Neil Shubin and Ted Daeschler: How Fish Came Ashore

Transcript:

Q: What do we know about the environment in which the transition to land occurred?

NS: Back in the Devonian, north central Pennsylvania was very different from today, where you now find a river valley with a mountain plateau and weather that gets cool in the fall. Back then, there was a much more tropical climate in Pennsylvania. It was south of the equator. Remember the continents are continually moving around. The area was a river valley, much like a broad delta.

And something very special was happening in those deltas. There were small streams with all kinds of diverse fish. There were tropical plants, and many of the species that we find were warm-adapted. In fact, what we're finding in the Devonian or a little bit before this time are probably some of the first forests. Life was beginning to settle on land -- plant life and invertebrate life, that is.

If we had a camera that could capture the Devonian world, many of the streams we would see would be small, shallow freshwater streams that were choked with these plants. What was happening within there was very interesting. It was a crucible for evolutionary, change largely because of the diversity of fish, many of them predatory. It was a fish-eat-fish world in the sense that you had fish getting larger and more predatory. You had fish developing more types of armor. And you had one very special type of fish that had a third strategy, which was to get out of the way of this of this arms race. And it's those fish that evolved the traits that enabled them to walk on this newly inhabitable land. Essentially what enabled those animals to get out of the way -- that is, to get out of the water -- were new features like limbs and changes in their vertebral column, in the heads and so forth.

TD: On the Allegheny plateau in north central Pennsylvania, the rocks are flat and most of them belong to what's called the Catskill Formation. We have to remember that when those rocks were deposited, the environment looked nothing like it looks like now. In fact, it was so different that at that time there wasn't even an Atlantic Ocean to the east. Instead, Europe was connected with North America, and there was a mountain range along that area. Those mountains were shedding sediments -- silt, sand, and clay -- off toward a seaway out in Ohio, to the west. The deposits that were left became what we call the Catskill Formation. They were streams and deltas, [and] it's the life in those streams that has become a very exciting part of the project in Pennsylvania to learn about the Devonian period.

Stream systems that were running across big, wide floodplains in north central Pennsylvania 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. The Earth was brown and muddy for the billions of years previous to this point in time. It was during the late Devonian that the land got green, especially in wet areas like these deltas that were shedding off and running into a seaway in the Ohio area. And so it would've been quite flat. If you were in a satellite looking down, you'd probably see green rims following the river courses. And, as I said, that productivity on land led to the productivity in the freshwater environments. So what was happening on land was new, and what was happening with the animals in the freshwater environments was also new.

Q: How did the transition happen?

NS: It's becoming very clear from discoveries made in Pennsylvania and elsewhere in the world that many of the traits that enabled animals to walk on land actually evolved in small freshwater streams. In these streams, animals were adapting in new ways to exploit this freshwater habitat. They evolved big, broad appendages, big, broad fins that enabled them to maneuver in weed-choked streams. Many of the features that animals later use to walk on land originally evolve for a different purpose -- to live in these freshwater streams.

Essentially what we're finding are clearly aquatic organisms, true fish in every sense of the word, that have features that are also seen in creatures that live on land. Some of the features correspond to bones in our limbs, including some that might be very similar to our fingers; features in the back; features in the skull. It's a very surprising discovery; not something we necessarily would have predicted, say, 15 years ago.

Apparently, what drove the transition in fish is that life in these small streams is very similar to life on land. Think about it: You're a big fish with giant teeth and you're making a living by eating little fish. But you're a huge fish, say, six feet long, and you're in a shallow stream choked with weeds and you need to crawl through those weeds and get into the mud. That is very similar to an environment that amphibians like a salamander live in today. So we're dealing with a mosaic environment in shallow streams; that is, there are a variety ways to exploit that habitat. One of those ways turns out to be a good way for the future, too, because it is later used to enable animals to walk on land.

TD: What makes the late Devonian, 370 million years ago, a particularly interesting fossil period is that there was such a wide diversity of basic kinds of fish in the world, many more basic kinds than we see on Earth today. There were the placoderm fishes, armored fishes that don't have any living descendants today. There were spiny sharks -- again, a whole group that left no descendants -- very primitive sharks, and very primitive forms of what we call ray-finned fishes, which did lead to a lot of the modern groups. And, of course, the fishes we're most interested in are the lobe-finned fishes. They were the ones that were developing structures in their fins that were beginning to look like limbs.

Lobe-finned fishes are a group of bony fishes. It's the structure in their fins -- particularly what we call paired fins, two in the front and two in the back -- that makes them unique and separates them from other kinds of bony fishes. Some were using those fins in developing extra-specialized bony structures, to perhaps push on the bottom of these streams, or perhaps to go up into more shallow water. And those sorts of adaptations for moving into those kinds of environments are a stepping stone, if you will, for eventually coming on land. Therefore, a very important transition was going on in these lobe-finned fishes in the Devonian.

Q: How did fish get onto land if they couldn't breathe?

TD: Very early in their evolution, fishes had lungs while they were still aquatic, so the transition to breathing air was no problem. Many kinds of fish have primitive lungs and some actually are able to breathe air -- we call it "air gulping." Among the lobe-finned fish there was not a big problem with breathing air. They had developed lungs first for use in water. As they may have moved into these stream systems, perhaps lungs became more and more important, especially in waters that may have had a lot of decaying plant material and low oxygen content. Coming up to the surface and breathing air may have been no problem at all. One thought is that the development of limbs was to help push off the bottom to get your nostrils above the water surface and take in some air.

Q: What are some of the scenarios that might have led from these fish with lobe fins to what we call tetrapods [fourlimbed creatures]?

TD: The transition from fin to limb seems to have happened in a rather complex environment. There were probably a number of factors favoring limb-like fin features for certain groups: being able to push off the bottom of the mud to get your nostrils above the surface and breathe air; being able to push along the bottom to move through plant-choked environments or into shallower water to capture prey, because these were carnivorous fish; or perhaps being able to escape the bigger predators behind you by moving into shallower water. There are lots of different ideas on why fins may have become useful for new purposes. But the fish that had a new tool kit in those fins were able to exploit new ecological opportunities in the stream systems 370 million years ago.

The discoveries that we've made in the Catskill Formation in north central Pennsylvania, in particular, have provided a lot of good new data, first of all about the diversity of the earliest limbed animals. We found a couple of new forms, and that's important. Added to the other forms around the world, we're seeing that at this point in time a lot of experimentation was going on among these earliest limbed animals. The different morphologies in all these different forms suggest they were doing different kinds of things.

Our work in Pennsylvania has not only given us a snapshot of some of the animals that lived here, especially those earliest tetrapod animals, but has also shown us the other creatures that were living around them. Maybe more importantly, we're finding spectacular plant material preserved around them, in the same layers. We're finding wonderful invertebrate material. These are soft-bodied things for the most part -- arthropods, like primitive scorpions that were coming up and living in these same stream or even land environments. So we can set that whole picture of fauna and flora in a geological context. We can try to interpret what the environment was like by judging the sediments that these stream systems left behind, and we can get some ideas of whether these were channels, or swamps, or what. So with geology and paleontology together, we have a really nice snapshot of what it would have been like here 370 million years ago.

NS: This is an important transition, because before this time period all life was in water. Earth was a brown and rocky place. Over many millions of years, plants and bugs inhabited land, and at this time period our distant ancestors evolved those features that enabled our success.

TD: Life finally was able to make the transition to land basically because of extrinsic factors -- features of the physical environment, such as oxygen and other atmospheric gas levels changing, and different sorts of geological phenomenon going on. Life on Earth is very old; but life on land is not nearly as old.

It was only in the last 400 million years or so that plants were able to establish a foothold on land. There were aquatic plants that began to colonize land and those were real pioneers because they were moving into an environment that was extremely harsh. There was no soil on the surface of the earth. It was rock and mud and silt. Just a brown wasteland, from our perspective today. Plants came first, then invertebrate animals, different sorts of arthropods -- many-legged small creatures, like insects -- and arachnids -- spiderlike insects. It was only after those animals and plants had colonized land and soils began to develop and there were perhaps damp microclimates within these early plant communities that the back-boned animals, the lobe-finned fishes particularly, began to exploit those new environments that had been set up on land.

Once you have that, the world's a different place. All of a sudden there are streams, small streams that have all kinds of plants inside them, and within these streams you have the opportunity for new evolutionary experiments. These small streams are like an engine or a crucible of evolutionary change. In fact, it's within these streams that different types of our ancestors evolved. These freshwater streams are a new environment. They did not exist a hundred million years before. With this novel environment, you have the possibility for evolution to go in a number of different directions.

New environments make a new set of rules that are experimented with by the vertebrate animals, and evolution chooses the ones that work best in these new environments. So indeed, a new environment spawns innovation in the animals that are beginning to exploit that new environment.

Q: Your discoveries have changed the way many scientists think about this transition. What were the old assumptions about this transition?

TD: Our work in the Catskill formation in Pennsylvania has provided a lot of new data about what has happened in the late Devonian period, 370 million years ago. We're able to see much more about the environments and the other animals that were living along with lobe-finned fishes that have been the focus of our attention.

In the past people looked at the rocks, may have found some of the fossils, and came up with one scenario: that perhaps those animals developing limbs were living in environments in which there were frequent drying periods. They may have lived in ponds that were drying up and they needed to use their fins to move into new wet areas to escape drying out their fins in these dying ponds. That was one idea. What we have been able to add to this idea, perhaps to replace this idea with, is that we're seeing wonderful wet environments in our work. We're seeing swampy, plant-rich places where these evolutionary changes were happening.

NS: One thing that was special about Red Hill, the road cut in Pennsylvania where we've found several key fossils, is that it is literally a slice of life of the Devonian 370 million years ago. You have the plants, you have vertebrate animals, and you have creepy crawlies, and it's rare to have all these things together. It enables a reconstruction of what life would actually have looked like at this time period from the climates to the organisms and so forth.

For a long time the paradigm was you have land and you have water and these are two discrete categories and the evolutionary jump between the two was a big one. The reality of the situation is very different, and it's actually more interesting. You have a lot of intermediate environments and a lot of intermediate forms living in these intermediate environments. It's helpful to think of the small streams of Red Hill and other places like that as being quasi-terrestrial and quasi-aquatic. Think about it, a shallow stream choked with weeds: If you're a big animal in this environment, it's kind of like land and it's kind of like water. It's no surprise that organisms with intermediate designs appeared in these intermediate environments.

Q: What was life like in these streams?

TD: There was a wide variety of fish in late Devonian environments, and we see many of them at the Red Hill site. We find a few different varieties of placoderms, which are armored fish, perhaps filling different niches but basically feeding on the mud on the bottom. They were eating the decaying plant material and such, which was provided by the plants that were beginning to colonize land; hence freshwater environments were beginning to be productive.

Of course animals were taking the opportunity to prey upon the placoderms. That's one of the ideas we have for why some of the early limbed animals at Red Hill had heavy jaws -- perhaps to crush placoderm plates. They'd exploit these fish that were exploiting the mud that was being fed by the terrestrial plants that were beginning to grow on land.

There are also other kinds of lobe-fin fishes. These early tetrapods were about a yard long, but some of the lobe-fin fishes that were related to tetrapods but didn't have the ability to get into more shallow water, were on the order of three or four yards long, with huge teeth. They were probably the top of the food chain there, the ultimate predators. And maybe the early tetrapods that we see had to fear those early predators. So besides being able to chase their prey into the shallows by using limblike fins, they were also able to escape the predators behind them by using these same limblike fins to get into the shallow waters. This was a really dynamic world. We were beginning to get a complex food chain in these new environments.

The lobe fin was an innovative new structure, which allowed an animal to do things like prop against the bottom, or perhaps push against the bottom and lift its head up to breathe air above the water surface. Perhaps it allowed the animal to move through plant-choked waters to more easily pursue prey up into the shallow waters. Perhaps it allowed the animal to enter those same shallow waters by pushing against the bottom and using its tail for propulsion to escape predators behind it. There are a lot of different ways that lobe fins were able to be used that were very different from the typical fish uses of their fin.

NS: The main mechanism of evolutionary change is a very simple one. No two organisms look alike. The fact that all of these individuals look different means that some are going to do better than others in their given environment. And in every generation having this process of variation and success leads to the changes we've seen throughout evolutionary history, in particular in the Devonian. If you look in a Devonian stream, no two fish look alike, no two fish do the same things. They feed in different ways, they swim in different ways. Some of these different ways are actually better than others, and over a period of time, sometimes very short, sometimes very long, we can see significant changes in how organisms look, survive, reproduce, and so forth.

Q: What do these recent discoveries tell us about how our knowledge of evolution is changing?

NS: It is really appealing to me at a personal level that discoveries at a road cut can really change our worldview in a very important way. For a long time many of us thought that evolution is progressive -- that is, evolutionary change goes in lockstep with climate or environmental change. It doesn't appear to be the case in a transition like this, the invasion of land. It's both more complex and more simple than we could have ever imagined. What seems to be happening is a series of experiments, like in those 370 million-year-old streams. You have all kinds of different forays into new types of ecologies, different ways that fish can make a living.

I think it's wrong for us to think of evolution as a ladder of progress, that you have environmental change and a ladder of evolutionary change leading to its ultimate destination -- mankind. Really what you have is a series of evolutionary stages. Organisms are trying to exploit their environment to the best of their ability, to make their living, if you will, in these environments. And there're almost as many ways of doing it as there are different types of organisms. It's in the Devonian that these experiments were happening at a very rapid pace, and we human beings are actually the descendants of one of the successful experiments from this time period.

Q: What about the idea that this transition is really one of the key personal moments in our own deep history?

TD: As limbed animals, we're very interested in what gave us these very useful appendages. It's interesting when you look at all of the limbed vertebrates. Appendages are used for such a wide variety of things. Sometimes appendages are even lost for the sake of a different lifestyle, like in snakes or whales that have significantly reduced appendages, especially their hind appendages. But one of the things that makes us unique as humans is the ability to use our hands to manipulate things in a very fine way, and it's something we inherited from primates. And primates inherited the appendages from more primitive mammals, and we go all the way back to lobed-fin fishes, which had the beginnings of all these structural features. We're very interested in those features for that reason.

NS: There are two types of bony fish on the earth today: the ray-fin fish and the lobe-fin fish. Ray-fin fish, as the name implies, have a fin composed mostly of rods, or rays. These rays form most of the surface area of the fin, and they're a special type of bone -- they're not the sort of bone that's present in our limbs. The ray-fin fish are very common today; for example, the common sole is a ray fin. Nowadays, he's dinner. In fact, most of the fish we eat are ray-fin fish. Yet in the Devonian, 370 million years ago, these fish were very rare. The more common fish in the Devonian were the lobe-fin fish. A lobe-fin fish has a fin composed of a fleshy lobe. Yet within this lobe is where most of the interesting evolutionary stuff happened. Many of the bones that gave rise to our limbs actually lie within the lobe of the fin.

For better or for worse, the lobe-fin fish are among the creatures most closely related to us. In fact, it's most closely related to all vertebrates that walk on land. What's special about the lobe is that most of the bones that enabled animals to walk on land [evolved from this]. Whether it's a bird flying, a whale swimming, or Mozart playing the piano, the bones that enable those creatures to do those functions originally evolved within the fin of a lobe-fin fish.

Now, the discovery of the fossil fin was really remarkable largely because it shows for the first time just how much like a tetrapod limb one of these fish fins can actually be. We can compare the fin of a 370-million-year-old fossil fish and the arm of a human. In a human arm you have first one bone, then two bones, the wrist, and the digits. In the fin, despite the fact it looks nothing like the arm of a human, what do you have? You have one bone, two bones, even little bones that can be compared to a wrist, and then rods that face away from the rest of the appendage itself, just like our fingers or toes. The whole arrangement of bones in both cases is very similar. This striking similarity between [the fin of] a fish that's 370 million years old and the arm of a [modern] human suggests that many of the bones now in use for daily functions, from our walking to birds' flying, originally were set up in a fish like this.

So what does this show? That many of the bones that evolved for functions in humans actually were already present in fish living in freshwater streams about 370 million years ago. Even though they don't look anything like the bones, and their size, shape, and functions are likely to be different, the pattern is essentially the same. And that's a very important feature in evolution that evolutionary biologists call homology -- a special sort of similarity. When we say things are homologous, we mean they're similar in an evolutionary sense; that is, they're derived from a common ancestor.

TD: We are just one of the animals that evolved from lobed-fin fishes, but we can trace our ancestry and the ancestry of all the other limbed animals back to that point 370 million years or a little bit more ago. We are lobed fin fishes.

Q: What do you mean by that?

TD: We're essentially very specialized fish. We have the same features as very primitive fish you find in the Devonian rocks. Yet we've added a few things -- actually, we've added a bunch of things. But yet we share with those fish so many characteristics that it's astounding. Not only the lobe, but also characteristics of the back, characteristics of the skull, behavioral characteristics, characteristics of the respiratory system, similar genes. It can go all the way down to the molecular level. Basically our body plan is that of a fish. We've added a few things here and there, and we like those things. But in an evolutionary sense, we view ourselves as very specialized lobed-fin fish. That's what we mean by that statement.