Session 3: Evidence of Evolution
Transcript of Part 1: Finding Tiktaalik, the Fossil Link Between Fish and Land Animals
00:00:07.22 My name is Neil Shubin, I'm from the University of Chicago. 00:00:10.01 And I'm going to talk today about finding Tiktaalik. 00:00:12.06 My life's quest began in graduate school, in the second year 00:00:16.26 when I saw this exact slide. This is the slide my professor 00:00:19.29 showed back in 1986, and it showed what we knew at the time as 00:00:25.17 the fish to tetrapod transition, which is the transition from life in water 00:00:29.19 to life on land, as seen by these cartoons of these fossil creatures. 00:00:33.12 The one on top is a fish from 390 million years ago, the one on the bottom 00:00:38.18 is a tetrapod from about 365 million years ago. I remember as a student 00:00:43.10 seeing this diagram for the first time and thinking, "Holy cow! 00:00:46.20 What a first class scientific problem!" How did fish evolve to walk on land? It 00:00:52.00 seemed so utterly impossible go to from the thing on top to the thing 00:00:54.28 on the bottom. And that's what became my quest. So like any 00:00:58.00 good paleontologist, I applied the paleontological toolkit to try to find 00:01:01.09 an intermediate, say some sort of creature between these two. 00:01:04.14 And so what do we do when we try to find a new place to discover fossils? 00:01:08.18 What we do is we look for places in the world that have three things. 00:01:11.27 The first is rocks of the right age, you need rocks of the right age 00:01:16.08 to answer whatever question interests you. So in this case, clearly 00:01:19.19 we want something between 390 and 365 million years old. 00:01:24.05 That's of a time period known as the Late Devonian. The other thing 00:01:28.08 that was really important, of course, is to have rocks of the right type. 00:01:31.00 Not every kind of rock holds fossils. Some are super heated, some are 00:01:35.06 highly pressurized. So we needed a particular kind of rock, sedimentary 00:01:39.15 rocks, and not only that, sedimentary rocks in the right environment 00:01:43.20 to hold the kind of intermediates we're looking for. So what's the 00:01:47.14 other variable? Rocks of the right age. Rocks of the right type. 00:01:50.10 Well, it does me no good if my wonderful rocks of the right age 00:01:53.27 and the right type are buried five miles underground, right? These things 00:01:57.08 have to be exposed at the surface. So what we do is we look for places 00:02:00.28 in the world where we have naked bedrock, okay? So that's it. If you 00:02:04.10 ever want to run your own expedition someday, look for places in the world 00:02:07.18 that have rocks of the right age, rocks of the right type, and rocks that 00:02:10.09 are exposed at the surface. There's another criterion, and in this case 00:02:14.06 was accessibility. And this is where I began my first scientific expeditions. 00:02:19.06 These were accessible, because I had just begun my first academic 00:02:24.12 job at the University of Pennsylvania, here at the southeastern corner 00:02:28.00 of the state of Pennsylvania. And I dug out a geological map of 00:02:31.15 the state. And I'm showing you here what is sort of relevant for our 00:02:35.21 question. If you look at this map of Pennsylvania, what you see is 00:02:39.00 everything in purple are Devonian age rocks. So I have lots of 00:02:43.22 rocks of the right age throughout the state of Pennsylvania. But what about 00:02:47.02 that second variable, rocks of the right type? Well if you want to 00:02:50.29 think about what Pennsylvania looked like in the Devonian, get Pittsburg, 00:02:54.29 Harrisburg, Philadelphia, out of your brain. And think amazon delta. 00:03:00.14 This is a cartoon of what Pennsylvania looked like 365 million years ago. 00:03:04.24 You had highlands to the east, an inland sea to the west, and a series 00:03:09.12 of rivers draining from east to west. Now if you're a paleontologist 00:03:14.01 wanting to find fossils at the cusp of the transition from life in water 00:03:18.01 to life on land, this is perfect. Because if you have the right exposures, 00:03:22.19 you can sample ancient seaways, ancient rivers and streams, even ancient 00:03:27.15 upland deposits. Now the problem is Pennsylvania doesn't have a whole lot 00:03:31.21 of exposures. So that third variable is very tough. So my initial 00:03:36.06 expeditions, if you will, became following the Pennsylvania Department of 00:03:40.24 Transportation around as they build new roads. And if we were really lucky, 00:03:45.02 what they would do is they would be digging a new road in a place of Devonian 00:03:48.23 rock. And that's what you see here. This is a set of rocks about an hour north 00:03:53.02 of State College Pennsylvania. And this expanse extends about half a mile 00:03:57.21 long. It's a big red hill of rock -- it's called Red Hill, no surprise. 00:04:01.12 But what you see in these layers is the strata, the ancient layers 00:04:05.21 of ancient streams. And this is really perfect. Almost as soon as we 00:04:10.02 hit this site in the early '90s, we started to find fossils. The first thing 00:04:15.07 we started to find were teeth the size of railroad spikes. So big monsters, almost the size of 00:04:19.15 your thumb. Then we started to find the jaws of these creatures. Here's the 00:04:22.27 front end of one of these jaws. These jaws would be the length of your arm. 00:04:25.27 Alright, so they're really big. These big animals are probably about 00:04:29.13 16 feet long. Then we started to find all kinds of other fish, like 00:04:33.01 this little mashed thing here. This is actually a mashed fossil 00:04:36.25 of a lobed fin fish, which is related to lungfish today. So that's a head 00:04:41.14 and a body. But then my colleague, Ted Daeschler, who's been my partner 00:04:45.07 in all these expeditions for the last number of decades. Ted found 00:04:49.11 early tetrapods. One day I had left, one week I had left to do 00:04:53.28 expeditions elsewhere, and he had just discovered a series of 00:04:56.04 bones. Arm bones, leg bones, from creatures that represented 00:04:59.16 early limbed animals. And working with the National Geographic Society, 00:05:03.18 Ted and I were able to reconstruct what this ancient road in Pennsylvania 00:05:07.16 looked like 365 million years ago. I mean we had some of the first 00:05:12.10 forests, you can see this, and ancient streams, see that in the middle 00:05:16.04 here is that big monster fish, about 16 foot long fish. And then you see 00:05:20.28 a series of other kinds of fish, in particular, you see the little creatures 00:05:23.21 with limbs crawling around both on land and in the water. 00:05:27.16 Now this was great, but Ted and I realized we had a problem. 00:05:31.23 Why? We're in rocks about 365 million years old and we're already finding 00:05:38.00 pretty advanced limbed creatures. Remember what motivated this 00:05:41.02 whole quest was this whole slide I saw in graduate school. 00:05:44.22 We're not finding intermediates, we're already finding pretty advanced 00:05:47.20 limbed animals, I want something in the middle here. Well, to give you 00:05:50.18 a sense of what we're after, let's just go through some of the 00:05:52.26 traits that we look for. When you look at the creature on top, it has a 00:05:56.06 conical head with eyes on either side. The creature on the bottom 00:06:00.01 has a flat head with eyes on top. So really one of the kind of features 00:06:03.28 that changes in this transition is the shape of the head. 00:06:08.08 The other thing that changes is if you look at the creature on top, 00:06:10.27 the fish on top, you'll see the head is connected to the shoulder girdle by a series 00:06:14.21 of bones. It doesn't have a neck. Fish don't have necks where the head 00:06:18.20 can bend independently of the body. Early limbed animals, early 00:06:22.23 tetrapods, have a neck where the head can swivel independently 00:06:26.16 and it can look around as its legs are on the ground. Finally, another big feature 00:06:31.23 which obviously gives it its name, tetrapods, four-legged animal, 00:06:35.02 is fish have fins and the earliest tetrapods have limbs with fingers 00:06:39.03 and toes and wrists and ankles. We're finding a lot of fingers, toes, and wrists, 00:06:43.08 and ankles, right? We're not finding any intermediate. What Ted and I 00:06:46.14 are looking for is a flat headed fish with say fins or something in that 00:06:50.18 intermediate zone. To get that, what we had to do was move back in time, 00:06:55.02 back in time about 15 million years using analyses that other colleagues had done. 00:07:00.06 So Ted and I pulled out the paleontological rule book and began looking for 00:07:04.21 places that have rocks that are the right age, rocks that are the right type, 00:07:07.11 and rocks that are exposed. Wed had an idea to work in Brazil. 00:07:11.16 We had an idea to work in western North America. Then everything 00:07:15.28 changed one day in my office, back in the late '90s, when we had a 00:07:20.28 geological debate. And to settle the debate, I pulled out a college 00:07:24.15 geology textbook, this one right here. It's now in its eleventh edition. 00:07:28.19 So that gives you a sense of how old I am. But anyway, so we 00:07:31.01 finished the debate and I was paging through the book, and we ran 00:07:35.08 into this figure. This is the figure that changed the traction of research 00:07:39.25 for me, about 15 years ago. What it is is what we look for in a nutshell 00:07:45.01 as paleontologist. You'll see the caption, it says upper Devonian sedimentary 00:07:49.11 facies. Rocks more or less of the right age and the right type. 00:07:52.27 But here you see a map of North America, and superimposed on that 00:07:56.18 map is an interpretation of the geology of the Devonian age 00:08:00.27 rocks. Now you'll notice in the western part of North America, 00:08:03.20 there are Devonian rocks mapped as ancient oceans, but then the authors 00:08:08.05 identify three different areas, shown in red here, that had rocks 00:08:12.03 that had formed in ancient rivers and streams. Ancient rivers and stream 00:08:15.06 environments, delta systems. The first one, you've seen before. 00:08:20.01 That's the one I just showed you from Pennsylvania, that's the ongoing Catskill 00:08:23.08 project that we're working on along the roadsides of Pennsylvania. 00:08:26.29 There's another one on top here in East Greenland, that was well studied. 00:08:31.01 That was discovered in the '20s by Danes and Swedes. And 00:08:34.29 had produced a number of early tetrapods or limbed animals, in fact 00:08:38.03 the cartoon I've been showing you is the cartoon derived from 00:08:40.29 one of the fossils from this area. You see where I'm going? 00:08:43.25 Extending 1500 kilometers east to west across the Canadian arctic, 00:08:48.04 was mapped Devonian aged rock, completely unexplored. Not only that, 00:08:53.05 that rock was perfect. It was mapped as being from ancient rivers 00:08:56.03 and streams, ancient delta systems. So Ted and I were really excited, 00:08:59.13 and just a little anecdote here is, we were so excited. It's all happening 00:09:02.18 one morning, so we ran to a Chinese food restaurant across the street 00:09:05.23 and had a fortune cookie. And that fortune cookie said, "Soon you'll 00:09:09.24 be at the top of the world." So yes, here we go! We had a fortune cookie, 00:09:13.02 off we go! So anyway, this is a colorized version of one of the scientific papers 00:09:17.15 and it shows the stratigraphic section right here, and as well as the 00:09:21.07 map. And you can see, super-imposed over the map of the Canadian 00:09:25.15 arctic is a huge area, shown in red here, of Devonian age rock 00:09:30.18 So you can see it's all over. This is 1500 kilometers of section. 00:09:35.23 So now we had a new problem, instead of driving in our station 00:09:39.23 wagons to central Pennsylvania, we're now working several hundred 00:09:42.28 miles from the North Pole, where the arrow is in this slide. 00:09:46.02 To give you a sense, the nearest town is about 300 miles away. 00:09:50.06 I'm going to show you a picture of that town, with about 170 permanent residents. 00:09:54.07 This is a picture of that town in spring, Beresford, Canada. This is a big city! 00:09:58.22 And so this is resupply. Anyway, as you can imagine, much of our 00:10:02.16 supply is based on planes and helicopters. And so we're highly 00:10:06.20 dependent on aircraft. Now the way this affects science, is that the supply chain 00:10:12.23 is very long. And fossils are rocks, they're very heavy. So that means we 00:10:16.29 can't take home everything we find. So much of what we do in the field 00:10:20.26 is trying to determine whether we have really good material that is 00:10:23.20 worth spending the money to freight it home. So we started in 1998, 00:10:30.27 we had that fortune cookie in 1998. Our first expedition was in 1999. 00:10:35.02 It took us about a year to get the permits, the money, all that good stuff. 00:10:37.22 And we started with great hope in the western part of the Canadian arctic. 00:10:41.06 And you can see what that area looks like. It looks kind of flat, doesn't it? 00:10:45.15 And you can see our camp just below me here. This is Devonian rock. 00:10:49.02 So we'd wake up in the morning and essentially walk all over these 00:10:51.25 Devonian rocks, looking for the bones that are weathering on the surface. 00:10:56.02 We realized we had a big problem, this is the wrong part of the Arctic. 00:10:58.28 We were in all these rocks, which were marine rocks formed in ancient 00:11:02.22 oceans. It was pretty clear based on other people's discoveries 00:11:06.12 that we needed to hit rocks in rivers and streams. So what that meant 00:11:11.07 is using the geological map, this is a version of what I showed you 00:11:14.29 before for Pennsylvania, it applied also to the arctic. We were in the 00:11:18.24 ancient seaways, shown in blue. We had to move upstream. To do 00:11:22.12 that geologically, it meant moving east. So we retooled, the next year we went back 00:11:27.23 and we moved east, here to Southern Ellesmere Island. 00:11:30.25 And as soon as we did that, you can see these beautiful landscapes 00:11:33.29 here, of mountains and areas. And these are all red beds that contain 00:11:38.09 Devonian rocks formed in ancient rivers and streams. 00:11:41.15 Took another few years, we eventually honed in on an individual 00:11:45.17 valley that had good fossil preservation. And this is that valley here. 00:11:49.04 What you see, in front of us, is a big hole in the middle of this 00:11:52.21 slide, that shows a layer that was preserving hundreds of fossil 00:11:56.13 fish, preserved one on top of the other. So, the arctic's a big place. Fossils 00:12:01.16 -- you know they're not very big, so you're trying to find the little 00:12:03.19 tiny patches of rock that will have those fossils inside. And what we use 00:12:08.01 are the tools of geology and sedimentology. Well, we worked this layer, 00:12:12.02 and everything changed one day in 2004, when my colleague, 00:12:16.09 Steve Gates, he's shown in blue here to the left, removed a rock 00:12:19.18 from this layer. And he saw a little notch, a little v, you can see that v 00:12:24.01 in the middle of this slide. If you look very carefully you can see it. 00:12:27.11 He says, hey guys, what's this? We run over to it and we saw what 00:12:31.04 we spent multiple years looking for and several decades of a quest. What we had 00:12:35.09 looking at us here is the snout of a fish, and not just any fish, the snout 00:12:39.22 of a flat headed fish. Remember I told you the conical head and the flathead 00:12:43.05 was the big piece of this transition? What I had here, in the middle of this 00:12:46.15 slide, was a flat headed fish sticking right out at me. With any luck at all, 00:12:50.11 the rest of the fish would be preserved in the rock. So we removed 00:12:53.29 this thing and they come home at the base of the helicopter, so this is a 00:12:57.08 first year graduate student for scale. And it comes back several hundred 00:13:00.17 miles to a supply base. And as we remove this first specimen, we 00:13:07.01 found several more. So at the end of the 2004 season, we had multiple 00:13:10.08 specimens of this flat headed fish. We come back to the lab, we're really 00:13:15.27 excited. We come back to the lab and the preparers start removing 00:13:19.05 the rock, grain by grain, to expose the fossils within. This is what 00:13:23.25 Steve's specimen looked like several months after preparation. 00:13:27.14 You can see what's emerging is sort of a flat head with eyes on top. 00:13:30.15 You can see the tool that's used here to remove the specimen, after 00:13:34.29 several more months, what's exposed. Look here we sort of have a flatheaded 00:13:38.14 animal with eyes on top, even a shoulder that looks separated from 00:13:42.00 the rest of the head. Remember what began this quest? In graduate 00:13:45.23 school I saw this slide, fish on top, tetrapod on bottom. 00:13:48.25 We wanted a flat headed fish. After several decades, using 00:13:52.05 tools of evolutionary biology, using tools of centrography, using the tools 00:13:55.22 of geology, this is the flat headed fish. This is Steve's specimen 00:13:59.19 as removed from that hole. Flathead, eyes on top, has a neck, 00:14:03.14 you can see the head is separated from the shoulder, it has fins, 00:14:06.22 it has fin rays, and it has scales on its back. Like a fish, it has 00:14:10.21 scales and fins, like an early limbed animal, it has eyes on top, 00:14:15.18 it has a neck. And indeed, when we crack open the fin, what 00:14:18.06 do we find? Bones that correspond with the upper arm, forearm, and even parts 00:14:22.13 of a wrist. This is a real mix of characteristics. Found at the right 00:14:25.28 part of the fossil record. So this creature we call Tiktaalik, Tiktaalik 00:14:30.05 roseae. And you can see here, what's sort of salient about it. It has a 00:14:33.16 mix of tetrapod and fish characteristics, like a lobed fin fish it has fins, 00:14:37.26 scales, and primitive jaws. Like an early tetrapod, it has a neck, a wrist, 00:14:43.16 a flathead, and so forth. So here's a creature with lungs and gills, here's a 00:14:49.26 creature with limbs that have limb bones in them but also fin bones, 00:14:52.25 as well. Fined rays. It's really a mix between fish and tetrapod. 00:14:56.19 And when we put it in the phylogenetic analysis, the evolutionary 00:14:59.29 tree, you can see Tiktaalik drawn right in the middle here. It really 00:15:04.03 shows a fine sequence of the emergence of tetrapod characteristics 00:15:08.11 throughout the Devonian. So what's the take home message here? 00:15:12.15 The take home message here is that we can predict likely and unlikely 00:15:16.04 places to find fossils. We can, in doing that, learn about the great 00:15:20.27 transitions in the history of life. In particular, this one, the transition 00:15:23.28 from life in water to life on land. But the story doesn't stop there, it begins there. 00:15:29.10 Why? Because we can trace the bones in a creature like Tiktaalik. 00:15:33.09 LIke the arm bone, the upper arm bone, the humerus of Tiktaalik, 00:15:36.17 we can trace it all the way to other mammals and to people. 00:15:39.23 This story of the origin of the neck, the origin of a wrist, is not 00:15:44.27 just some esoteric event in the history of life. It's an event 00:15:48.19 embedded in our own bodies. The origin of the neck, the first time we see 00:15:52.09 in Tiktaalik and its evolutionary cousins, is something that is to become 00:15:55.09 our own neck. Likewise with the arm bones. So every time you 00:15:58.11 shake your head, every time you bend your wrists, you can say 00:16:01.07 "Thank you, Tiktaalik and its evolutionary cousins that lived in the 00:16:04.22 Devonian streams 375 million years ago."