulling open one drawer, I spotted in the middle, sitting in a little cardboard tray like so, a fossil the likes of which had...never have been found anywhere in the world.

NARRATOR: The paleontologist, Per Ahlberg, had found a new piece of evidence in a 400 million-year-old detective story: how and why creatures first left the water to live on land.

NEIL SHUBIN (University of Chicago): For a long time, all life was in water—in the seas, in the ocean. And it's not until about 370 million years ago that we start to find the first animals venturing out on land. Fins evolved into limbs at some point in that time period.

NARRATOR: For over a century scientists have searched the world for fossils that can help them unravel the mystery. Now a series of new discoveries is shaking up long-held views on how evolution fashioned this profound transformation, how it happened that fish left the water for land and became the ancestors of us all.

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NARRATOR: In north-central Pennsylvania, near the Appalachian towns of Hyner and Renovo, stretches an ancient sandstone formation known as Red Hill. It was here that paleontologists Neil Shubin and Ted Daeschler discovered extraordinary new clues in one of evolution's most enduring mysteries: how ancient creatures left the water to walk on land.

NEIL SHUBIN:The reason why I am in this business is because of a sense of discovery. I mean that's really what I like to do. Being a paleontologist is great because if you look at rocks of the right age, of the right type, and if you're really lucky, sometimes you can find a fossil which will fill one of these big gaps in evolution, one of these big transformations.

TED DAESCHLER (Academy of Natural Sciences): It's a detective story and you're finding evidence out there. We're breaking rocks and we're looking for little pieces of evidence to help piece together this, this story of how limbs developed from fins.

NARRATOR: The story began to take shape back in the 18th century with a simple but crucial observation, a vast array of animals showed striking similarities. They all had four limbs. They are tetrapods.

PER AHLBERG:We are tetrapods—to wit: one, two, three, four. Horses are tetrapods evidently enough.

NARRATOR: So are dogs. So are lions, tigers and bears.

PER AHLBERG:So is a bird: two hind legs, two wings at the front which are modified front legs.

A snake is a tetrapod. It has no legs any more, but it's quite clear that they derived from a lizard ancestry which had both fore limbs and hind limbs.

NARRATOR: Non-tetrapods have a wide variety of body plans. Some have hundreds of legs. Some have none. But all tetrapods, beneath the skin, share similar features.

They all have backbones with special interlocking spurs. It's as true of us as it was of the dinosaurs. All tetrapods have a pelvis attached to the backbone to support their weight. They all have a ribcage to protect the heart and lungs. And they all breath air through nostrils. Their limbs invariably consist of a single bone nearest the body, the humerus, then a pair of bones, the radius and ulna, leading to feet or hands—which never seemed to have more than five fingers or toes.

It's true of dinosaurs, human beings, and even whales—for under their flippers, whales have five fingers.

NEIL SHUBIN:If mammals, reptiles, birds and amphibians all have this common structure, what does that mean? That means they all must have descended from an ancestor that already had this structure.

Our question is, "What did that ancestor look like? Where did it come from?"

NARRATOR: Sometime during the four-billion-year history of life on Earth there were primitive tetrapods from which all four-limbed, air-breathing creatures descended—amphibians and reptiles, birds and mammals.

And even further back in time, there were water-dwelling creatures, fish, that were the ancestors of those first tetrapods.

The fish likely belonged to a group known as lobe-fins. Like this modern-day lungfish, the ancient lobe-fins had lungs as well as gills and a unique structure in their fins that looked like a precursor to arms and legs.

NEIL SHUBIN:There are two types of bony fish on the Earth today, ray-fin fish and lobe-fin fish. Now, ray-fin fish are very common, as represented by this common sole here. This creature's dinner. And, in fact, most of what we have for dinner are the ray-fin fish.

The reason why we call them ray-fin fish is because their fins are composed of a very special sort of bone. You can see them here. These are the rods. These are the rays that make up most of the surface area of the fin. The type of bone that makes up these rays is not present in our limbs. Now, this monster here, for better or for worse, is one of the fish that's most closely related to us. It's a lobe-fin fish. And the reason why we call it a lobe-fin fish is because its fin is composed mostly of this thing here, which you can see is this fleshy lobe. Now, from an evolutionary perspective this lobe is very important because many of the bones that make up our limbs actually first evolved within this lobe.

NARRATOR: Lobe-fin fishes were common during a time in Earth's history called the Devonian Period, a hundred-fifty million years before the age of dinosaurs began. Tetrapod's fossils were plentiful in rock layers younger than the Devonian, but older rock layers yielded no tetrapods at all—only more primitive creatures like sponges, worms and some fish.

So the water-to-land transition must have occurred during the Devonian Period, between about 410 and 360 million years ago.

PER AHLBERG:Imagine for a moment that you were able to go back to the world just before the beginning of the move onto land. Let's say you go back to the world of 500 million years ago, and you stand on the shore. What do you find?

Well, let's say it's low tide. So you walk down onto the tidal flats. You find it's really not that different from today. There are rock pools, and maybe anemones and stuff growing in them. There are seaweeds draped over the rocks. There are little arthropod things, crustacean-like creatures and so on scuttling around. The overall picture, the system is there. And it's not so different from today.

But turn your back to the sea and walk inland and what do you find? A barren, empty wasteland: no greenery, no trees, no insects. Wind keening over the rocks. A barren land that could not possibly support you.

Then, in a time frame between about 450 and 350 million years ago, group after group of organisms start making their way onto land, assembling the immensely complex land ecosystem which we inhabit today.

NARRATOR: In 1881, on the Gaspé Peninsula of Quebec, a crucial discovery was made by a Canadian farmer. He stumbled upon the most perfect fossils of lobe-fin fishes ever seen, in rock from the Devonian period. Called Eusthenopteron, the layout of its fin bones showed a striking resemblance to the bone structure of tetrapod limbs, and with a clarity never before seen.

KEITH THOMSON (Oxford University): Here in these fossils the limb was just laid out simply beautifully, and it was so easy to turn it in your mind into a tetrapod limb. These bones, the one and the two bones, they were, they were laid out, and there were these bits in the, the, the ankle and the, the wrist and so on—absolutely fantastic, beautiful material. Clinched it really.

NARRATOR: What they thought they'd clinched was the sort of fish from which we all came, an ancestor with fins that were verging on limbs. But could they find creatures that looked more like tetrapods—animals with the beginnings of true arms and legs?

Despite years of searching, fossil hunters had never found tetrapods in Devonian rock layers. Perhaps, there were none. Or perhaps none had ever become fossilized.

KEITH THOMSON: The chances of any animal becoming a fossil are extraordinarily remote. And the only way that we have a lot of fossils is that there's been an incredible amount of time and an unbelievable number of animals.

The animal originally had some hard part that was preserved even though the soft parts were dissolved away by bacterial action. Then it got buried in a place where bacterial action stopped, usually in mud or something like that. And then it has to be covered up pretty soon with some kind of a sediment of sandstone, silt or maybe volcanic ash. And then it will gradually get further and further into the Earth. And then the next thing is, because a fossil is something that's dug up, it has to be...come near the surface and somebody has to find it.

NARRATOR: In 1931, fossil hunters got lucky. A team of Swedish scientists on a geological expedition to Greenland came upon a 360 million-year-old Devonian creature that definitely was not a fish. It had the telltale bone structure of a tetrapod. It appeared to be a milestone along the evolutionary path from living in water to living on land.

They named it Ichthyostega, meaning "fish-plate," since the roof of its skull was still fish-like.

KEITH THOMSON: People had been looking for this, in a way, ever since Darwin, ever since 1859. This transition is the one that so intrigued everybody—going from the water to the land—and no evidence of it. And then, boom, they found it. Terribly, terribly exciting really. Very, very important.

NARRATOR: It fell to Erik Jarvik to analyze this crucial new discovery. He spent years digging hundreds of fossilized bones out of the rock to try to reconstruct the anatomy of the creature. Jarvik was a brilliant anatomist, incredibly painstaking. He began working on Ichthyostega in 1948, but didn't complete the work until two years before his death in 1998. And during Jarvik's nearly 50 years of research no one else was able to study the fossil.

PER AHLBERG:If a particular research group has collected material and is actively engaged in studying that material, then other people don't muscle in and take it away.

KEITH THOMSON: It was extremely frustrating to everybody that they were holding this to their chest as it were, and only letting out little bits of information while everybody else was going crazy trying to write and work in a field in which you know somebody else has a lot of information and you don't know what it is.

NARRATOR: Jarvik did release two papers during the course of his research comparing Ichthyostega to the fish Eusthenopteron. He was convinced that Ichthyostega had been a true tetrapod with a ribcage, a pelvis attached to a backbone with interlocking spurs, and four limbs with five digits each that would have enabled the creature to walk on land.

Ichthyostega was then the most primitive tetrapod ever found. Still, there must have been several intermediate creatures between it and a fish like Eusthenopteron.

KEITH THOMSON: No one was ever really satisfied with Eusthenopteron as the immediate ancestor of something like Ichthyostega. So I think people have been, not only been looking for things that are a little less fish-like than Eusthenopteron, they've also been looking for things that are a little more fish-like from Ichthyostega. So wanted to close the gap from both sides.

NARRATOR: Somewhere in the rocks there may be other fossils of four-legged creatures from the Devonian Period that would help fill in the evolutionary sequence from water to land. There may be more what are often called "missing links." Charles Darwin called them "transitional forms" in his 1859 masterwork on evolution.

One striking transitional fossil was found in 1861, the archaeopteryx, a reptile with feathers. It was widely hailed as proof that birds had dinosaur ancestors. But few such clearly transitional forms have been discovered.

PER AHLBERG:Presumably the transitional forms are very rapidly out-competed by their more...by their own more advanced descendants. So these transitional episodes in the history of life tend to be brief and involve, it seems, relatively low numbers of species and probably low numbers of individuals.

NARRATOR: Some transitions occur when there are dramatic environmental changes. Creatures not well suited to the new environment die out. But, just by chance, some members of a population will be able to survive: those with thicker fur if a climate turns colder; those with longer bills if a major food source develops deeper flowers.

STEPHEN JAY GOULD (Harvard University): Most of evolution is stability and the production of new species that are pretty like the ones that came before. We have 500,000 species of beetles just for starters. But every once in a while you do have a transition to a very different kind of environment. Or you do have the invention evolutionarily of a very different kind of organ or structure that allows the occupation of a part of the ecological world that wasn't inhabited before. And that catches our attention. So there was a point when you didn't have organisms on land, so what was it about the evolution of creatures that lived in the sea that allowed them to get onto land?

NARRATOR: The most popular explanation for why fish evolved to walk on land was proposed by Harvard paleontologist Alfred Sherwood Romer. Romer based his hypothesis, known as the "drying pond scenario," on a view long held by geologists that the red color of Devonian rock meant it had been a time of severe drought.

PER AHLBERG:The drying pond scenario basically ran like this: there were lobe-fin fishes living in the rivers and lakes of the Devonian continents, but because of the seasonal droughts that were supposed to be happening, during the dry season a lot of these pools would be drying out. And fishes stuck in a drying pond would be faced with the choice either of just sitting it out glumly in the mud and hoping for rain, or else boldly setting out overland in search of another and perhaps more permanent water body.

The idea was that lobe-fin fishes became gradually better and better at using their lobe fins to drag themselves across the mud.

NARRATOR: Romer suggested that limbs evolved as fish adapted to making this desperate march across land. These would have become the first tetrapods, our ancestors.

Throughout the early 20th century a mere handful of fossils shaped our view of Devonian creatures. Then, quite remarkably, from the waters off South Africa's east coast, the Devonian Period suddenly came alive. Just before Christmas 1938, Marjorie Courtenay-Latimer, the curator of a small museum in East London, South Africa, was called down to the docks to examine a most unusual catch.

MARJORIE COURTENAY-LATIMER (East London Museum, South Africa): Twenty-second December 1938 was a wonderful day. I came onto this most beautiful fish. It was just on...just on 5 foot. It was silver and gold and green and blue and had white, kind of, flecks on it. And to my horror it had these limb-like fins. And I thought to myself, "What on earth can this be? I've never seen a fish like this."

Somehow I was going to preserve it. Somehow, whatever happened, I had to save it. That was this intuition that I had: "It must be saved at all costs."

NARRATOR: Unable to have the fish frozen, she left it with a taxidermist until Dr. J.L.B. Smith, a prominent South African scientist, could come identify it.

MARJORIE COURTENAY-LATIMER: He stood at the head of the table, and he said, "Well, lass," he said, "this fish will be on the lips of every scientist in the world. It's a coelacanth."

NARRATOR: Coelacanths are a group of lobe-finned fishes that lived during the Devonian Period and were thought to have gone extinct over 76 million years ago.

KEITH THOMSON: It was absolutely fantastic because it was...it's living and it, it's exactly like having found a live dinosaur or a live archaeopteryx. And it was all those things like The Lost World by Arthur Conan-Doyle and whatnot come true. It was absolutely amazing.

ARCHIVE FILM NARRATION: Meet Professor Smith of Grahamstown, South Africa with a model of that famous fish, the coelacanth.

J.L.B. SMITH (on tape): Coelacanths are close relatives of the fish that scientists consider was the ancestor of all land animals. The coelacanths have lived for probably 350 million years, and in that time they have changed but little. As you see, the fins are more like paddles than ordinary fins.

NARRATOR: Smith was convinced the Coelacanth could actually move about on the ocean floor using its lobed fins.

J.L.B. SMITH (on tape): I have no doubt that this fish crawls about on the bottom quite easily.

ARCHIVE FILM NARRATOR: Yes, the Professor says the fish is a kind of ancestor of man. Poor fish.

KEITH THOMSON: It was found in December so that means it was the middle of the summer. Huge fish, five feet or so. No way to preserve it, so it was preserved in a taxidermist mount, a skin mount, with the bones of the head and the skin. And everything else was thrown away which of course was a tragedy because all sorts of information went with it.

NARRATOR: With little left to study, Smith was determined to find another coelacanth, alive, and offered a reward to local fishermen.

It took 13 years, but finally a coelacanth was found off the Comoros Islands near Madagascar. It didn't walk on the bottom, but it was later seen that its fins moved in an alternating left-right pattern, just like tetrapods do when walking.

After studying the complete creature, Smith realized that coelacanth had less in common with tetrapods than he had thought. Most of its organs were distinctly fish-like. Coelacanths must have survived virtually unchanged since branching off from an ancestral fish some 360 million years ago.

But Eusthenopteron, also a fish with limb-like fins, was on a different evolutionary branch, one that produced four-legged creatures like Ichthyostega.

The next question was, "What animals filled the 20-million-year gap between Eusthenopteron, the fish, and Ichthyostega, the tetrapod?

The answer came in 1981, from a motorcycle enthusiast who was also Per Ahlberg's advisor at Cambridge University Museum.

PER AHLBERG:I would be sitting there working in the morning, and if I'd got in early I'd suddenly hear this sound from the courtyard, which was the Associate Director of Vertebrates arriving for her daily work at the museum.

JENNY CLACK (Cambridge University): I was always somebody who was interested in natural history, really from as far back as I can remember. And in England we have little I Spy books, where you tick off what you seen. And 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 fall flora and fauna, or look for fossils.

NARRATOR: Jenny Clack had long dreamed of solving the mystery of how and why creatures first walked on land, but it seemed a remote possibility.

JENNY CLACK:I had just finished my thesis when I started work here and was looking around for another project and a colleague of mine said, "Don't worry, something will turn up." And I didn't believe him.

NARRATOR: What turned up was the notebook of a geology student who had visited Greenland years earlier. In one corner he'd made an extraordinary note: he'd found remains of Ichthyostega, the creature Erik Jarvik had discovered.

JENNY CLACK:He'd noted, "Ichthyostega bones and skull bones common." And early tetrapod specimens are not common anywhere, particularly not Devonian tetrapods on a mountain in Greenland.

Even when they'd been collected in the thirties they weren't common. They were chance finds after days of walking the scree. And to see this in his notebook just set the bells ringing: "We have to go there."

NARRATOR: It took six years to raise the funds for the expedition, but finally Jenny Clack headed off. Accompanying her was her Ph.D. student, Per Ahlberg.

PER AHLBERG:We were very excited to be going at last, but this was of course also coupled with a certain trepidation. This was a big undertaking. It was an expensive expedition involving air support, helicopter time, all sorts of things.

JENNY CLACK:It's at least 100 miles from the nearest permanent habitation, and that's an airstrip which is only manned during summer.

PER AHLBERG:And of course it was possible that we were going to find almost nothing, or at least nothing new. So the potential was there on the one hand for a spectacular success and on the other hand for a considerable embarrassment.

JENNY CLACK:The landscape is vast. You have no sense of scale, because there are no trees. And so something will look as though it will take you half an hour to reach—it actually takes you several hours.

NARRATOR: Even with maps and detailed notes Ahlberg and Clack feared they'd never find the right spot.

JENNY CLACK:The notes in the books say 825 meters. And, in fact, that was wrong. We'd been starting too high up the mountain. And eventually we thought, well, first of all we thought, "Are we on the right mountain?" And then we checked. And yes, it was the right mountain. So we decided that we would start from lower down.

NARRATOR: When they explored the mountain at a lower elevation, Clack saw something.

JENNY CLACK:It was covered with dirt and soil. It very nearly got thrown on the scrap heap, but, fortunately, we brushed some of the dirt off and we could see part of a skull.

NARRATOR: They had found the most complete tetrapod specimens since Erik Jarvik's expedition 56 years earlier: fossils of a four-legged Devonian creature called Acanthostega.

Clack returned to Cambridge with dozens of fossils. At last, someone other than Jarvik would be able to do original work.

But the true importance of the trip did not emerge until Acanthostega had undergone several years of painstaking preparations. Clack recalls it was her colleague, Mike Coates, who first saw a hand emerge from the rock.

JENNY CLACK:The first thing he found on this block was a finger. This digit here. So we've got a number of finger bones aligned along the edge of this block. Then he continued with the preparation and he found the next finger which is here with its end curled over and then a third, similarly with this crooked finger end and a fourth, again with that. And then there's a gap. And then he went on to find another finger. Individual finger bones are really quite clear. And that makes a total of five. But he still had all this area here to prepare, so instead of stopping he went on to clean up the rest of this area. And lo and behold here is another digit. So that makes six, and he expected to finish there. And then, to his amazement, here's a seventh and finally an eighth. What?

PER AHLBERG:My initial thought, before I had seen the specimen, was that there might be a problem here. If the specimen is preserved in such a way that the two fore-limbs are lying on top of each other it's easy to see how you could produce something that would look like a hand with more than five digits. And I wondered whether something like that was going on—whether there was in fact an interpretation problem. But of course once I'd seen the specimen it was perfectly plain that that was not so, that you did indeed have a fore-limb with a humerus, radius, ulna, and eight little fingers in a row.

NARRATOR: Acanthostega had eight fingers on one hand, suddenly calling into question one of the most basic assumptions behind the previous hundred years of research.

PER AHLBERG:Until that day I had assumed, like everyone else, that five was the primitive number of digits for a tetrapod limb. The old explanations for the origin of the structure, after all one of the most fundamental and defining structures of being a tetrapod, and, in our own way, of being human, was in the bin.

NARRATOR: Ahlberg and Clack now believe our earliest ancestors with legs must have had numerous digits, and then somehow evolution reduced them to five over the eons.

And upon further examination, Acanthostega called another fundamental assumption into question. Its limbs were not made for walking.

JENNY CLACK:If you look at the limbs, what you find is that the joints are all orientated, angled, so that the limb would have stretched out just to the sides. On the end of the humerus, the radius and ulna just fit in grooves along the end of the bone, not, as they would in a later animal, underneath there. So it's not supporting weight like this. There's just no way that it could have brought its leg underneath to take any weight.

Similarly with the hind limb, which we found a bit later on—similar kind of arrangement, no ankle to speak of, just a paddle-like limb.

NARRATOR: Acanthostega's legs would have been useless for walking. And what's more, it could never have lived out of the water for long. It breathed primarily with gills, like a fish. The evolution of legs was apparently not triggered by the need to walk on land.

JENNY CLACK:So the thing that has really changed is that rather than the fish going onto the land while it's, it's still got fins, we've turned that completely on its head, so now we've got tetrapods in the water, still in the water, while they've got limbs with digits.

NARRATOR: Stunned by these revelations, Clack checked her findings against a fragment of Ichthyostega which she'd found in Greenland. Her team prepared the specimen and counted the toes.

JENNY CLACK:Seven. Why didn't Jarvik see this?

NARRATOR: There was more that Jarvik had not seen. Clack found that Ichthyostega's legs were also not made for walking. They, too, were more like paddles.

Ichthyostega lived around the same time as Acanthostega, 360 million years ago. And while they both had limbs and gills, Acanthostega was a bit more fish-like, especially in the structure of its tail. These differences show how evolution was experimenting, tinkering with different body plans that would eventually result in all modern four-limbed animals.

But what drove these changes?

It had been widely accepted that fish evolved legs to move between bodies of water during times of drought, the drying pond scenario. But now that explanation no longer fit the facts.

The next piece of the puzzle was unearthed in the United States. During the Devonian Period, North America was part of a huge landmass called Laurasia, made up of present-day Europe, Asia and Greenland. Lying near the equator, Laurasia had great tropical sandstone formations, which became home to fossils of myriad life forms. And when the continents drifted to their present positions, over the next 400 million years, these fossils were scattered across the globe.

Here in Pennsylvania, a wide stretch of Devonian sandstone runs through the hills. Most of the range is forested, the ancient sandstone layers, buried. But there are places where the Devonian layer was exposed when the hills were blasted away to build new highways.

Paleontologist Ted Daeschler began combing this area, known as Red Hill, while a graduate student of Neil Shubin. No one had found tetrapod fossils here despite years of searching, and Daeschler was hoping for a change of luck in a newly excavated road-cut.

TED DAESCHLER: Well, sometimes you'll open up a rock and it really changes the whole way that you may be thinking about a certain problem, about something in evolution that you've learned. But you might change what the next student will learn. In fact, science works by building on the ideas of others.

NARRATOR: In 1995 Daeschler opened up a Red Hill rock and discovered a nearly perfect bone. It was the first piece of a Devonian tetrapod ever found on the North American continent.

TED DAESCHLER: We named it Hynerpeton, which means crawling animal from Hyner. And actually the town down below us here is Hyner, Pennsylvania. And the first part of the body that we found, actually the original material and all we had to work with for a while, was its shoulder girdle. And a shoulder girdle actually is a very interesting part to find if you're looking at some of the earliest limbed animals because it shows you where that limb attached into the body. And we could tell from the shoulder girdle of Hynerpeton that it was an animal with very muscular limbs. It's not like the shoulder girdle of a fish at all.

NARRATOR: More able to carry its own weight than either Acanthostega or Ichthyostega, Hynerpeton could possibly have walked on land.

Daeschler and his colleagues had noticed something else. Among the swathes of red sandstone were patches of green material.

TED DAESCHLER: Okay, the majority of the rock out here at Red Hill, of course, is red. Climbing up through sandstones, siltier sandstones and mudstones, right into this zone. Up here, we start with a green layer. It's reduced probably because of all of the plant material that's buried within the rock here.

NARRATOR: Finding these fossilized plants prompted the Red Hill team to rethink some old ideas. Perhaps the late Devonian environment wasn't as drought-ridden as experts had thought for nearly a century. Perhaps it was more like a rainforest.

TED DAESCHLER: The most common thing we're finding is a tree-like plant. It actually has a long, tall trunk and some people say these got up to 30 meters tall. So these were truly the first canopy, sort of, producing plants. We also find fern-like plants and a variety of other things. And so we're really seeing a diversity from a site like Red Hill.

Well, these stream systems that were running across big, wide floodplains 370 million years ago would have created big, muddy channels. And in between those channels there would have been forests, in fact, these were some of the first forests on Earth. Plants had finally taken hold of land environments, and that's a very important change when you think about it. The Earth was brown and muddy for the billions of years previous to this point in time. And it was during the late Devonian, really, that the land got green...and especially in wet areas like these deltas, that were shedding off and running into a seaway out in Ohio.

So what was happening on land was new, and what was happening with the animals in the fresh water environments was also new.

NARRATOR: The Earth may once have been barren, but the findings in Pennsylvania suggest that by the end of the Devonian, the Earth was densely forested and etched with rivers. These were bordered by something completely new, swamp. The first four-limbed creatures may have evolved in this wholly new ecosystem, just the kind of environmental shift that can trigger major evolutionary change.

KEITH THOMSON: For the very first time in Earth history animals and plants are living on land in a significant, permanent way. And a lot of open niches in that waiting to be exploited.

NARRATOR: Some of these new niches were the margins of this watery world. In the tangle of vegetation creatures with limbs and fingers would have a real advantage over those with fins.

TED DAESCHLER:I think we have to think of these fins or, or limbs, or "flims" as something that would be used by the animal for moving through more complex environments like swamps, or environments that...where there may have been trees down in channels, or just shallow water, to pursue prey or to escape the guy who's trying to prey upon you.

NARRATOR: And there was definitely something to escape from. The Red Hill team found evidence of a predator of terrifying proportions. A fish Keith Thomson had named Hyneria.

TED DAESCHLER:Hyneria is the most common lobe-finned fish at this site. It's also the biggest. It's probably two or three meters long. This, this is a single tooth from a large hyneria, and these were carnivorous, obviously.

KEITH THOMSON: And with a few predators like that around swimming in the channels of an estuary of a mud flat delta system, you could see why some of the lobe-fin fishes might find it very judicious to hop out into the growth...into the undergrowth, and find somewhere else to live.

NARRATOR: The revised picture of the Devonian environment opened up new ways of thinking about the forces that drove the evolution from fish to tetrapods. Limbs seem to have evolved not after a fish ventured onto land, but before. They were useful to navigate through swamps, to avoid predators, or, perhaps, to lay eggs on shore, out of harm's way. Limbs and fingers evolved because they gave the creatures who had them a distinct advantage in the swampy Devonian world. Using them to walk on dry land was a happy accident.

NEIL SHUBIN:What the tetrapod story shows us is that evolution is not goal-directed. It wasn't trying to evolve limbs. What is evolution in this case? It's just tinkering. What we're seeing is in these streams, 370 million years ago, a bunch of different types of fish tinkering with new ways of making a living, living in that new environment. It just so happened, one of those tinkered solutions was extremely successful. It led to all later life that was to live on land.

NARRATOR: More of nature's tinkering would be revealed when Per Ahlberg, now of London's Natural History Museum, visited eastern Europe. He began rummaging around in a forgotten drawer of an obscure fossil collection at a museum in Latvia, and a piece of bone caught his eye. It turned out to be a jaw fragment from a Devonian creature that showed both fish and tetrapod characteristics.

PER AHLBERG:I picked this piece up, had a look at it, turned it around, and, really, I knew within about half a minute that here was something absolutely extraordinary.

NARRATOR: Ahlberg named the creature Livoniana, after the region in Latvia where it was found. And to confirm his suspicions that it really was a transitional fossil, he ran it through something called a "cladistic analysis." It's a database that lists all the anatomical features that distinguish fish from tetrapods.

Some are obvious: does the specimen have limbs or fins? Lungs, gills or both? Others reflect minute shifts in the position of blood vessels or bones. Such fine detail allows scientists to identify a creature from just a fossil fragment and place it on an evolutionary tree relative to other animals.

As an example of the kinds of distinctions cladistic analysis can make, Ahlberg has placed a lobe-finned fish jaw on one side, the Livoniana jaw in the middle, and a tetrapod jaw on the other side. The differences are subtle, but significant.

PER AHLBERG:What you can see if we look at the endpoints is that these two jaws differ in quite a lot of ways. First of all, if we look at this pit in the fish jaw which is particularly important feature, a deep hollow that goes all the way through to underlying bones, the bone you get in the bottom there is a different one to the ones that are coming up on the surface here. In Livoniana that's the same place. The pit has almost disappeared surrounding now a blood vessel hole here, which we didn't have in the other jaw. If we look at the tetrapod, we have the pit now gone altogether. And here's that lower blood vessel hole there in the tetrapod. So, in this respect, Livoniana kind of agrees with the tetrapod. On the other hand, we find that in the fish a bone from the outer surface of the jaw comes round down to here and ends, and it's, it abuts against another bone up here called the pre-articula.

In the tetrapod the bone from the outer face comes all the way up here. It forms a big tongue extending backwards, so, and it comes all the way up here beneath it. So quite a different arrangement.

Livoniana here has got the junction and the bone exposed on the surface just like the fish, so in this case Livoniana agrees with the fish. So as you can see, depending on which characteristic you look at, it either lies sort of halfway between or it agrees with the tetrapod or it agrees with the fish—exactly what you would expect from an intermediate form.

NARRATOR: The analysis shows Livoniana to be clearly a transitional creature. It fits on an evolutionary tree about midway between fish and tetrapods.

And it has one very odd feature: there are seven rows of teeth. That makes it unlike any other creature we know of and suggests that it may have been one of a host of evolutionary experiments, most of which met with extinction, but one of which was the ancestor of us all.

In recent years, Darwin's 400 million-year-old detective story has become much less shrouded in mystery. In the Devonian world of forest and rivers bordered by swamp, a whole new way of life was born. The distinction between being in the water and out of it became blurred. From this swampy place, our ancestor came crawling onto dry land. It was one of the most momentous events in all of evolution, for one day that creature's descendants would inherit the Earth.

For other momentous events in the history of life on Earth, from the evolution of the first simple plants to the arrival of modern humans, visit NOVA's Website, on PBS.org or America Online, Keyword PBS.

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Coming in March on NOVA: an expedition trapped for three years in an icy hell; one man determined to bring his men back alive. Polar exploration was littered with dead bodies. He could not rest until he had saved them: Shackleton's Voyage of Endurance.

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PRODUCTION CREDITS

The Missing Link

Produced by
Matthew Barrett
David Espar

Narrated by
Peter Thomas

Edited for NOVA by
David Espar

Associate Producers
Jennifer Lorenz
Jackie Higgins

Camera
Brian Dowley
Vaughan Matthews
Mark Molesworth
Steve Downer
Brand Jordaan

Sound Recordists
David Keene
Alex Sullivan
Roger Stamp
Tony Bensusan

Original Music
Ray Loring

Animation
Chris Pullman
Gaye Korbet
Bruce Walker
Alisa Placas
Daryl Myers
Frank Capria

Online Editor
Ed Ham

Colorist
Mark Kueper

Audio Mix
Heart Punch Studio

Research
Jenny Foster

Production Coordinator
Emily Samson

Horizon Unit Manager
Alex Barraki

Horizon Executive Editor
John Lynch

Horizon Series Editor
Bettina Lerner

Archival Material
ABC/British Movietonews
Discovery Communications
National Geographic Television
New Zealand Natural History Films
Getty Images
Swedish Museum of Natural History
J.L.B. Smith Institute of Ichthyology
Max Planck Institute, Germany

Special Thanks
Powderhouse Productions
Clear Blue Sky Productions
Dr. Farish Jenkins, Harvard University
Carl Zimmer
Latvian Museum of Natural History
Geological Museum, Copenhagen
Dr. Michael Coates, Univ. of Chicago
Sarah Finney

NOVA Series Graphics
National Ministry of Design

NOVA Theme
Mason Daring
Martin Brody
Michael Whalen

Post Production Online Editor
Mark Steele

Closed Captioning
The Caption Center

Production Secretaries
Queene Coyne
Linda Callahan

Publicity
Jonathan Renes
Diane Buxton
Katie Kemple

Senior Researcher
Ethan Herberman

Unit Managers
Sarah Goldman
Jessica Maher
Sharon Winsett

Paralegal
Nancy Marshall

Legal Counsel
Susan Rosen Shishko

Business Manager
Laurie Cahalane

Assistant Editor, Post Production
Dan Van Roekel

Post Production Assistant
Patrick Carey

Associate Producer, Post Production
Nathan Gunner

Post Production Supervisor
Regina O'Toole

Post Production Editors
David Eells
Rebecca Nieto

Supervising Producer
Lisa D'Angelo

Senior Science Editor
Evan Hadingham

Senior Series Producer
Melanie Wallace

Managing Director
Alan Ritsko

Executive Producer
Paula S. Apsell

A BBC/WGBH Boston Co-Production

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NOVA | Transcripts | The Missing Link

PRODUCTION CREDITS