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Great Transitions: The Origin Of Tetrapods

20 minutes


(male narrator) Our planet is teeming with many kinds of animal life, including groups with defining structures, such as the four legs of land animals, the feathered wings of birds, and the dexterous hands of primates. Understanding the origins of these structures and of the groups that possess them, has long been a central quest of biology. Charles Darwin asserted that each kind of animal must have evolved from preexisting, earlier kinds of animals that lacked those structures. He boldly predicted that buried in the crust of the earth, were animals that connected one major group to another. Such transitional fossils would be intermediate in form between earlier and later groups. What made Darwin's prediction so bold, was when he stated it, in the Origin of Species, no such fossils had been found. His critics immediately latched on to the admission that transitional animals are somehow missing from the fossil record. Over the decades, it's become a talking point for those with closed minds to the science of evolution. But, in fact, since Darwin, paleontologists have unearthed hundreds of transitional creatures that have enabled us to reconstruct the origins of many groups. And even so, searching for such fossils remains a challenging adventure. Finding the right one can change the way we think about the origins of living creatures. For me, paleontology has always been about filling gaps in the story of life. One of the biggest ones was understanding the origin of animal limbs with fingers and toes. Paired limbs are a feature of many animals. But there's little at first that suggests the limbs of different species are related. On frogs, they're springy. On elephants, not so much. They're feathered on eagles, not on bats. But inside the limbs of mammals, amphibians, reptiles, and birds, one finds a common architecture. Here's a dog. Dogs run and jump. What do you have? One bone, two bones, little bones, and then digits: the equivalents of the fingers or toes. And, of course, here's a bird--it flies. It's limb has been modified into a wing and it has one bone, two bones, lots of bones, and then digits. The amazing fact is every four-limbed animal walking the earth today has this fundamental pattern of one bone, two bones, little bones, fingers. That pattern suggest a connection between these very different groups of animals. And it's not the only feature they share. They also will have a backbone-- they're vertebrates. The history of vertebrates has been captured in rock we can accurately date. Fossils reveal when each animal group first emerged. The youngest group is the birds. Go further back and you'll find the first mammals and then the first reptiles and the first amphibians. And then you get to 370 million years ago. Suddenly, there are no four-limbed creatures, or tetrapods, anywhere to be found. Where the first tetrapods came from has always been a great mystery of biology. It's not like there weren't animals or vertebrates around 400 million years ago, there were. But they were Did four-legged animals come from fish? Fish might seem unlikely candidates to be the earliest ancestors of frogs, horses, and humans. They don't even have limbs. They have fins. Despite their different external appearances, there are revealing similarities. First, fish and tetrapods are vertebrates. And early in life, when they are embryos, they look remarkably similar. Finally, DNA analysis shows that fish are tetrapods' closest relatives. All of this suggests four-legged animals did indeed come from fish. But how did a fish with fins give rise to tetrapods with four legs? As a scientist, I wanted to find fossils that could help answer that question. I knew it wasn't going to be easy. The world is big, the earth giant, and fossils tiny. How do you find those things? We run through a checklist. We look for rocks of the right age. For the origin of dinosaurs, there's one age of rock. For the origin of land-living creatures, another age. Then you look for rocks of the right type, the kinds of rocks that are likely to hold fossils. Many things must happen for an animal to be fossilized. It has to be in the kind of setting where sediments form. Soon after dying, it has to be buried before its remains are ravaged by decay, weather, or scavengers. The dirt and mud burying it has to harden sufficiently to protect what's left for thousands or millions of years. After which, something, say erosion has to bring the embedded remains to the surface. And then someone who cares about such things, like me or my longtime colleague Ted Dashler, has to wander by and find it. The fossils I wanted to find existed in the Devonian Era, between 365 and 385 million years ago. Where could we find the right rocks from that era? I remember sitting in the office. We were bantering about something geological. We had a college textbook. We were just thumbing through the diagrams and boom, there was this figure that changed our lives. I remember seeing that and saying to myself, "This is what we're looking for." A map of North America highlighted three areas of Devonian rock just the right type to hold fossil fish moving onto land. Two of those areas had already been worked on, so we focused on the third, the Canadian Arctic. My heart was racing when I saw that. I'm sure yours was too. No paleontologist had worked on that, expressly looking for early tetrapods. Then you dug out aerial photos. That's when I got kinda terrified.

[both laughing]

I thought, "You've gotta be kidding me." Look at all this snow. How do you work there?

[engines growl]

The arctic presents some unusual challenges. You're far from help. You have to bring everything. You have to move fast because the season is short. You don't want to be there when the weather turns. When the helicopter drops you off the first time, you say, "What am I doing here?" You're thinking of polar bears. Everything white becomes a polar bear at first. The last thing on your mind are fossils. It's hard to believe that this frozen terrain was once a warm, watery world, swimming with life. There's this huge disconnect between the present and past. What we see is a valley with rocks that are tilted and stacked on each other. That's not how it was historically. These valleys have been carved by glaciers that have moved back and forth. Those red and green rocks extended across the valley. They were straight. They weren't tilted. Inside, the rocks tell us that this valley, 375 million years ago, was a giant floodplain. That floodplain was filled with rivers that swelled their banks and sometimes shrunk, but in those conditions formed swamps and streams of all different sizes. Inside those streams was diverse life... including, we suspected, a fish with features that would ultimately enable animals to walk on land. Even if it had been there, could we find evidence buried on one of the nameless hillsides that had built up and eroded over the past 375 million years? So, how do you find fossils? I pick up a lot of stuff. Sometimes it's just a piece of rock. Sometimes it's bird poop. Sometimes it's a leaf. But occasionally, it's a jaw with teeth in it. You learn to differentiate white which is not bone from white which is bone, teeth, or scale. Then you apply that search image to other rocks. Here's another scale here that stands out. Then we go, if you look around-- one right here. This is the underside of a skull. You never know where you're gonna hit it. That's why we keep on lookin'. But that first expedition ended without finding what we had come for. As our second trip drew to a close, we were still searching. Then, a bit before our scheduled departure, we had a real scare. The team had separated. Everybody needs to return back to camp by radio call. "Any of you see Jason?" "No, I ain't seen Jason." Suddenly it became, "Where's Jason?" This is our youngest member. We were looking out for him. My heart was beginning to race. Then I hear footsteps outside the tent. There's Jason. His eyes are like globes. "I think I found it." Every pocket was burgeoning with bones. He's laying them out on the table. It's daylight 24 hours a day, so we ran to Jason's site. As we came to this bluff and looked down, we saw why Jason was excited. Beneath our feet, were fossil fishbones--fragments. Thousands of them. It was a whole aquarium of different species. It got better because, as we walked up the hill and followed that carpet of fragments, it stopped. Meaning it likely came from one layer. With luck, we'd find that layer and see what's inside. Hard as we tried, we couldn't discover what was buried in Jason's hill before we had to leave. We kept coming back to it in following years to dig, chip, and search. The second week of July, 2004 we're working in series in this hole, where my head is next to Farish's feet, and Farish's feet is next to Steve Gatesy. We're digging and Steve says, "Hey, what's this? " Ted and I ran over to see. What we saw...was this V here. It was covered with rock. As soon as we saw this V and saw these teeth, it became clear that this V we're seeing is the tip of a snout and that this was the snout of a flatheaded fish. It was sticking out of the rock. With luck, the rest of the creature would be encased in the rock. And here it is. What's wonderful is that we have pretty much the whole thing. And it's put together: the head is connected to the body. The body is connected to the fins. So we know that this fin comes from this body. Put together, we see this creature's about four feet long. Some of the biggest were about nine feet long. What's really amazing is that this is an animal that Darwin would have predicted, a real mix of characteristics: a combination of fish-like and tetrapod-like features. Like a fish, it has scales on its back. It also has fins with fin rays. Like a tetrapod, it has a flat head with eyes on top. When we look inside the body, we see these huge interlocking ribs that suggested that it had lungs. When you put the body and the head together, you see a neck where the head moved independently. It could use the neck to peer outside the water, find prey, and avoid predators. So, we've many bones of these animals, including this one which is a hip bone. It reveals that the hind fins were already evolving into legs while these animals were living in water. I get excited when I see the front fins of Tiktaalik. Here's one from a larger specimen. You see the shoulder and some of the fin bones inside. You see a version of the one-bone, two-bone pattern that's inside our own arms. You have one bone, two bones, even a version of a wrist. Once those fins were strong enough to lift its body out of the water, a new frontier opened. Over millions of years, the two pairs of fins in fish-like Tiktaalik would lead to the two pairs of limbs in every tetrapod. What does this all mean? It means that our arms and legs are derived from the paired fins of our fishy ancestors. How fast did this transition happen? We know that Tiktaalik didn't exist in a vacuum. There are other creatures, other transitional fossils that are more fish-like and others more tetrapod-like. These creatures existed for over 15 million years. This means that this great transition from fish to tetrapod didn't happen in a single step, but happened gradually over time. The discovery of Tiktaalik made headlines because it is one of the earliest fossils that illuminate the transition from water to land. That's just one key transition fossils have shed light on. One of the first was the evolution of birds from feathered dinosaurs. That's one of the best documented transitions in the story of life. What it and other well-studied examples tell us is that what first seemed to be huge leaps, are almost always products of a series of smaller evolutionary steps. That's true for when our fish ancestors first came to land, when dinosaurs took to the air, and when we first stood upright.

(male) ...had an ancestor who lived 3.2 million years ago.

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Charles Darwin once boldly predicted that buried deep in the earth are transitional fossils of creatures with intermediate features between ancestral animal groups and the modern animal groups. Since Darwin’s time, many transitional fossils have been discovered, and they provide crucial insights into the origin of key structures and the creatures that possess them. And University of Chicago paleontologist and award-winning author Neil Shubin provides a first-hand account of the painstaking search for the transitional fossil of Tiktaalik, a creature with a mix of features common to fish and four-legged animals.

Media Details

Runtime: 20 minutes

Great Transitions
Episode 1
20 minutes
Grade Level: 7 - 12
Great Transitions
Episode 2
19 minutes
Grade Level: 7 - 12
Great Transitions
Episode 3
20 minutes
Grade Level: 7 - 12

Viewer Comments

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    Debra S. (Cottonwood Heights, UT)
    March 9th, 2016 at 02:42 PM

    This is a great film reviewing the fossil origin of tetrapods. Great for a biology class.