1 00:00:10,905 --> 00:00:17,932 In an obscure museum in eastern Europe a fossil hunter has made a momentous discovery. 2 00:00:20,118 --> 00:00:24,489 The young palaeontologist was randomly sifting through a collection of drawers 3 00:00:24,670 --> 00:00:27,325 that hadn't been examined for 30 years. 4 00:00:28,115 --> 00:00:33,346 I was going through these drawers finding drawer after drawer of very much the sort of fossil I would expect to find, 5 00:00:33,476 --> 00:00:39,646 really nothing particularly exciting and then pulling open one drawer I spotted in the middle, 6 00:00:39,647 --> 00:00:41,943 sitting in a little cardboard tray like soap, 7 00:00:42,695 --> 00:00:48,865 a fossil the likes of which would never have been found anywhere in the world. 8 00:01:14,398 --> 00:01:20,049 Of all the tales of life on Earth there is one more fabulous than all the others 9 00:01:20,352 --> 00:01:23,777 the story of how we got our legs. 10 00:01:30,757 --> 00:01:37,433 Scientists believe that long ago a fish came onto the land, grew legs and started to walk. 11 00:01:37,984 --> 00:01:40,893 It is one of the most crucial events in the history of life 12 00:01:41,151 --> 00:01:44,612 because that animal is our ancestor, 13 00:01:50,301 --> 00:01:53,222 but how and why that fish grew legs 14 00:01:53,422 --> 00:01:56,061 is one of the biggest mysteries in evolution. 15 00:01:56,694 --> 00:02:00,729 It baffled the finest minds in science for over a century. 16 00:02:03,345 --> 00:02:09,283 All they had to guide them were the theories of Charles Darwin, evolution's founding father. 17 00:02:12,890 --> 00:02:16,541 Darwin said the answer would lie out there in the rocks. 18 00:02:16,849 --> 00:02:20,427 Somewhere there would be fossils that would explain everything. 19 00:02:20,955 --> 00:02:25,325 Palaeontologists would go scurrying all over the world trying to find them, 20 00:02:25,706 --> 00:02:28,448 but such early fossils are rare. 21 00:02:28,947 --> 00:02:34,650 There's still so very, very few pieces of evidence. This is like one of those terrible 22 00:02:34,753 --> 00:02:39,255 classic murder mysteries, you know, that goes on for 30 or 40 years and people 23 00:02:39,256 --> 00:02:42,921 slowly trying to pick up a little bit of evidence, a little bit there. 24 00:02:44,107 --> 00:02:49,445 But the more scientists looked the more they realised one crucial fossil was missing. 25 00:02:50,101 --> 00:02:56,791 This would become a challenge to the whole Theory of Evolution and the story of how we got our legs 26 00:02:56,956 --> 00:03:01,006 would become the quest for the ultimate missing link. 27 00:03:20,324 --> 00:03:25,328 The whole quest began 150 years ago with one simple observation. 28 00:03:26,005 --> 00:03:34,290 A vast array of animals are, in fact, related. They all have four legs, they are tetrapods. 29 00:03:38,560 --> 00:03:42,721 We are tetrapods, to whit one, two, three, four. 30 00:03:43,619 --> 00:03:48,440 Every mammal, every dog, cat is, is a tetrapod, four legs. 31 00:03:48,441 --> 00:03:52,070 A horse is a tetrapod evidently enough, so is a bird. 32 00:03:52,071 --> 00:03:58,338 All the reptiles, all the frogs, the salamanders and even the snakes which don't have legs at all, 33 00:03:58,686 --> 00:04:03,231 whales which don't seem to have legs, they're all tetrapod animals. 34 00:04:08,107 --> 00:04:14,888 Scientists became bewitched by the fact that under the skin all tetrapods are basically the same. 35 00:04:15,585 --> 00:04:20,479 They all had spines kept firm by special interlocking spurs. 36 00:04:21,249 --> 00:04:24,475 It is as true of us as it was of the dinosaurs. 37 00:04:26,399 --> 00:04:30,486 All tetrapods had a pelvis attached to the backbone to support their weight. 38 00:04:34,665 --> 00:04:38,037 They all had a ribcage to protect their heart and lungs 39 00:04:39,082 --> 00:04:41,941 and they all breathed air through nostrils. 40 00:04:43,958 --> 00:04:48,174 Their limbs invariably consisted of one single bone at the top. 41 00:04:49,237 --> 00:04:52,884 A pair of bones underneath leading to feet or hands 42 00:04:53,086 --> 00:04:57,521 which scientists noted never seemed to have more than five fingers or toes. 43 00:04:59,190 --> 00:05:07,438 It was true of dinosaurs, human beings and even whales, for under their flippers they have five fingers. 44 00:05:08,739 --> 00:05:13,963 This similarity convinced scientists all tetrapods must come from just one type of creature, 45 00:05:14,201 --> 00:05:16,657 a single common ancestor. 46 00:05:18,160 --> 00:05:21,936 To prove it they thought they needed just two fossils. 47 00:05:22,284 --> 00:05:29,304 They needed the first tetrapod, the very first land walking creature with four legs with five toes 48 00:05:29,560 --> 00:05:34,821 and they needed the fish from which it came, a fish that could grow legs. 49 00:05:35,535 --> 00:05:40,686 Find these two fossils, compare them and in the differences between them 50 00:05:40,687 --> 00:05:44,993 we would learn the reason why a fish had developed legs. 51 00:05:45,891 --> 00:05:48,329 They had one huge clue to set them off: 52 00:05:48,824 --> 00:05:56,173 they knew this evolution must have happened 400 million years ago, in the Devonian era. 53 00:06:06,364 --> 00:06:12,119 The Devonian is so long ago that barely any rock can now be found from it, let alone fossils, 54 00:06:12,432 --> 00:06:17,692 but academics knew that before it nothing walked and after it everything did, 55 00:06:17,985 --> 00:06:23,795 so the evolution of legs must have happened then and they thought they knew something else. 56 00:06:24,052 --> 00:06:29,111 Textbooks said it was a time of blistering heat when almost nothing could live on the land, 57 00:06:29,661 --> 00:06:31,237 not even plants. 58 00:06:32,593 --> 00:06:37,230 It meant most life survived not on land but in the water. 59 00:06:41,684 --> 00:06:47,385 This was the age of fish and one of them, the one with the beginning of legs, 60 00:06:47,660 --> 00:06:50,703 was the first fossil they needed to find. 61 00:06:53,801 --> 00:06:58,933 Finding that fish fossil proved easy. By the turn of the 19th century 62 00:06:59,079 --> 00:07:03,056 all eyes were on a Devonian group called the lobe-finned. 63 00:07:03,570 --> 00:07:10,461 Lobe-finned uniquely have a bone structure in their fins that seems to be a precursor to our legs and arms 64 00:07:10,772 --> 00:07:19,460 and one particular lobe-finned, the long extinct eusthenopteron had all the leg bones except the feet and toes. 65 00:07:22,703 --> 00:07:29,118 Here in these fossils the limb was just laid out simply beautifully and it was so easy to turn it 66 00:07:29,247 --> 00:07:31,574 in your mind into a tetrapod when 67 00:07:32,345 --> 00:07:37,477 these bones, the 1 and the 2 bones, they were, they were laid out and there were these 68 00:07:37,623 --> 00:07:41,399 bits in the, the, the ankle and the, the wrist and so on. 69 00:07:50,765 --> 00:07:54,907 Absolutely fantastic, beautiful material, clinched it really. 70 00:07:57,858 --> 00:08:04,585 What they thought they'd clinched was the fish from which we all came, the ancestor without legs, 71 00:08:05,648 --> 00:08:12,319 and early last century scholars developed a theory to explain why it might have evolved legs and started walking. 72 00:08:15,380 --> 00:08:19,449 That brutal Devonian sun must have been the cause of droughts. 73 00:08:20,000 --> 00:08:24,142 Fish would have been trapped in drying pools and faced death. 74 00:08:24,472 --> 00:08:29,549 To survive a few eusthenopterons must have dragged themselves on their fins, 75 00:08:29,622 --> 00:08:34,736 as mudskippers do today, out of the puddles in search of deeper water. 76 00:08:45,917 --> 00:08:48,336 Some could then have evolved on land. 77 00:08:48,648 --> 00:08:55,338 Their fins became legs, they grew five fingers and toes, they started to walk. 78 00:08:55,539 --> 00:09:02,779 They became tetrapods, our ancestors. Limbs had developed so they could do what we all do: 79 00:09:03,035 --> 00:09:05,052 walk on land. 80 00:09:07,856 --> 00:09:11,101 The drying pools theory seemed to explain everything, 81 00:09:11,504 --> 00:09:15,280 both why we had grown our legs and from what we'd evolved. 82 00:09:15,555 --> 00:09:20,082 All they needed to confirm it was to find that very first tetrapod, 83 00:09:20,375 --> 00:09:24,206 the creature into which the fish had evolved once it was on the land. 84 00:09:25,012 --> 00:09:31,501 If it was, as they'd predicted, an animal with five fingers and toes then the whole thing would make sense. 85 00:09:34,652 --> 00:09:40,059 Many would seek this mythical beast and for years no one found it, 86 00:09:44,696 --> 00:09:49,828 In the 1930s a team of Swedes made a series of trips to Greenland, 87 00:09:50,103 --> 00:09:54,208 one of the few places in the world with outcrops of Devonian rock. 88 00:09:55,913 --> 00:10:00,532 Their mission: to find that first creature with legs. 89 00:10:02,640 --> 00:10:05,371 Among the party Erik Jarvik, 90 00:10:07,883 --> 00:10:11,530 Jarvik's team found what everyone had been searching for 91 00:10:12,153 --> 00:10:17,322 fossils of the very first creature with legs rather than fins. 92 00:10:18,001 --> 00:10:20,823 They called it ichthyostega. 93 00:10:23,078 --> 00:10:28,246 People had been looking for this in a way ever since Darwin, ever since 1859. 94 00:10:28,814 --> 00:10:34,862 This transition is the one that so intrigued everybody going from the water to the land and no evidence of it 95 00:10:34,863 --> 00:10:41,332 and then boom, they found it. Terribly, terribly exciting, really very, very important. 96 00:10:42,048 --> 00:10:45,530 It fell to Jarvik to analyse and describe the new discovery. 97 00:10:46,136 --> 00:10:49,636 This meant years of digging the fossilised bones out of the rock 98 00:10:49,765 --> 00:10:54,347 and then trying to reconstruct the anatomy of this strange creature. 99 00:10:56,876 --> 00:11:01,476 Jarvik was a brilliant anatomist, but he was also painstaking. 100 00:11:01,806 --> 00:11:07,598 He started in 1948, but did not finish until 1996. 101 00:11:08,038 --> 00:11:12,968 In those 48 years no one else was able to analyse the fossil. 102 00:11:13,811 --> 00:11:20,171 To be honest it shouldn't take that long I mean frankly. I mean you have to be really sort of 103 00:11:20,318 --> 00:11:26,275 anal to take that long to describe anything, especially when the world is just hanging on this. 104 00:11:26,569 --> 00:11:32,525 But Jarvik did produce two preliminary papers. These did confirm that existing theory. 105 00:11:32,965 --> 00:11:39,252 Ichthyostega was an Identikit land walking tetrapod with five fingers and toes. 106 00:11:41,799 --> 00:11:44,952 The mystery of how we got our legs was solved. 107 00:11:45,355 --> 00:11:52,870 After eusthenopteron the fish had struggled on its fins onto land. It had evolved into ichthyostega, 108 00:11:52,980 --> 00:11:58,955 the first tetrapod with legs. It was just as science had predicted. 109 00:12:10,666 --> 00:12:16,696 But then the doubts crept in. Only Jarvik had ever analysed the ichthyostega fossil 110 00:12:16,934 --> 00:12:24,009 and you just had to take his word for it, but worse, mutterings began that there was a gaping hole in the story. 111 00:12:24,595 --> 00:12:29,141 Eusthenopteron could not be the immediate ancestor of ichthyostega. 112 00:12:29,416 --> 00:12:32,458 The difference between them was too vast. 113 00:12:33,759 --> 00:12:40,651 Ichthyostega was a fully formed tetrapod with a ribcage, pelvis attached to the backbone, 114 00:12:41,274 --> 00:12:46,168 a spine with interlocking spurs and limbs with fingers and toes. 115 00:12:47,267 --> 00:12:51,721 The fish was still a fish, despite its primitive leg bones. 116 00:12:52,033 --> 00:12:56,486 It showed little sign of evolving any of these other tetrapod features. 117 00:12:58,520 --> 00:13:03,194 To prove one had become the other they needed more evidence. 118 00:13:04,349 --> 00:13:10,507 What this really adds up to is that these changes seem to be too gross to have happened in one step. 119 00:13:10,690 --> 00:13:13,879 There must be missing animals in here. 120 00:13:15,016 --> 00:13:20,680 For a start somewhere out there should be other species of Devonian tetrapods 121 00:13:20,973 --> 00:13:27,443 which might shed some light on the theory, but above all they needed to find an intermediate animal, 122 00:13:27,791 --> 00:13:32,630 one that showed the changes between fish and tetrapod actually happening, 123 00:13:33,656 --> 00:13:40,309 some strange beast that could walk, but was half fish/half tetrapod 124 00:13:40,895 --> 00:13:42,655 that was a missing link. 125 00:13:46,027 --> 00:13:50,481 The missing link had to be what Charles Darwin called a transitional form. 126 00:13:50,884 --> 00:13:57,390 These lie at the very heart of his Theory of Evolution because they show how one animal can mutate into another. 127 00:14:00,341 --> 00:14:05,033 Transitions occur in evolution when there is a dramatic environmental change. 128 00:14:11,686 --> 00:14:15,388 Creatures that cannot adapt to the new environment die out. 129 00:14:19,842 --> 00:14:24,131 but chance mutations often turn out to be the key to survival. 130 00:14:24,388 --> 00:14:31,261 There is an explosion of bizarre forms as a host of mutants experiment with living in the new environment. 131 00:14:35,532 --> 00:14:41,965 These freaks of nature will die out quickly and just some will become transitional forms, 132 00:14:41,983 --> 00:14:48,013 creatures that are half one animal and half another which bridge the old way of living and the new. 133 00:14:51,513 --> 00:14:56,444 Transitional forms are the most crucial fossils in all evolution. 134 00:14:58,955 --> 00:15:03,226 They're important to the zoologist, to the palaeontologist because they, 135 00:15:03,648 --> 00:15:10,008 they show you what the process actually was. I think they're very important to the public as, 136 00:15:10,063 --> 00:15:16,807 as being direct evidence that there was a process of change that you can document. 137 00:15:20,327 --> 00:15:24,450 But transitional forms are also the rarest of beasts. 138 00:15:24,725 --> 00:15:29,435 By their very nature they were few in number and lived for just a short intermediate time 139 00:15:29,655 --> 00:15:32,056 until a wholly new animal evolved. 140 00:15:32,863 --> 00:15:39,351 In fact for years there was only one accepted transitional form - the archaeopteryx 141 00:15:39,553 --> 00:15:45,290 which was a dinosaur with feathers that marked the transition to birds. 142 00:15:46,041 --> 00:15:53,648 Presumably the transitional forms are very rapidly outcompeted by their more, by their own more advanced descendants 143 00:15:54,142 --> 00:15:58,669 so these transitional episodes in the history of life tend to be brief 144 00:15:59,182 --> 00:16:04,993 and involve, it seems, relatively low numbers of species and probably low numbers of individuals. 145 00:16:08,016 --> 00:16:13,569 The trouble was without anything half fish/half tetrapod between eusthenopteron and ichthyostega 146 00:16:13,845 --> 00:16:19,435 the story of how we got our legs remained incomplete and lurking out there 147 00:16:19,766 --> 00:16:24,458 was a group dedicated to pointing out such gaps in the story of evolution 148 00:16:24,622 --> 00:16:28,013 with the aim of its ultimate destruction. 149 00:16:35,124 --> 00:16:37,617 Duane Gish is a scientist. 150 00:16:37,726 --> 00:16:41,722 My Masters degree is chemistry from UCLA 151 00:16:41,905 --> 00:16:46,340 and my doctorate is in biochemistry from the University of California at Berkeley. 152 00:16:46,615 --> 00:16:51,454 After I finished my doctorate I went to Cornell University Medical College for three years 153 00:16:51,636 --> 00:16:53,139 where I worked with Dr… 154 00:16:53,304 --> 00:16:59,060 He's also a creationist, and believes that the world was created in six days, 155 00:16:59,224 --> 00:17:05,053 just as it says in the Bible according to God's plan, and therefore the whole Theory of Evolution 156 00:17:05,145 --> 00:17:07,601 he thinks must be wrong. 157 00:17:08,645 --> 00:17:12,678 Evolution has no plan, no purpose, it's a random process. 158 00:17:16,032 --> 00:17:21,201 Creationists believe that all animals were made fully formed by God 159 00:17:22,190 --> 00:17:26,479 One main line of attack is the rarity of transitional forms. 160 00:17:26,754 --> 00:17:33,169 If you cannot show how a fish evolved into a tetrapod then, they argue, evolution never happened at all. 161 00:17:33,480 --> 00:17:39,272 The fossil record is enormously better now than it was in Darwin's time, but it hasn't solved Darwin's problem. 162 00:17:40,280 --> 00:17:45,174 In our museum today we have about a quarter of a million different fossil species. 163 00:17:45,504 --> 00:17:50,214 If evolution's true, tens of thousands of those things should be of obvious intermediates, 164 00:17:50,948 --> 00:18:00,918 but they're not there. Every major type of creature appears fully formed, no ancestors and no transition form. 165 00:18:01,358 --> 00:18:07,003 The irritation for palaeontologists was that no one had yet unearthed a transitional form between fish 166 00:18:07,077 --> 00:18:12,228 and our earliest ancestor with legs. One just had to be found, 167 00:18:13,932 --> 00:18:22,382 and then, for a brief moment, it seemed one had been, when in 1938 a miracle happened. 168 00:18:33,837 --> 00:18:36,733 East London in South Africa. 169 00:18:37,063 --> 00:18:40,417 63 years ago, just before Christmas, 170 00:18:40,418 --> 00:18:46,318 the curator of the local museum was idly sifting through a fisherman's catch down on the waterfront. 171 00:18:46,960 --> 00:18:52,348 Marjorie Courtenay-Latimer was then just 31 years old. 172 00:18:53,063 --> 00:18:57,572 22nd December 1938 was a wonderful day. 173 00:18:58,507 --> 00:19:05,197 I came upon this most beautiful fish. It was just on, just on 5ft. 174 00:19:06,791 --> 00:19:13,334 It was silver and gold and green and blue and had white kind of flecks on it 175 00:19:13,737 --> 00:19:18,704 and to my horror it had these slim-like fins 176 00:19:19,766 --> 00:19:25,961 and I thought to myself what on earth can this be, I've never seen a fish like this. 177 00:19:28,105 --> 00:19:33,622 She rushed the fish off to have it preserved. Then she asked Dr. J.L.B. Smith, 178 00:19:33,677 --> 00:19:37,050 a South African academic, to identify it. 179 00:19:37,930 --> 00:19:43,703 He stood at the head of the table and he said, "Well lass," he said, 180 00:19:44,747 --> 00:19:51,474 "this fish will be on the lips of every scientist in the world. It's a coelacanth." 181 00:19:59,739 --> 00:20:05,604 Coelacanths were a breed of Devonian lobe-finned fish that were thought to have been extinct 182 00:20:05,769 --> 00:20:08,720 for over 76 million years. 183 00:20:08,721 --> 00:20:12,129 It was absolutely fantastic because it was... it's living 184 00:20:12,350 --> 00:20:19,809 and it, it's exactly like having found a live dinosaur or a live archaeopteryx. 185 00:20:20,616 --> 00:20:23,237 The scientific community was transfixed. 186 00:20:23,476 --> 00:20:29,946 For decades the coelacanth had been touted as a possible transitional form between fish and tetrapods, 187 00:20:30,184 --> 00:20:35,169 but no one had really known enough about it. It existed only as a fossil. 188 00:20:36,489 --> 00:20:39,568 Smith proclaimed the coelacanth a transitional form 189 00:20:39,733 --> 00:20:44,132 and as proof he announced that it would actually walk on the bottom of the sea. 190 00:20:44,297 --> 00:20:49,777 I have no doubt that this fish crawls about on the bottom quite easily. 191 00:20:50,674 --> 00:20:56,338 Yes, the Professor says the fish is a kind of ancestor of man. Poor fish. 192 00:20:57,657 --> 00:21:00,993 But he knew he would have to find one alive and walking 193 00:21:00,994 --> 00:21:05,099 to prove the coelacanth was the elusive transitional form. 194 00:21:05,392 --> 00:21:11,917 He looked for 13 years until another one was found, and it didn't walk - it swam. 195 00:21:12,192 --> 00:21:14,300 It was just another fish. 196 00:21:14,959 --> 00:21:20,238 Well it was thrown out. There were quite a lot of nasty letters send to J.L.B. too 197 00:21:20,714 --> 00:21:23,298 to say that he was having a daydream. 198 00:21:26,817 --> 00:21:31,528 There were still no transitional forms, nothing to show that a fish with fins 199 00:21:31,529 --> 00:21:36,458 had walked over land and evolved into our first ancestor with legs, 200 00:21:36,824 --> 00:21:44,999 nothing to silence the creationists and there it rested for 30 years. 201 00:21:45,329 --> 00:21:51,927 Evolutionists still clung to the theory that a fish evolved into the first five-fingered land walking ancestor 202 00:21:52,092 --> 00:21:57,022 as a result of drought, even though they knew there was a gap in the story, 203 00:21:57,316 --> 00:22:02,008 but without new evidence no one could come up with a better idea. 204 00:22:09,028 --> 00:22:14,471 But then in 1981 along came palaeontology's avenging angel. 205 00:22:15,827 --> 00:22:21,363 Jenny Clack had long dreamed of embarking on the quest to find out why we first walked on the land, 206 00:22:21,693 --> 00:22:25,688 but when she arrived in Cambridge it seemed a remote hope. 207 00:22:27,447 --> 00:22:33,844 I had just finished my thesis when I started work here and was looking around for another project 208 00:22:34,339 --> 00:22:40,479 and a colleague of mine said don't worry, something will turn up and I didn't believe him. 209 00:22:42,165 --> 00:22:48,159 What turned up was the notebook of a geology student who had visited Greenland in 1970. 210 00:22:50,652 --> 00:22:53,364 In one corner he'd made an extraordinary note 211 00:22:53,438 --> 00:22:57,232 that showed though he knew about rocks, he knew little of fossils. 212 00:22:57,323 --> 00:23:03,463 He'd written that he had found remains of ichthyostega, Jarvik's legendary first tetrapod 213 00:23:03,609 --> 00:23:08,027 of which only one complete specimen existed anywhere in the world. 214 00:23:08,045 --> 00:23:14,570 He's noted, "Ichthyostega bones and skull bones common," and 215 00:23:16,421 --> 00:23:24,760 early tetrapod specimens are not common anywhere, particularly not Devonian tetrapods on a mountain in Greenland 216 00:23:25,384 --> 00:23:29,508 and to see this in his notebook just set the bells ringing. 217 00:23:31,121 --> 00:23:32,990 We have to go there. 218 00:23:40,816 --> 00:23:45,489 By 1987 Clack was off to this apparent goldmine of tetrapods, 219 00:23:45,746 --> 00:23:51,684 taking along a student, Per Ahlberg. It was a big undertaking. 220 00:23:53,389 --> 00:24:00,005 And of course it was possible that we were going to find almost nothing, or at least nothing new, 221 00:24:00,244 --> 00:24:03,580 so the potential was there on the one hand for a spectacular success 222 00:24:03,581 --> 00:24:05,981 and on the other hand for a considerable embarrassment. 223 00:24:10,123 --> 00:24:13,202 At first it looked more like embarrassment. 224 00:24:13,624 --> 00:24:19,214 As the week wore on, or the first two weeks wore on we still hadn't found 225 00:24:19,360 --> 00:24:24,510 the, the locality that we were looking for. We were beginning to think are we on the right mountain? 226 00:24:28,891 --> 00:24:31,219 Then Clack saw something. 227 00:24:34,114 --> 00:24:40,053 It was covered with dirt and soil. It very nearly got thrown on the scrapheap, 228 00:24:41,134 --> 00:24:46,046 but fortunately we brushed some of the dirt off and we could see part of a skull. 229 00:24:50,886 --> 00:24:55,431 Clack's team had not found the transitional form between fish and tetrapods, 230 00:24:55,706 --> 00:24:58,767 but she had found something almost as rare. 231 00:24:59,189 --> 00:25:05,109 It was another species of Devonian tetrapod, a species called acanthostega, 232 00:25:05,530 --> 00:25:12,678 different from Jarvik's but clearly sharing the same 2ncestry and, therefore, also related to humans. 233 00:25:13,100 --> 00:25:19,130 Acanthostega was only the second complete Devonian tetrapod ever to be found. 234 00:25:23,969 --> 00:25:27,928 Clack returned to Cambridge with dozens of tetrapod fossils. 235 00:25:28,167 --> 00:25:34,655 It meant that at last someone else would be able to do original work in the field of how we got our legs, 236 00:25:37,038 --> 00:25:40,960 but the true importance of the trip did not emerge until 1990 237 00:25:41,308 --> 00:25:46,257 when a colleague, Mike Coats, started work on the specimen she'd almost thrown away, 238 00:25:46,770 --> 00:25:48,346 the acanthostega. 239 00:25:49,373 --> 00:25:56,522 He started to dig its hand out of the rock and he expected, of course, to find just five fingers. 240 00:25:57,988 --> 00:26:05,099 The first thing he found on this block was a finger. This digit here. 241 00:26:05,777 --> 00:26:11,404 So we've got a number of finger bones aligned along the edge of this block. 242 00:26:11,808 --> 00:26:18,131 Then he continued with the preparation. He found the next finger which is here with its end curled over 243 00:26:19,084 --> 00:26:26,635 and then a third similarly with this crooked finger end and a fourth, again with that, 244 00:26:28,321 --> 00:26:36,019 and then there's a gap and then he went on to find another finger. Individual finger bones are really quite clear 245 00:26:36,533 --> 00:26:43,259 and that makes a total of five, but he still had all this area here to prepare so instead of stopping 246 00:26:43,479 --> 00:26:51,470 he went on to clean up the rest of this area and lo and behold here is another digit, so that makes six 247 00:26:52,113 --> 00:27:02,175 and he expected to finish there and then to his amazement here's a seventh and finally an eighth. 248 00:27:03,568 --> 00:27:04,741 What? 249 00:27:10,056 --> 00:27:15,957 But it was true. Acanthostega had eight fingers on one hand. 250 00:27:16,398 --> 00:27:23,693 The very earliest tetrapods did not, as all the textbooks claimed, have five fingers after all. 251 00:27:24,316 --> 00:27:31,776 It suddenly raised the question: if the most basic assumption behind the previous 100 years' research was wrong 252 00:27:32,766 --> 00:27:34,965 then what else might be wrong? 253 00:27:35,827 --> 00:27:42,553 Until that day I had assumed, like everyone else, that five was the primitive number of digits for a tetrapod limb. 254 00:27:43,121 --> 00:27:48,088 The old explanations for the origin of the structure, after all one of the most 255 00:27:48,271 --> 00:27:51,717 fundamental and defining structures of being a tetrapod, 256 00:27:52,743 --> 00:27:56,959 and in our own way of being human, was in the bin. 257 00:28:02,786 --> 00:28:06,727 Scientists now believe our earliest ancestors with legs 258 00:28:06,947 --> 00:28:13,050 must have started out life with numerous digits and then evolution reduced them to five 259 00:28:13,270 --> 00:28:18,951 over the aeons that followed and the shocks just kept getting bigger. 260 00:28:20,583 --> 00:28:26,796 Another fundamental assumption, that we had evolved legs for the express purpose of walking, 261 00:28:27,346 --> 00:28:29,601 just could not be true. 262 00:28:32,698 --> 00:28:39,736 If you look at the limbs what you find is that the joints are all angled 263 00:28:39,863 --> 00:28:45,875 so that the limb would have stretched out just to the sides. 264 00:28:48,130 --> 00:28:54,471 There's just no way that it could have brought its leg underneath to take any weight. 265 00:28:54,819 --> 00:29:00,886 Similarly with the hind limb, which we found a bit later on, similar kind of arrangement, no ankle to speak of, 266 00:29:01,344 --> 00:29:03,562 just a paddle like limb. 267 00:29:05,945 --> 00:29:09,813 Acanthostega's legs would have been useless for walking 268 00:29:10,014 --> 00:29:16,924 and what's more, although it was a fully formed tetrapod it could never have lived out of the water. 269 00:29:17,620 --> 00:29:20,095 It had gills just like a fish. 270 00:29:21,268 --> 00:29:27,994 It meant the evolution of our legs could never have had anything to do with walking on land. 271 00:29:30,231 --> 00:29:34,886 Jenny Clack's discovery of the acanthostega had changed everything. 272 00:29:35,399 --> 00:29:41,063 The old explanation that we'd evolved our legs after a fish came onto land 273 00:29:41,411 --> 00:29:43,629 just could not be true. 274 00:29:44,985 --> 00:29:49,274 So the thing that has really changed is that rather than the fish 275 00:29:50,557 --> 00:29:55,762 going onto the land while it's, it's still got fins we've turned that completely on its head, 276 00:29:55,890 --> 00:30:02,929 so now we've got tetrapods in the water, still in the water, while they've got limbs with digits. 277 00:30:04,834 --> 00:30:06,997 Stunned by these revelations 278 00:30:07,198 --> 00:30:11,011 Clack decided to check her findings against ichthyostega, 279 00:30:11,194 --> 00:30:17,206 Jarvik's iconic first tetrapod, a fragment of which she'd also collected in Greenland. 280 00:30:17,590 --> 00:30:21,952 Her team prepared the specimen and counted the toes. 281 00:30:23,602 --> 00:30:27,433 Seven. Why didn't Jarvik see this? 282 00:30:29,412 --> 00:30:32,088 There was more that Jarvik did not see. 283 00:30:32,492 --> 00:30:37,074 Clack discovered that ichthyostega did not, as Jarvik had shown, 284 00:30:37,183 --> 00:30:42,242 have legs made for walking. they too were paddles. 285 00:30:42,755 --> 00:30:48,638 Jarvik, who died three years ago, had simply got it wrong and no one will ever know why. 286 00:30:56,025 --> 00:31:02,989 Clack's discoveries meant the whole quest as to how and why we had evolved limbs would have to start again. 287 00:31:03,870 --> 00:31:08,103 Now they didn't just need the transitional form to link fish to tetrapods 288 00:31:08,397 --> 00:31:10,230 though that was still missing 289 00:31:10,486 --> 00:31:15,472 but they also needed a whole new reason why limbs had evolved. 290 00:31:16,406 --> 00:31:20,713 Why would any creature need legs that weren't for walking? 291 00:31:27,880 --> 00:31:32,535 In a valley bypassed by the American Dream, they would find the answer, 292 00:31:32,847 --> 00:31:36,073 the real story of how we got our legs. 293 00:31:43,825 --> 00:31:48,480 Renovo, Pennsylvania is what happens at the end of the railway line 294 00:31:48,829 --> 00:31:51,358 when the trains don't come any more, 295 00:31:53,887 --> 00:31:57,424 but the town is rich in something rare and precious. 296 00:31:58,065 --> 00:32:03,289 Beneath the trees are swathes of red Devonian sandstone. 297 00:32:12,893 --> 00:32:17,988 So in 1993 along came a lone fossil hunter. 298 00:32:19,344 --> 00:32:21,690 He is called Ted Daeschler. 299 00:32:46,379 --> 00:32:50,924 Daeschler made his name finding two more new Devonian tetrapod species 300 00:32:51,126 --> 00:32:54,077 in a roadside cutting called Red Hill, 301 00:32:56,020 --> 00:33:00,785 but the thing that took both him and Red Hill into fossil hunting legend 302 00:33:01,170 --> 00:33:04,634 was when he examined one of the site's peculiar features. 303 00:33:05,001 --> 00:33:09,986 In amongst all the red rock was a broad green layer. 304 00:33:11,416 --> 00:33:15,888 OK, the majority of the rock out here at Red Hill of course is red. 305 00:33:16,108 --> 00:33:22,505 Climbing up through sandstones, silt here, sandstones and mudstones 306 00:33:22,982 --> 00:33:26,904 right into this zone up here we start with a green layer. 307 00:33:27,107 --> 00:33:34,511 It's reduced probably because of all of the plant material that's buried within the rock here. 308 00:33:36,179 --> 00:33:37,901 Fossil plants. 309 00:33:38,323 --> 00:33:43,821 By finding them alongside Devonian tetrapods Daeschler had made a vital breakthrough. 310 00:33:44,188 --> 00:33:47,579 He could now reconstruct for the very first time 311 00:33:47,726 --> 00:33:51,776 the true environment in which the first tetrapods had evolved. 312 00:33:52,143 --> 00:33:56,670 The Devonian wasn't the barren, treeless landscape spoken of in textbooks. 313 00:33:56,982 --> 00:33:59,548 It was more like a rainforest. 314 00:34:01,399 --> 00:34:06,586 The most common thing we're finding is a tree-like plant. It actually has a long, tall trunk 315 00:34:06,587 --> 00:34:13,716 and some people say these got up to 30m tall, so these were truly the first canopy sort of producing plants. 316 00:34:15,566 --> 00:34:22,458 We also find fern-like plants and a variety of other things and so we're really seeing a diversity 317 00:34:22,806 --> 00:34:27,003 from a site like Red Hill and I think it's important because it's showing us that 318 00:34:27,186 --> 00:34:30,687 these were actually complex environments. 319 00:34:31,677 --> 00:34:36,791 The textbooks had got it wrong again. The Earth may once have been barren, 320 00:34:37,047 --> 00:34:45,479 but by the end of the Devonian it was very, very wet and densely forested with huge permanent rivers. 321 00:34:45,991 --> 00:34:53,341 These were bordered by something completely new: swamp, that grey area between land and water. 322 00:34:54,367 --> 00:34:58,748 The first tetrapods had evolved in this wholly new eco-system, 323 00:34:59,279 --> 00:35:05,822 precisely the kind of thing that could indeed be a spur to major evolutionary change. 324 00:35:08,168 --> 00:35:15,316 Oh it's completely new. By the time you get to the end of the Devonian for the very first time in Earth history 325 00:35:15,517 --> 00:35:19,989 animals and plants are living on land in a significant, permanent way. 326 00:35:19,990 --> 00:35:27,303 That's, that's brand new and a lot of open niches in that waiting to be exploited. 327 00:35:28,164 --> 00:35:31,977 Those new niches were the margins of this watery world. 328 00:35:32,472 --> 00:35:39,235 In the tangle of vegetation limbs with fingers would have given tetrapods a unique advantage over fish. 329 00:35:39,693 --> 00:35:44,899 I think we have to think of these fins or, or limbs, or flims as 330 00:35:44,900 --> 00:35:50,837 something that would be used by the animal for moving through more complex environments like swamps, 331 00:35:51,185 --> 00:35:56,134 or environments that, where there may have been trees down in channels, 332 00:35:56,280 --> 00:36:04,143 or just shallow water to pursue prey or to escape the guy who's trying to prey upon you. 333 00:36:05,810 --> 00:36:09,164 And there was most definitely something from which to escape. 334 00:36:15,652 --> 00:36:21,756 Over and over again Daeschler and his team found evidence of a fish called hyneria, 335 00:36:22,030 --> 00:36:25,366 a predator of terrifying proportions. 336 00:36:29,729 --> 00:36:34,549 Hyneria is the most common, lobe-finned fish at this site. It's also the biggest. 337 00:36:34,642 --> 00:36:36,951 It's probably two or three metres long. 338 00:36:37,025 --> 00:36:43,055 This, this is a single tooth from a large hyneria and these were carnivorous obviously. 339 00:36:50,826 --> 00:36:56,105 When you see the tetrapods in the same fauna with these hyneria it, it really does 340 00:36:56,306 --> 00:37:03,216 make you think well maybe escaping from large, large predators like this hyneria was, 341 00:37:03,326 --> 00:37:07,028 was a pretty important thing for these early tetrapods. 342 00:37:13,168 --> 00:37:16,907 The mystery of why we had evolved limbs was finally solved. 343 00:37:16,980 --> 00:37:23,523 They were not for walking, but navigating through swamps, just like this hellbender salamander does today, 344 00:37:23,926 --> 00:37:27,262 but there was still a hole in the story. 345 00:37:34,611 --> 00:37:40,770 There was still no fossil that showed the process of change from fish to tetrapod actually happening. 346 00:37:41,044 --> 00:37:47,753 The transitional form, something half fish/half four legged animal, was still missing. 347 00:37:55,286 --> 00:38:00,638 And then Per Ahlberg made his fateful visit to Latvia. 348 00:38:03,240 --> 00:38:09,802 In that forgotten museum drawer he may have found what everyone had been looking for. 349 00:38:10,535 --> 00:38:15,942 His trained eye told him the fossil was a fragment of jaw right from the time of transition 350 00:38:16,143 --> 00:38:19,662 and it was certainly part fish and part tetrapod. 351 00:38:20,615 --> 00:38:28,991 The information we get from this one paltry little bone is quite overwhelming. 352 00:38:36,250 --> 00:38:39,274 He named the jaw livoniana 353 00:38:42,060 --> 00:38:45,048 and to prove it really was a transitional form 354 00:38:45,323 --> 00:38:48,860 he ran it through something called a cladistic analysis. 355 00:38:49,044 --> 00:38:54,945 A computer is programmed with all the anatomical features that distinguish fish from tetrapods. 356 00:38:55,220 --> 00:38:59,582 Some are obvious, such as does the specimen have limbs or fins? 357 00:38:59,820 --> 00:39:04,036 Others are tiny shifts in the position of blood vessels or bones. 358 00:39:04,329 --> 00:39:10,433 Such minute details allow scientists to identify a creature from just a fossil fragment 359 00:39:10,726 --> 00:39:15,656 and place it in an evolutionary tree relative to other animals. 360 00:39:26,031 --> 00:39:30,650 For this comparison a lobe-finned fish jaw is on one side, 361 00:39:31,933 --> 00:39:38,439 livoniana is in the middle and a Devonian tetrapod jaw is on the other side. 362 00:39:39,575 --> 00:39:44,084 What you can see if we look at the end points s that these two jaws differ in quite a lot of ways. 363 00:39:44,303 --> 00:39:51,158 First of all if we look at this pit in the fish jaw which is particularly important feature, 364 00:39:51,159 --> 00:39:55,471 a deep hollow that goes all the way through to underlying bones. The bone you get in the bottom there 365 00:39:55,472 --> 00:39:58,678 is a different one to what are coming up on the surface here. 366 00:39:59,264 --> 00:40:03,407 In Devonian that's the same place. The pit has almost disappeared 367 00:40:03,626 --> 00:40:07,787 surrounding now a blood vessel hole here which we wouldn't have in the other jaw. 368 00:40:08,208 --> 00:40:14,788 If we look at the tetrapod we have the pit now gone altogether 369 00:40:15,063 --> 00:40:18,673 and here's that lower blood vessel hole there in the tetrapod, 370 00:40:18,930 --> 00:40:24,135 so in this respect livoniana kind of agrees with the tetrapod. 371 00:40:26,262 --> 00:40:30,147 On the other hand, we find that in the fish 372 00:40:30,404 --> 00:40:36,122 a bone from the outer surface of the jaw comes round down to here and ends 373 00:40:36,452 --> 00:40:41,382 and it's, it abuts against another bone up here called the pre-articular. 374 00:40:41,713 --> 00:40:43,435 In the tetrapod 375 00:40:43,820 --> 00:40:48,971 the bone from the outer face comes all the way up here. It forms a big tongue 376 00:40:49,099 --> 00:40:54,396 extending backwards so and it comes all the way up here beneath it, so quite a different arrangement. 377 00:40:54,561 --> 00:41:00,371 Livoniana here has got the junction and the bone exposed on the surface 378 00:41:00,610 --> 00:41:04,770 just like the fish, so in this case livoniana agrees with the fish, 379 00:41:04,862 --> 00:41:07,391 so as you can see depending on which characteristic you look at 380 00:41:07,392 --> 00:41:10,709 it either lies sort of halfway between or it agrees with the tetrapod 381 00:41:10,710 --> 00:41:15,877 or it agrees with the fish. Exactly what you would expect from an intermediate form. 382 00:41:23,393 --> 00:41:28,415 Ahlberg believes livoniana really is an elusive transitional form, 383 00:41:28,726 --> 00:41:32,667 almost exactly half fish and half tetrapod. 384 00:41:33,491 --> 00:41:36,607 It is certainly the only fossil yet discovered 385 00:41:36,753 --> 00:41:41,977 that shows the process of change between the two actually taking place. 386 00:41:43,278 --> 00:41:51,948 It also has one freakish feature: there are seven rows of teeth. It is unlike any other creature we know of. 387 00:41:52,754 --> 00:41:57,831 This suggests it must be one of the host of mutants that made this change, 388 00:41:58,069 --> 00:42:02,541 just one of which would eventually become our ancestor. 389 00:42:04,594 --> 00:42:08,461 Livoniana is a real missing link. 390 00:42:19,274 --> 00:42:25,194 Darwin's 360 million year old riddle about how we developed our legs 391 00:42:25,635 --> 00:42:31,243 has been solved. It was vegetation on land and in the water 392 00:42:31,244 --> 00:42:35,880 that let flourish an explosion of mutations among lobe-finned fish. 393 00:42:38,539 --> 00:42:44,037 The most successful of these mutations, the one that stood the test of time, 394 00:42:44,202 --> 00:42:49,005 was the development of limbs with fingers and toes. 395 00:42:52,285 --> 00:42:57,856 It was out of this swampy area that our earliest ancestor came crawling over land. 396 00:42:58,058 --> 00:43:04,785 It was not pre-ordained, but chance, a series of evolutionary accidents, 397 00:43:05,298 --> 00:43:14,242 but it just so happened that that creature's children would indeed inherit the earth one day.