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The Missing Link-如何長腳-真耶、假耶、幻想

(2016-10-14 10:16:02)


分类: 神/哲/科/進/心理/大脑相关研

The Missing Link EVOLUTION- THE LINK TO OUR PAST DocumentaryHistory science 

BBC2 9.00pm Thursday 1st February 2001


放此参考,看官需批判 究竟足以称为科学或进化论者一厢情愿的科幻小说

想想看, 鱼塘水快乾了,鱼不改变(突变)不能活,於是那长出肺的,活了下来,成为我们的祖先。在历史那重要一刻,水里用鳃呼吸的鱼,从来不知道可以用肺中红血球交换氧气方式呼吸的鱼,在那一刻,长出肺。需不需要先长一半肺,鳃呢?总不会运气那麽好,一变就变成用肺呼吸吧?



Per Ahlberg believes this jawbone offers new insight to how fish may have formed limbsNARRATOR (JOHN SHRAPNEL): In an obscure museum in eastern Europe a fossil hunter has made a momentous discovery. The young palaeontologist was randomly sifting through a collection of drawers that hadn't been examined for 30 years.

DR. PER AHLBERG (The 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 particularly exciting and then pulling open one drawer I spotted in the middle, sitting in a little cardboard tray like soap, a fossil the likes of which would never have been found anywhere in the world.

NARRATOR: Per Ahlberg may have found a vital piece of evidence in a 360 million year old detective story: the mystery of how our earliest ancestors first crawled out of the water and onto the land.

PER AHLBERG: The feeling was rather like that of the doors of Tutankhamen's tomb swinging open. The elation of understanding what was not previously known, an overpowering experience of oh, so this is what it's like.

NARRATOR: Of all the tales of life on Earth there is one more fabulous than all the others - the story of how we got our legs. Scientists believe that long ago a fish came onto the land, grew legs and started to walk. It is one of the most crucial events in the history of life because that animal is our ancestor, but how and why that fish grew legs is one of the biggest mysteries in evolution. It baffled the finest minds in science for over a century. All they had to guide them were the theories of Charles Darwin, evolution's founding father. Darwin said the answer would lie out there in the rocks. Somewhere there would be fossils that would explain everything. Palaeontologists would go scurrying all over the world trying to find them, but such early fossils are rare.

PROF. KEITH THOMSON (Oxford University Museum): There's still so very, very few pieces of evidence. This is like one of those terrible classic murder mysteries, you know, that goes on for 30 or 40 years and people slowly trying to pick up a little bit of evidence, a little bit there.

NARRATOR: But the more scientists looked the more they realised one crucial fossil was missing. This would become a challenge to the whole Theory of Evolution and the story of how we got our legs would become the quest for the ultimate missing link. The whole quest began 150 years ago with one simple observation. A vast array of animals are, in fact, related. They all have four legs, they are tetrapods.

PER AHLBERG: We are tetrapods, to whit one, two, three, four.

KEITH THOMSON: Every mammal, every dog, cat is, is a tetrapod, four legs.

PER AHLBERG: A horse is a tetrapod evidently enough, so is a bird.

KEITH THOMSON: All the reptiles, all the frogs, the salamanders and even the snakes which don't have legs at all, whales which don't seem to have legs - they're all tetrapod animals.

NARRATOR: Scientists became bewitched by the fact that under the skin all tetrapods are basically the same. They all had spines kept firm by special interlocking spurs. It is as true of us as it was of the dinosaurs. All tetrapods had a pelvis attached to the backbone to support their weight. They all had a ribcage to protect their heart and lungs and they all breathed air through nostrils. Their limbs invariably consisted of one single bone at the top. A pair of bones underneath leading to feet or hands which scientists noted never seemed to have more than five fingers or toes. It was true of dinosaurs, human beings and even whales, for under their flippers they have five fingers. This similarity convinced scientists all tetrapods must come from just one type of creature, a single common ancestor. To prove it they thought they needed just two fossils. They needed the first tetrapod, the very first land walking creature with four legs with five toes and they needed the fish from which it came, a fish that could grow legs. Find these two fossils, compare them and in the differences between them we would learn the reason why a fish had developed legs. They had one huge clue to set them off: they knew this evolution must have happened 400 million years ago, in the Devonian era. The Devonian is so long ago that barely any rock can now be found from it, let alone fossils, but academics knew that before it nothing walked and after it everything did, so the evolution of legs must have happened then and they thought they knew something else. Textbooks said it was a time of blistering heat when almost nothing could live on the land, not even plants.

PER AHLBERG: A barren, empty wasteland, no greenery, no trees, no insects, wind keening over the rocks.

NARRATOR: It meant most life survived not on land but in the water. This was the age of fish and one of them, the one with the beginning of legs, was the first fossil they needed to find.

KEITH THOMSON: And really we are just one of the ways of being a fish. You see it's just a modified fish.

NARRATOR: Finding that fish fossil proved easy. By the turn of the 19th century all eyes were on a Devonian group called the lobe-finned. Lobe-finned uniquely have a bone structure in their fins that seems to be a precursor to our legs and arms and one particular lobe-finned, the long extinct eusthenopteron had all the leg bones except the feet and toes.

KEITH THOMSON: 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 when these bones, the 1 and the 2 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 fish from which we all came, the ancestor without legs, and early last century scholars developed a theory to explain why it might have evolved legs and started walking. That brutal Devonian sun must have been the cause of droughts. Fish would have been trapped in drying pools and faced death. To survive a few eusthenopterons must have dragged themselves on their fins, as mudskippers do today, out of the puddles in search of deeper water. Some could then have evolved on land. Their fins became legs, they grew five fingers and toes, they started to walk. They became tetrapods, our ancestors. Limbs had developed so they could do what we all do: walk on land. The drying pools theory seemed to explain everything, both why we had grown our legs and from what we'd evolved. All they needed to confirm it was to find that very first tetrapod, the creature into which the fish had evolved once it was on the land. If it was, as they'd predicted, an animal with five fingers and toes then the whole thing would make sense. Many would seek this mythical beast and for years no one found it, but eventually there was a man who did. In the 1930s a team of Swedes made a series of trips to Greenland, one of the few places in the world with outcrops of Devonian rock. Their mission: to find that first creature with legs. Among the party Erik Jarvik, one of the least loved figures in all palaeontology.

KEITH THOMSON: Erik Jarvik was tall and you could only describe him as dour. He had an eye problem one eye pointed off at an angle so this I think produced when he was in public a shyness and diffidence. He was extremely opinionated in the sense of having a very narrow view of what was important and dismissing everything else.

NARRATOR: And then a wonder. Jarvik's team found what everyone had been searching for - fossils of the very first creature with legs rather than fins. They called it ichthyostega.

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 Jarvik to analyse and describe the new discovery. This meant years of digging the fossilised bones out of the rock and then trying to reconstruct the anatomy of this strange creature. Jarvik was a brilliant anatomist, but he was also painstaking. He started in 1948, but did not finish until 1996. In those 48 years no one else was able to analyse the fossil.

KEITH THOMSON: To be honest it shouldn't take that long. I mean frankly. I mean you have to be really sort of anal to take that long to describe anything, especially when the world is just hanging on this.

NARRATOR: But Jarvik did produce two preliminary papers. These did confirm that existing theory. Ichthyostega was an Identikit land walking tetrapod with five fingers and toes. The mystery of how we got our legs was solved. After eusthenopteron the fish had struggled on its fins onto land. It had evolved into ichthyostega, the first tetrapod with legs. It was just as science had predicted. But then the doubts crept in. Only Jarvik had ever analysed the ichthyostega fossil and you just had to take his word for it, but worse, mutterings began that there was a gaping hole in the story. Eusthenopteron could not be the immediate ancestor of ichthyostega. The difference between them was too vast. Ichthyostega was a fully formed tetrapod with a ribcage, pelvis attached to the backbone, a spine with interlocking spurs and limbs with fingers and toes. The fish was still a fish, despite its primitive leg bones. It showed little sign of evolving any of these other tetrapod features. To prove one had become the other they needed more evidence.

PER AHLBERG: What this really adds up to is that these changes seem to be too gross to have happened in one step. There must be missing animals in here.

NARRATOR: For a start somewhere out there should be other species of Devonian tetrapods which might shed some light on the theory, but above all they needed to find an intermediate animal, one that showed the changes between fish and tetrapod actually happening, some strange beast that could walk, but was half fish/half tetrapod that was a missing link. The missing link had to be what Charles Darwin called a transitional form. These lie at the very heart of his Theory of Evolution because they show how one animal can mutate into another. Transitions occur in evolution when there is a dramatic environmental change. Creatures that cannot adapt to the new environment die out. but chance mutations often turn out to be the key to survival. There is an explosion of bizarre forms as a host of mutants experiment with living in the new environment. These freaks of nature will die out quickly and just some will become transitional forms, creatures that are half one animal and half another which bridge the old way of living and the new. Transitional forms are the most crucial fossils in all evolution.

KEITH THOMSON: They're important to the zoologist, to the palaeontologist because they, they show you what the process actually was. I think they're very important to the public as, as being direct evidence that there was a process of change that you can document.

NARRATOR: But transitional forms are also the rarest of beasts. By their very nature they were few in number and lived for just a short intermediate time until a wholly new animal evolved. In fact for years there was only one accepted transitional form - the archaeopteryx which was a dinosaur with feathers that marked the transition to birds.

PER AHLBERG: Presumably the transitional forms are very rapidly outcompeted 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: The trouble was without anything half fish/half tetrapod between eusthenopteron and ichthyostega the story of how we got our legs remained incomplete and lurking out there was a group dedicated to pointing out such gaps in the story of evolution with the aim of its ultimate destruction. Duane Gish is a scientist.

DR DUANE GISH (Institute for Creation Research): My Masters degree is chemistry from UCLA and my doctorate is in biochemistry from the University of California at Berkeley. After I finished my doctorate I went to Cornell University Medical College for three years where I worked with Dr…

NARRATOR: He's also a creationist, and believes that the world was created in six days, just as it says in the Bible according to God's plan, and therefore the whole Theory of Evolution he thinks must be wrong.

DUANE GISH: Evolution has no plan, no purpose, it's a random process.

NARRATOR: Creationists believe that all animals were made fully formed by God and since you cannot alter His work the very idea of evolution is ungodly.

CREATIONIST: What I'm not saying is everybody who believes in evolution is a Nazi. By no means, alright. What I'm saying though is if there is no God, OK, then morality is something you make up. You can tell what you want do for right or wrong. A lot of people will still perhaps be good, some won't.

NARRATOR: One main line of attack is the rarity of transitional forms. If you cannot show how a fish evolved into a tetrapod then, they argue, evolution never happened at all.

DUANE GISH: The fossil record is enormously better now than it was in Darwin's time, but it hasn't solved Darwin's problem. In our museum today we have about a quarter of a million different fossil species. If evolution's true, tens of thousands of those things should be of obvious intermediates, but they're not there. Every major type of creature appears fully formed, no ancestors and no transition form.

CREATIONIST: So we think there was an Ice Age that followed the Genesis flood maybe…

NARRATOR: Of course transitional forms were inevitably few in number and lived but a short time. The chances of one being fossilised is highly remote, but even so the contest between the creationists and science is being played out for the very highest stakes.

PER AHLBERG: What would happen if they won, if they truly won is that we would descend into another Dark Age. It's an unimaginably dark prospect.

CREATIONIST: …science has put into itself right now…

NARRATOR: The irritation for palaeontologists was that no one had yet unearthed a transitional form between fish and our earliest ancestor with legs. One just had to be found, and then, for a brief moment, it seemed one had been, when in 1938 a miracle happened. East London in South Africa. 63 years ago, just before Christmas, the curator of the local museum was idly sifting through a fisherman's catch down on the waterfront. Marjorie Courtenay-Latimer was then just 31 years old.

DR MARJORIE COURTENAY-LATIMER (East London Museum, South Africa): 22nd December 1938 was a wonderful day. I came upon this most beautiful fish. It was just on, just on 5ft. It was silver and gold and green and blue and had white kind of flecks on it and to my horror it had these slim-like fins and I thought to myself what on earth can this be, I've never seen a fish like this.

NARRATOR: She rushed the fish off to have it preserved. Then she asked Dr. J.L.B. Smith, a South African academic, to 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 were a breed of Devonian lobe-finned fish that were thought to have been extinct for over 76 million years.

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.

NARRATOR: The scientific community was transfixed. For decades the coelacanth had been touted as a possible transitional form between fish and tetrapods, but no one had really known enough about it. It existed only as a fossil.

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

PROF. SMITH: Coelacanths are close relatives of the fish that scientists consider was the ancestors of all land animals.

NARRATOR: Smith proclaimed the coelacanth a transitional form and as proof he announced that it would actually walk on the bottom of the sea.

PROF. SMITH: 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.

NARRATOR: But he knew he would have to find one alive and walking to prove the coelacanth was the elusive transitional form. He looked for 13 years until another one was found, and it didn't walk - it swam. It was just another fish.

MARJORIE COURTENAY-LATIMER: Well it was thrown out. There were quite a lot of nasty letters send to J.L.B. too to say that he was having a daydream.

NARRATOR: There were still no transitional forms, nothing to show that a fish with fins had walked over land and evolved into our first ancestor with legs, nothing to silence the creationists and there it rested for 30 years. Evolutionists still clung to the theory that a fish evolved into the first five-fingered land walking ancestor as a result of drought, even though they knew there was a gap in the story, but without new evidence no one could come up with a better idea. But then in 1981 along came palaeontology's avenging angel.

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

NARRATOR: Jenny Clack had long dreamed of embarking on the quest to find out why we first walked on the land, but when she arrived in Cambridge it seemed a remote hope.

DR JENNY CLACK (Cambridge University Museum of Zoology): 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 in 1970. In one corner he'd made an extraordinary note that showed though he knew about rocks, he knew little of fossils. He'd written that he had found remains of ichthyostega, Jarvik's legendary first tetrapod of which only one complete specimen existed anywhere in the world.

JENNY CLACK: He's noted, "Ichthyostega bones and skull bones common," and early tetrapod specimens are not common anywhere, particularly not Devonian tetrapods on a mountain in Greenland and to see this in his notebook just set the bells ringing. We have to go there.

NARRATOR: By 1987 Clack was off to this apparent goldmine of tetrapods, taking along a student, Per Ahlberg. It was a big undertaking.

JENNY CLACK: I regarded this as pretty much of a fool's errand actually. I thought it was far too difficult, certainly scary.

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, 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 air-strip which is only manned during the 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.

NARRATOR: At first it looked more like embarrassment.

JENNY CLACK: As the week wore on, or the first two weeks wore on we still hadn't found the, the locality that we were looking for. We were beginning to think are we on the right mountain?

NARRATOR: Then Clack saw something.

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

NARRATOR: Clack's team had not found the transitional form between fish and tetrapods, but she had found something almost as rare. It was another species of Devonian tetrapod, a species called acanthostega, different from Jarvik's but clearly sharing the same ancestry and, therefore, also related to humans. Acanthostega was only the second complete Devonian tetrapod ever to be found. Clack returned to Cambridge with dozens of tetrapod fossils. It meant that at last someone else would be able to do original work in the field of how we got our legs, but the true importance of the trip did not emerge until 1990 when a colleague, Mike Coats, started work on the specimen she'd almost thrown away, the acanthostega. He started to dig its hand out of the rock and he expected, of course, to find just five fingers.

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. 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?

NARRATOR: But it was true. Acanthostega had eight fingers on one hand. The very earliest tetrapods did not, as all the textbooks claimed, have five fingers after all. It suddenly raised the question: if the most basic assumption behind the previous 100 years' research was wrong then what else might be wrong?

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: Scientists now believe our earliest ancestors with legs must have started out life with numerous digits and then evolution reduced them to five over the aeons that followed and the shocks just kept getting bigger. Another fundamental assumption, that we had evolved legs for the express purpose of walking, just could not be true.

JENNY CLACK: If you look at the limbs what you find is that the joints are all angled so that the limb would have stretched out just to the sides. 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, although it was a fully formed tetrapod it could never have lived out of the water. It had gills just like a fish. It meant the evolution of our legs could never have had anything to do with walking on land. Jenny Clack's discovery of the acanthostega had changed everything. The old explanation that we'd evolved our legs after a fish came onto land just could not be true.

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 decided to check her findings against ichthyostega, Jarvik's iconic first tetrapod, a fragment of which she'd also collected 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 did not see. Clack discovered that ichthyostega did not, as Jarvik had shown, have legs made for walking. they too were paddles. Jarvik, who died three years ago, had simply got it wrong and no one will ever know why. Clack's discoveries meant the whole quest as to how and why we had evolved limbs would have to start again. Now they didn't just need the transitional form to link fish to tetrapods - though that was still missing - but they also needed a whole new reason why limbs had evolved. Why would any creature need legs that weren't for walking? In a valley bypassed by the American Dream, they would find the answer, the real story of how we got our legs. Renovo, Pennsylvania is what happens at the end of the railway line when the trains don't come any more, but the town is rich in something rare and precious. Beneath the trees are swathes of red Devonian sandstone. So in 1993 along came a lone fossil hunter. He is called Ted Daeschler. Daeschler made his name finding two more new Devonian tetrapod species in a roadside cutting called Red Hill, but the thing that took both him and Red Hill into fossil hunting legend was when he examined one of the site's peculiar features. In amongst all the red rock was a broad green layer.

DR TED DAESCHLER (Philadelphia Academy of Natural Sciences): OK, the majority of the rock out here at Red Hill of course is red. Climbing up through sandstones, silt here, 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: Fossil plants. By finding them alongside Devonian tetrapods Daeschler had made a vital breakthrough. He could now reconstruct for the very first time the true environment in which the first tetrapods had evolved. The Devonian wasn't the barren, treeless landscape spoken of in textbooks. 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 30m 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 and I think it's important because it's showing us that these were actually complex environments.

NARRATOR: The textbooks had got it wrong again. The Earth may once have been barren, but by the end of the Devonian it was very, very wet and densely forested with huge permanent rivers. These were bordered by something completely new: swamp, that grey area between land and water. The first tetrapods had evolved in this wholly new eco-system, precisely the kind of thing that could indeed be a spur to major evolutionary change.

KEITH THOMSON: Oh it's completely new. By the time you get to the end of the Devonian for the very first time in Earth history animals and plants are living on land in a significant, permanent way. That's, that's brand new and a lot of open niches in that waiting to be exploited.

NARRATOR: Those new niches were the margins of this watery world. In the tangle of vegetation limbs with fingers would have given tetrapods a unique advantage over fish.

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 most definitely something from which to escape. Over and over again Daeschler and his team found evidence of a fish called hyneria, a predator of terrifying proportions.

TED DAESCHLER: Hyneria is the most common, lobe-finned fish at this site. It's also the biggest. It's probably two or three metres long. This, this is a single tooth from a large hyneria and these were carnivorous obviously. When you see the tetrapods in the same fauna with these hyneria it, it really does make you think well maybe escaping from large, large predators like this hyneria was, was a pretty important thing for these early tetrapods.

NARRATOR: The mystery of why we had evolved limbs was finally solved. They were not for walking, but navigating through swamps, just like this hellbender salamander does today, but there was still a hole in the story. There was still no fossil that showed the process of change from fish to tetrapod actually happening. The transitional form, something half fish/half four legged animal, was still missing. Nothing to silence the creationists.

DUANE GISH: Now why do we have all these various opinions? It's the lack of transitional forms, stupid.

NARRATOR: And then Per Ahlberg made his fateful visit to Latvia. In that forgotten museum drawer he may have found what everyone had been looking for. His trained eye told him the fossil was a fragment of jaw right from the time of transition and it was certainly part fish and part tetrapod.

PER AHLBERG: The information we get from this one paltry little bone is quite overwhelming.

NARRATOR: He named the jaw livoniana and to prove it really was a transitional form he ran it through something called a cladistic analysis. A computer is programmed with all the anatomical features that distinguish fish from tetrapods. Some are obvious, such as does the specimen have limbs or fins? Others are tiny shifts in the position of blood vessels or bones. Such minute details allow scientists to identify a creature from just a fossil fragment and place it in an evolutionary tree relative to other animals. For this comparison a lobe-finned fish jaw is on one side, livoniana is in the middle and a Devonian tetrapod jaw is on the other side.

PER AHLBERG: What you can see if we look at the end points 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 what are coming up on the surface here. In Devonian that's the same place. The pit has almost disappeared surrounding now a blood vessel hole here which we wouldn'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-articular. 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: Ahlberg believes livoniana really is an elusive transitional form, almost exactly half fish and half tetrapod. It is certainly the only fossil yet discovered that shows the process of change between the two actually taking place. It also has one freakish feature: there are seven rows of teeth. It is unlike any other creature we know of. This suggests it must be one of the host of mutants that made this change, just one of which would eventually become our ancestor. Livoniana is a real missing link. Darwin's 360 million year old riddle about how we developed our legs has been solved. It was vegetation on land and in the water that let flourish an explosion of mutations among lobe-finned fish. The most successful of these mutations, the one that stood the test of time, was the development of limbs with fingers and toes. In this new world of forest and rivers bordered by swamp a whole new way of life was born, for in these shallows the distinction between being in the water and out of it became blurred. It was out of this swampy area that our earliest ancestor came crawling over land. It was not pre-ordained, but chance, a series of evolutionary accidents, but it just so happened that that creature's children would indeed inherit the earth one day.


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