The Missing Link | Full Documentary | NOVA | PBS

[Music] in an obscure museum in eastern europe a fossil hunter has made a startling discovery [Music] while randomly sifting through a set of drawers he found a collection that hadn't been examined for 30 years 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 which have never been found anywhere in the world [Music] the paleontologist pierre alberg 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 [Music] 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 the time period 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 [Music] major funding for nova is provided by the park foundation dedicated to education and quality television [Music] this program is funded in part by the northwestern mutual foundation [Music] some people already know northwestern mutual can help plan for your children's education are you there yet northwestern mutual financial network [Music] science it's given us the framework to help make wireless communications clear sprint pcs is proud to support nova [Music] and by the corporation for public broadcasting and by contributions to your pbs station from viewers like you thank you [Music] [Applause] [Music] [Applause] [Music] [Applause] [Music] in north central pennsylvania near the appalachian towns of heiner and renovo stretches an ancient sandstone formation known as red hill it was here that paleontologist neil shubin and ted destler discovered extraordinary new clues in one of evolution's most enduring mysteries how ancient creatures left the water to walk on land the reason why i'm 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 it's a detective story and you're finding evidence out there where we're breaking rocks and we're we're looking for little pieces of evidence to help piece together this the story of how limbs developed from fins 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 we are tetrapods to it one two three four horses are tetrapods evidently enough so are dogs so are lions tigers and bears [Music] so it's a bird two hind legs two wings for the front which are modified front legs a snake is a tetrapod it has no legs anymore but it's quite clear that they're derived from a lizard ancestry which had both four limbs and hind limbs non-tetrapods have a wide variety of body plans [Music] some have hundreds of legs some have none [Music] 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 [Music] they all have a rib cage to protect the heart and lungs and they all breathe 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 seem 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 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 sometime during the 4 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 loeb fins this modern-day lungfish the ancient lobe fins had lungs as well as gills and the unique structure in their fins that looked like a precursor to arms and legs there are two types of bony fish on the earth today rayfin fish and lobe-finned fish now ray finn fish are very common as represented by this common soul here this this creature is dinner and in fact most of what we have for dinner are the ray-finned fish the reason why we call them ray-finned 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-finned fish and the reason why we call it a low-fin fish is because it's finn is composed mostly of this thing here which you can see is this fleshy lobe now from an evolutionary perspective this lobe is is very important because many of the bones that make up our limbs actually first evolved within this lobe [Music] lobe-fin fishes were common during a time in earth's history called the devonian period 150 million years before the age of dinosaurs began tetrapod fossils were plentiful in rocklers 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 [Music] imagine from him that you were able to go back to the world just before the beginning of the move on to 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 flat 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 some scuttling around the overall picture the system is there and it's not so different from today but turn your back on the sea and walk in land and what do you find a barren empty wasteland [Music] no greenery no trees no insects wind keening over the rocks a barren land that could not possibly support you [Music] 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 in 1881 on the gaspe peninsula of quebec a crucial discovery was made by a canadian farmer [Music] he stumbled upon the most perfect fossil of loeb thin fishes ever seen in rock from the devonian period called eustanopteron the layout of its fin bone showed a striking resemblance to the bone structure of tetrapod limbs and with the clarity never before seen 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 the 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 wrist and so on absolutely fantastic beautiful material clinched it really what they thought they'd clenched 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 look 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 the chances of any animal becoming a fossil are extraordinarily remote and the only way that we have a lot of fossils this that has 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 [Music] in 1931 fossil hunters got lucky [Music] 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 tell-tale bone structure of a tetrapod [Music] 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 people have 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 it fell to eric 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 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 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 is going crazy trying to write and work in a field in which you know somebody else has a lot of information you don't know what it is [Music] jarvik did release two papers during the course of his research comparing ichthyostega to the fish eustanopteron he was convinced that ichthyostega had been a true tetrapod with a rib cage 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 the fish like eustonopteron no one was ever really satisfied with jesus on optima as the immediate ancestor of something like ichthyostega so i think people have not only been looking for things that are little less fish-like than usual uptron they've also been think looking for things to do a little more fish-like for make their steak so wanting to close the gap from both sides [Music] 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 [Music] there may be more but 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 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 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 most of evolution is stability and the production of new species that are pretty like the ones that came before 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 where 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 the most popular explanation for why fish evolved to walk on land was proposed by harvard paleontologist alfred sherwood romer [Music] 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 the drying pond scenario basically ran like this there were lobe-finned fishes living in the rivers and lakes of the devonian continent 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 [Music] the idea was that lothian fishes became gradually better and better at using their lobe fins to drag themselves across the mud [Applause] romer suggested that limbs evolved as fish adapted to making this desperate march across land these would have become the first tetrapods our ancestors [Music] 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 [Music] just before christmas 1938 marjorie courtney latimer the curator of a small museum in east london south africa was called down to the docks to examine a most unusual catch 22nd 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 that white kind of flex on it and to my horror it had these slim like fins and i thought to myself what 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 the intuition that i had it it must be saved by all at all costs unable to have the fish frozen she left it with the taxidermist until dr j l b smith a prominent south african scientist could come identify it he stood at the head of the table and he said well less he said this fish will be on the lips of every scientist in the world it's a silicate coelacanths are a group of lobe-finned fishes that live during the devonian period and were thought to have gone extinct over 76 million years ago 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 will not come true it's absolutely amazing meet professor smith of grahamstown south africa with a model of that famous fish the sealer cap coelacanths are close relatives of the fish that scientists consider as the ancestor of all land animals the scenic ants have lived for probably 350 million years and in that time they have changed but little as you see the fins are more like panels than ordinary fins smith was convinced the coelacanth could actually move about on the ocean floor using its lobed fins i have no doubt that this fish falls about on the bottom quite easily yes the professor says the fish is a kind of ancestor of man poor fish 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 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 [Music] coelacanths must have survived virtually unchanged since branching off from an ancestral fish some 360 million years ago but used to knocked iran also a fish with limb 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 eustanopteron the fish and ichthyostega of the tetrapod [Music] the answer came in 1981 from a motorcycle enthusiast who was also pierre alberg's advisor at cambridge university museum i would be sitting there working in the morning if i got in early and suddenly hear this in the courtyard which was the associate curator of vertebrates arriving for her daily work at the museum [Music] 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've seen and i used to have a whole collection of those and a whole collection of the little observers books and i used to go on holiday specifically so that i could either look at the local fall flora and fauna or look for fossils jenny clack had long dreamed of solving the mystery of how and why creatures first walked on land but it seemed a remote possibility 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 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 victostega the creature eric jarvik had discovered he'd noted igthiastiga 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've been collected in the 30s they weren't common they were chance vines after days of walking the screen and to see this in his notebook just set the bells ringing we have to go there it took six years to raise the funds for the expedition but finally jenny clack headed off accompanying her was her phd student peer alberg we were very excited to be going at last but this is of course also coupled with a certain trepidation this is a big undertaking it's an expensive expedition involving air support helicopter time all sorts of things [Music] it's at least 100 miles from the nearest permanent habitation and that's an airstrip which is only manned during summer [Music] 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 spectacular success and on the other hand for considerable embarrassment [Music] 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 even with maps and detailed notes alberg and clack feared they'd never find the right spot the notes in the book say 825 metres and in fact that was wrong we've 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 [Music] when they explored the mountain at a lower elevation clack saw something it was covered with dirt and soil it very nearly got thrown on scrap heap um but fortunately we brushed some of the dirt off and we could see part of a skull they had found the most complete tetrapod specimens since eric jarvik's expedition 56 years earlier fossils of a four-legged devonian creature called acanthostega [Music] clack returned to cambridge with dozens of fossils at last someone other than jarvik would be able to do original work [Music] but the true importance of the trip did not emerge until acanthustega had undergone several years of painstaking preparations crack recalls it was her colleague mike coates who first saw a hand emerge from the rock 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 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 full limbs 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 it was in fact an interpretation problem but of course once i had seen the specimen it was perfectly plain that that was not so that you did indeed have a fall in with a humorous radius ulna and eight little fingers in a row [Music] acanthustega had eight fingers on one hand suddenly calling into question one of the most basic assumptions behind the previous hundred years of research until that day i had assumed like everyone else that five was the primitive number of digits for a tetrapod lim 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 alberg 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 acanthustega called another fundamental assumption into question its limbs were not made for walking 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 you know 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 [Music] there's just no way 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 [Music] acanthustega'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 so the thing that has really changed is that rather than the fish going on to the land while they're still 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 stunned by these revelations clack checked her findings against a fragment of ichthyostega which he'd found in greenland her team prepared the specimen and counted the toes seven why didn't jarvik see this 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 [Music] ichthyostega lived around the same time as a kanthastega 360 million years ago and while they both had limbs and gills acanthustega was a bit more fish-like especially in the structure of its tail [Music] these differences show how evolution was experimenting tinkering with different body plans that would eventually result in all modern four limbed animals [Music] 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 [Music] the next piece of the puzzle was unearthed in the united states during the devonian period north america was part of a huge land mass called larasia made up of present-day europe asia and greenland lying near the equator larasia 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 destler 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 destler was hoping for a change of luck in a newly excavated road cut sometimes you will 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 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 in 1995 beschler 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 we named it hynerpaton which means crawling animal from heiner and actually the the town down below below us here is heiner pennsylvania and the first part of the body that we found actually the original material and all we had to work with was 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 hynerbiton that it was an animal with very muscular limbs it's not like the shoulder girdle of a fish at all [Music] more able to carry its own weight than either acanthustega or ecthiostega hynerbaton could possibly have walked on land destler and his colleagues had noticed something else [Music] among the swaths of red sandstone were patches of green material [Music] okay majority of the rock out here at red hill of course is red climbing up through sandstones siltier sandstones and mudstones but right into this zone up here we start with a green layer it's reduced um probably because of all the plant material that's buried within the rock here 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 rain forest the most common thing we're finding is a tree-like plant 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 [Applause] well these stream systems that were running across big wide flood plains 370 million years ago would have created big muddy channels and in between those channels there would have been forests in fact they 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 freshwater environments was also new [Music] 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 [Music] swamp first four limbed creatures may have evolved in this holy new ecosystem just the kind of environmental shift that can trigger major evolutionary change [Music] for the very first time in earth history animals and plants living on land in a significant permanent way and a lot of open niches in that waiting to be exploited 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 i think we have to think of these fins or limbs or phlegms 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 [Music] and there was definitely something to escape from the red hill team found evidence of a predator of terrifying proportions a fish keith thompson had named hyneria 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 hinarius and these were carnivorous obviously and with a few predators like that around swimming in the 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 and into the undergrowth and find somewhere else to live [Music] 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 seemed 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 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 was to live on land more of nature's tinkering would be revealed when pear alberg now of london's natural history museum visited eastern europe [Music] 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 i picked this piece up had a look at it turn it round and really i knew within about half a minute that here was something absolutely extraordinary alberg named the creature livoniana after the region and latvia where it was found [Music] 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 alberg has placed the lobe thin fish jaw on one side the lavoniana jaw in the middle and the tetrapod jaw on the other side the differences are subtle but significant 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 stool which is a 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 livonia 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 all together and here is 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 around down to here and and ends and it's it butts 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 livonia 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 [Music] the analysis shows livoniana to be clearly a transitional creature it fits on an evolutionary tree about midway between fish and tetrapods [Music] 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 [Music] [Music] in recent years darwin's 400 million-year-old detective story has become much less shrouded in mystery [Music] in the devonian world of forests and rivers bordered by swamp a whole new way of life was born [Music] the distinction between being in the water and out of it became blurred from this swampy place our ancestor came crawling onto dry land [Music] it was one of the most momentous events in all of evolution for one day that creature's descendants would inherit the earth [Music] [Music] 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 [Applause] [Music] educators can order this or any other nova program for 19.95 plus shipping and handling call wgbh boston video at 1-800-255-9424 [Music] [Music] nova is a production of wgbh boston [Music] [Applause] major funding for nova is provided by the park foundation dedicated to education and quality television [Music] science it's given us the framework to help make wireless communications clear sprint pcs is proud to support nova this program is funded in part by the northwestern mutual foundation people already know northwestern mutual can help plan for your children's education are you there yet northwestern mutual financial network and by the corporation for public broadcasting and by contributions to your pbs station from viewers like you thank you this is pbs [Applause] me [Music] you

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Length: 56min 48sec (3408 seconds)

Published: Sun Jan 02 2022