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."