The Four-Winged Dinosaur

The Four-Winged Dinosaur

PBS Airdate: February 26, 2008

NARRATOR: One day, long ago, life in a forest came to a sudden halt, snuffed out and buried by volcanic ash. One hundred thirty million years later, fossils are all that remain, and some are like nothing ever seen before. Strangest of all is a creature called Microraptor. It had wings like a bird, but it had them on its arms and its legs. It was a four-winged, feathered dinosaur, and no one has any idea how it worked.

JACQUES GAUTHIER (Paleontologist, Yale University): What's this dinosaur doing with these aerodynamic feathers coming off their hind limbs?

NARRATOR: Rival teams have been trying to figure it out, piecing together clues about what it looked like and how it functioned from crushed and distorted fossils. Each team has built a model with very different results.

JACQUES GAUTHIER: When the animal is squashed flat, as these are, well, it leaves a little room for interpretation.

NARRATOR: Scientists have argued for decades over whether and how dinosaurs evolved into flying birds. Now, they think Microraptor could solve the puzzle, but they still don't agree on the solution.

LARRY MARTIN (Paleontologist, The University of Kansas): You have just shown me the death of the dinosaur-origin of birds.

JACQUES GAUTHIER: Birds are dinosaurs. That's why they look like dinosaurs.

FARISH JENKINS (Anatomist, Harvard University): I'm getting whiplashed by two lines of evidence, and I'm going to be disturbed this evening, thinking about this.

NARRATOR: One team is about to fly their model in a wind tunnel. But who's got the real Microraptor? It's the battle of the four-winged dinosaurs, up next on NOVA.

Major funding for NOVA is provided by David H. Koch. And...

Discover new knowledge: HHMI.

And by the Corporation for Public Broadcasting, and by contributions to your PBS station from viewers like you. Thank you.

NARRATOR: One day, in 2002, a courier made a delivery to the Institute of Vertebrate Paleontology in Beijing.

The dinosaur specialist, Xu Xing, was unprepared for what he saw when he opened the box.

XU XING (Paleontologist, Institute of Vertebrate Paleontology and Paleanthropology): My first feeling is, "Amazing, a beautiful fossil."

NARRATOR: It was a nearly complete skeleton of a small dinosaur, with a feature he'd never seen before, long feathers attached to the arms.

XU XING: This animal has feathers like feathers in flying birds. I thought, "Oh, we have evidence suggesting some dinosaurs could fly."

NARRATOR: For over a century, scientists have searched for the origin of birds and flight, and the evidence keeps pointing to the same improbable conclusion. Somehow, flying birds evolved from Earthbound dinosaurs more than a hundred million years ago. Exactly how it happened is still one of the great mysteries in the history of life. And the solution may be in this box.

Mark Norell and Mick Ellison are from the American Museum of Natural History in New York, but they spend much of their time in China, hunting dinosaurs.

They're on their way to Liaoning Province in the northeastern corner of China, a region that's produced some of the most spectacular fossils ever seen. It's an overnight train ride to the kind of place paleontologists dream about.

Today, it's a harsh and often barren landscape. One hundred thirty million years ago, during the age of dinosaurs, it was a forest teeming with life, some familiar, and some not. It was also a place where volcanic eruptions created a virtual Pompeii. Now the trees are gone, but remnants of ancient life remain in the petrified layers of volcanic ash.

Norell has spent much of the past 10 years immersed in the history recorded in these slabs of rock. They open like the pages of a book, sometimes with exquisitely preserved fossils inside. These are more than snapshots of the distant past, they're high resolution records of life in a temperate forest during the Cretaceous period. They're especially valuable for the number and variety of small animals they add to the fossil record: the fish, frogs, turtles and reptiles that lived in and around the freshwater lakes.

MARK NORELL (Paleontologist, American Museum of Natural History): It's the first look at a forest like this and the creatures that would live in a forest like this, where we have tremendous diversity, but it's 130 million years ago. And it's the first time we've ever had that.

NARRATOR: Victims were buried quickly in fine ash, preserving detail rarely seen in fossils: skin impressions on a reptile, fur on a small mammal.

MARK NORELL: ...wish I could find something. Like, you'll go through hundreds if not thousands of layers, and you won't find anything. But occasionally, on some layers, you'll find lots of stuff, either tons of fish or lots of insects or, if you're really lucky, you know, once in a while–I mean it's never happened in my life–but once in a while, people find feathered dinosaurs.

NARRATOR: The first turned up more than a decade ago with what looked like a feathery fringe along its back. Then came others with feather impressions that were unmistakable. These weren't giants like T. rex. Some were no bigger than crows. But their skeletons showed they belonged to well-known dinosaur clans.

They were runners, not fliers. Their arms were too short to function as wings. They were just what most scientists predicted the ancestors of birds would look like before they evolved flight: small, carnivorous dinosaurs with feathers.

The dinosaur-bird connection first came to light in the1860s, when quarrymen in Germany discovered a fossil called Archaeopteryx. It had the feathered wings of a bird, but the teeth and long bony tail of a reptilian ancestor. Some thought the ancestor might be a dinosaur.

But the idea didn't catch on until a century later, when paleontologist John Ostrom led an expedition to Montana and discovered a new kind of dinosaur. It was a lightly-built meat-eater with a killing claw on each foot. They called it Deinonychus, "Terrible Claw."

A few years later, Ostrom was studying Archaeopteryx, when it dawned on him that the skeleton looked like a miniature version of Deinonychus. He found dozens of similarities, enough to convince almost everyone that dinosaurs like Deinonychus were not only the ancestors of birds, they were very much like birds themselves.

The new-look dinosaurs made their widescreen debut as the raptor villains of Jurassic Park. Playing the role of cunning predators, they forever dispelled the old image of dinosaurs as dim-witted, cold-blooded reptiles.

JACQUES GAUTHIER: That's a radical change in the way we looked at these creatures: big, slow, lumbering, now seen as active, dynamic animals.

NARRATOR: Twenty years ago, Jacques Gauthier worked out the details of the family tree, based on changes in anatomy that appeared as dinosaurs evolved from their earliest ancestors.

JACQUES GAUTHIER: Just by using the sequence in which those characteristics appear, we reconstruct the history of life.

NARRATOR: Gauthier found that dinosaurs and crocodiles parted ways 240 million years ago when the ancestors of dinosaurs evolved the ability to stand up and run with their legs straight under their bodies.

As the family branched out, the meat-eaters, called theropods, became more and more bird-like, acquiring traits like a three-toed foot, a three-fingered hand, a wishbone and many others. Each new feature was passed along to the descendants.

Finally, a branch led in one direction to Deinonychus and in the other to Archaeopteryx and birds.

With birds nested at the top of the dinosaur tree, linked to their ancestors by hundreds of inherited traits, Gauthier says there can be no doubt about the family ties.

JACQUES GAUTHIER: Birds are dinosaurs. That's why they look like dinosaurs.

NARRATOR: And some dinosaurs looked like birds. We now know there's one detail the makers of Jurassic Park got wrong. The raptors should have had feathers. The fossils from China proved it.

Since the first discoveries, Liaoning fossil collectors have turned up feathered dinosaurs from many different dinosaur clans. All are from branches of the tree close to birds and Archaeopteryx. Some are from the same clan as Deinonychus; others are from more primitive groups like Oviraptors and Tyrannosaurs. And that means all the dinosaurs in this part of the tree had feathers, wherever they lived in the world.

MARK NORELL: We would assume that an Oviraptor that we find in Mongolia or a Tyrannosaur that we find in Montana would have also had feathers. So everywhere you go in the world, all these advanced theropods were covered with feathery body coverings.

NARRATOR: Feathers would have been useful to the ancestors of birds long before they took to the air. There's nothing better for insulation, which may have been their original function. They can be used to bluff an enemy, attract a mate or simply to show off.

But long feathers attached to the arm and finger bones are the signature of flying birds. And they were missing from the feathered dinosaurs of Liaoning. There were no clues to how Earth-bound dinosaurs achieved flight, until the strange fossil arrived on Xu Xing's doorstep.

As he examined it that first day, it looked like it could be a missing link, with a bird's wing and a dinosaur's long tail and legs. But it had something else that he'd never seen in any dinosaur or bird.

XU XING: I noticed something strange: long feathers attached to the foot. This is so bizarre. You don't know anything, any animal has this long feather attached to the foot.

NARRATOR: A two-winged dinosaur would be a spectacular find, but four wings border on science fiction. Nothing in the world today has feathers on its feet. And this was the first clue that such a thing ever existed.

Xu named it Microraptor.

XU XING: And we started thinking of, "What's this means?" And then it brings the big story about the origin of flight.

NARRATOR: The origin of flight in birds is a puzzle that seems to defy solution. The aerial skills of modern fliers evolved in small steps over millions of years, and the fossil record provides few clues to how it happened. It's one of the oldest debates in paleontology, and for decades it's been a standoff between two opposing theories.

One has argued that flight must have evolved from the ground, up. The ancestors were running dinosaurs, already feathered, probably to conserve body heat. Over time the feathers could have been adapted for flight, as bodies became smaller and the running leaps of dinosaurs evolved into the powered, flapping flight of birds.

But not everyone bought it.

LARRY MARTIN: It's always been a hard sell because with a terrestrial origin of flight, flight that originated from running fast on the ground, you're always working against gravity. But an arboreal origin of flight, where you fall out of the tree, you accumulate air speed whether you want to or not.

NARRATOR: In the arboreal scenario, the bird ancestor was a small climbing animal that evolved flight by gliding from the treetops. And that seemed to rule out dinosaurs, which, presumably, couldn't climb trees.

LARRY MARTIN: The ground-up origin of flight didn't make good physical sense, but it seemed to be essential to the dinosaur-origin of birds. And that made us suspect that the dinosaur-origin of birds was wrong, too.

NARRATOR: Fossil bird expert Larry Martin has been a thorn in the side of dinosaur paleontologists for decades.

LARRY MARTIN: People are going to simply have to explain how you can originate birds from dinosaurs, which apparently can't get up in trees.

NARRATOR: Years later, he's still at it.

LARRY MARTIN: So, if you really love dinosaurs, then my arguing that birds aren't dinosaurs is sort of like taking away Christmas.

MARK NORELL: I get really frustrated by this just because we shouldn't be having to deal with something which has been settled for 20 years. I don't know, when Magellan got back from sailing around the world, if he was frustrated, too, by people who still said the Earth was flat.

NARRATOR: It was hard to imagine what kind of evidence could break this deadlock, until Microraptor turned up. A four-winged dinosaur is a clue that no one expected. For Martin, it's evidence that he was right, at least about the origin of flight.

LARRY MARTIN: I think that Microraptor completely destroyed all hope for a terrestrial origin of flight. This is an animal that probably couldn't even walk on land comfortably, let alone run. And so you're looking at something that's probably completely arboreal.

NARRATOR: Xu thought the same thing: with feathers on its feet, Microraptor must have lived in the trees. But he's also quite sure that it was a dinosaur. Numerous details of the skeleton say so, from the long bony tail to the sickle claws on its feet.

Xu also believes it must have been able to fly.

XU XING: I noticed the feather shape. This animal has feathers like feathers in flying birds.

NARRATOR: The wing feathers are asymmetrical, with veins wider on one side of the shaft than the other, which forms airfoil contour, like an airplane wing. Only flying birds have asymmetrical flight feathers.

But unlike modern birds, Microraptor had not yet evolved the specialized shoulder anatomy for powered, flapping flight. It was more likely a glider, like a flying squirrel, so Xu concluded that gliding came before flapping in the evolution of birds. He presented Microraptor to the world as a synthesis: flight began with gliding from the treetops, and the glider was a dinosaur.

XU XING: Those feathers will change the whole idea how dinosaur evolved into flying birds.

NARRATOR: It looked like Xu had finally cracked the origin of flight. But, within days, another scientist announced that he'd solved it and come to the opposite conclusion.

KEN DIAL (Biologist, The University of Montana): We think we have the answer, and we didn't make it up. Birds told us.

NARRATOR: Ken Dial is a well-known experimental biologist and a strong believer in the dinosaur-origin of birds. He's convinced that the powered, flapping flight of modern birds really did begin on the ground with a running start.

Dial studies living birds called chukars. They can run within hours of birth, although they can't fly until they're a few weeks old. He found that long before they can fly, young chukars use their developing wings to scramble up inclined surfaces to a safe refuge. Over time, the wing beat that helps them scramble becomes the power stroke that helps them fly. There's no gliding involved. Powered flight comes first, even on the way down.

Dial believes this is a primitive behavior common to all birds, reflecting their evolutionary history.

KEN DIAL: I think you're looking at the evolutionary process through this developmental process. And what these animals do, we think, is paralleling the evolution of what transpired between the theropod dinosaurs to modern-day birds.

NARRATOR: It makes sense, unless the ancestor couldn't run because of feathers on its feet. But Dial says speculation about fossils is not enough to prove him wrong.

KEN DIAL: If we're going to live in a world of interpreting fossils because it's just fun to think of all the different things they might have done, well, that's great. It's just...I think you're stepping out of the bounds of reality, 90-some-odd percent of the time. That doesn't mean you shouldn't do it. You can't do it; I just don't think it's great science.

NARRATOR: Science is built on testable ideas, and it's hard to run experiments on a fossil.

But what if Microraptor could be resurrected from this slab of stone? What would it reveal? Xu wants to find out and so does Norell.

MARK NORELL: This is a really spectacular animal, and I want to figure out as much as we can about functions that it might have had and how this will relate to the origin of flight in modern birds.

NARRATOR: Together they decide to dig deeper, to see what they can learn. And the process begins at Xu's lab in Beijing.

There are now at least 30 specimens of Microraptor in China. Xu has one of the best collections, and they're going to need all of them. They plan to build a model to see how four wings might have worked. To do that, they need to understand its skeleton and the range of motion in its arms and legs. Unfortunately, the fossils were crushed under tons of volcanic ash, making it difficult sometimes to interpret the true shape of the bones.

MARK NORELL: In the best of all possible worlds, we'd make our reconstruction from a wonderful three-dimensionally preserved specimen. But this isn't the best of all possible worlds. We don't have that.

NARRATOR: Many bones are damaged, deformed and sometimes missing altogether. They'll need to work with multiple specimens, look for the best preserved examples of each bone, and combine them all into a composite that fills in the blanks in the original fossil.

The specimens have to remain intact and stay in China, so they'll use high-resolution photographs for reference.

At the American Museum of Natural History in New York, where Norell is Curator of Paleontology, a scientific sculptor starts work on a bone-by-bone reconstruction of the entire skeleton.

Jason Brougham is an artist and a trained anatomist.

JASON BROUGHAM (Reconstruction Artist, American Museum of Natural History): We had more than 16 specimens to look at, so, many of these bones, I could see from not only both sides, but from end-on, and from all different positions.

NARRATOR: Some specimens have already been analyzed in detail by other scientists.

JASON BROUGHAM: So I would just sit there with my micrometer and check and make sure that what I was sculpting not only looked like the picture but that it met those measurements.

NARRATOR: Since the specimens are all slightly different sizes, each bone is scaled to match the dimensions of the original fossil, a process that takes months.

JASON BROUGHAM: It's hard work to sit down and look at every bone from every angle. It's grueling in fact. My wife says I have a high tolerance for tedium. I guess that might be true.

NARRATOR: By chance, Microraptor is also under investigation at the University of Kansas. Larry Martin and David Burnham have built their own model based on a single specimen and a unique method of preparation.

DAVID BURNHAM (Paleontologist, The University of Kansas): The specimen was found with this side exposed. I embed it in optically clear plastic; I flip it over; I clean down through the matrix on this side, exposing the skeleton from the other side, ending up with a specimen that can be examined from both sides.

NARRATOR: Burnham made molds of each side of the slab, cast the exposed bones and cut out the pieces. Some had good preservation, and some were crushed. When he assembled the bones, the result looked more like a crocodile than a dinosaur.

DAVID BURNHAM: What really surprised me, when I was mounting this skeleton is when I put the back leg into the hip socket, it just popped right into joint. And as you can see, when that happened the leg was mounted in a sprawled posture. The only time it felt like it was in socket was in this position, splayed out.

NARRATOR: A sprawling Microraptor is the smoking gun Martin has been looking for all these years, to prove that dinosaurs were not the ancestors of birds.

All dinosaurs have hip joints that support the legs more or less straight under the body. It's one of their defining characteristics. Sprawling posture is the signature of more primitive reptiles, including the earliest common ancestors of crocodiles and dinosaurs.

Martin has long believed that birds arose directly from these pre-dinosaur reptiles, making them distant cousins, not direct descendants of dinosaurs. And he says Microraptor's primitive sprawl proves him right.

LARRY MARTIN: You have just shown me the death of the dinosaur-origin of birds.

NARRATOR: News of Martin's latest salvo in the bird-origin debate does not surprise Mark Norell.

MARK NORELL: People who are on the other side of it creatively reinterpret all new evidence that's put into it, to fit their own thing. You know, I knew that they would still be at it for years to come. And they are.

NARRATOR: And, as for the sprawling Microraptor skeleton?

MARK NORELL: I haven't seen it. I don't know. Beyond that, though, I would question whether, if that's true, that it actually is Microraptor, because I have seen definitive Microraptor specimens.

NARRATOR: Those are the ones the American Museum team has used for its reconstruction.

JULIA CLARKE (Paleontologist, American Museum of Natural History): The primary specimen is not particularly well preserved, but did you look at other ones?

One interesting thing with this process has been to see the artist take those fossils and render these often flattened elements into three dimensions. But you have to have a great relationship of trust and collaboration between scientists and artists to really have those three-dimensional representations, kind of, be trustworthy and accurate.

So we're articulating the whole hind limb from these elements, is that correct?

JASON BROUGHAM: Yeah.

NARRATOR: Norell doubts that Microraptor was anything other than a typical dinosaur with legs straight under the body. But for a second opinion, they send the bones to Harvard's Museum of Comparative Zoology.

Anatomists Farish Jenkins and Steve Gatesy are specialists in the function of limbs and joints in fossil animals.

In his first paper, Xu himself assumed that Microraptor would have to splay its legs out to the side to form the rear wings. But Jenkins says that doesn't fly.

FARISH JENKINS: It's just...it anatomically is not possible given this limb. You can see the articular relationships, there's no question about it. This is a normal hip socket, and here's a good, well-developed femoral head. And in a normal standing position, it would be about like that. You can abduct the femur away from the body–that would be normal–but if you reach about 45 degrees, you begin to hit bone on bone, the femur on the pelvis. So, beyond 45 degrees, this trochanter here acts as a lever, and out she comes.

STEVE GATESY (Anatomist, Brown University): It just has to come out.

FARISH JENKINS: It has to come out. This animal doesn't work like the cover showed it to work.

NARRATOR: Xu didn't have the benefit of the three-dimensional model when he wrote the paper, and he concedes that he was probably wrong about the posture. He never questioned the dinosaur-origin of birds, and upright posture makes sense for the ancestor, given that all birds are built the same way.

But back in Kansas, Martin and Burnham are standing by their conclusion that Microraptor was a flat out sprawler, and that this puts the ancestry of birds outside of dinosaurs.

LARRY MARTIN: Microraptor is absolutely deadly to the dinosaur-origin of birds. Wasn't even a biped; it was quadrupedal. It was on all fours. And it was arboreal, and that probably couldn't be a dinosaur.

NARRATOR: They think the evidence is clear.

DAVID BURNHAM: When I put this leg into the hip, you know, I can visually look at it and determine where it fits. But there's a certain tactile sensation, and I can feel when this bone plugs into the socket.

NARRATOR: So which model is more reliable? The cast made directly from a fossil? Or the sculpture based on 16 specimens?

DAVID BURNHAM: My criticism of the sculpting method is that somebody is carving that. You know, they're, they may miss a little process or a little bump, or a little shape that the computer doesn't think's important or the artist doesn't think's important. And it's probably not even a scientist themselves that is sculpting these things. It's usually done by artists.

JASON BROUGHAM: Granted, it's a sculpture. It's a hand-made, you know, piece of art. But it was held to pretty high standards, you know, and very exacting measurement. And it was crosschecked against multiple specimens. As far as it being a subjective process, well, if you think that measuring, you know, bones, down to the hundredth of a millimeter is subjective, you should have been where I was, you know, for all those months working on it.

NARRATOR: In a final showdown, Martin and Burnham examine the sculpted bones from the American Museum team.

LARRY MARTIN: The only difference from our pelvis is this area right here. You see what they've done. This is pushed in. I would argue that this is fudged for vertical posture as much as you could possibly do it, and you still don't have it. Now, let's see if they can get.... Guess what? They can't sprawl. If they try to sprawl it, it pulls all the way out. I'll be doggone!

NARRATOR: At Harvard, Farish Jenkins looks at Martin and Burnham's cast.

FARISH JENKINS: What we've got is a very well preserved femur in three-dimensional preservation. So this is preserved quite well. That's not the case here. This pelvis has been crushed. It's been squashed, actually flat, and made much more shallow by this crushing. And so, if I fit the femoral head in there it doesn't even fit all the way into the socket. There's not enough socket left. Now even if you tried to splay the leg on this animal, the femoral head is now out of the socket. So even in this deformed specimen, there is no evidence that the animal could put the hind limb in a horizontal plane. It simply doesn't work.

NARRATOR: Neither side trusts the other's evidence, so the standoff continues. And with crushed fossils as the starting point, people are more likely to believe what they already believe.

JACQUES GAUTHIER: When the animal is squashed flat, as these are, well, it leaves a little room for interpretation. And then, so I said before, science is objective, but people are scientists, and they aren't.

NARRATOR: Still, the question remains, if Microraptor couldn't splay its hind limbs, how did it use its leg feathers in flight?

A wing deflects the air flowing over the upper surface, creating lower pressure above than below, which generates the lift necessary for flight. Microraptor could spread its forelimbs out to the side to make an aerodynamic wing, just as birds do. But if the hind limbs couldn't do the same thing, what did they do?

JACQUES GAUTHIER: They certainly can't flap with their hind limbs. It has the same hook-up as you do. It goes up vertically and goes 90 degrees and goes into your, into your hip bone. Can you flap your hind leg in that direction? Try sticking your leg out in that direction and see what you do to it, you know. So yeah, there are these feathers hanging off of it and they're aerodynamic. What are they doing?

NARRATOR: Jenkins and Gatesy can't figure it out.

FARISH JENKINS: Well let's just try to put this together. And you've got a wing that's shooting down. How's that work?

STEVE GATESY: And we can lengthen it, shorten it. I don't even know whether we can turn it this way or this way.

FARISH JENKINS: This is very problematic. Coming off the back? You've got to be mad!

STEVE GATESY: That's what I see, right?

NARRATOR: Martin and Burnham think they have the answer.

LARRY MARTIN: Microraptor has a fully developed wing on its hind legs. And what that means is the hind legs have to be able to sprawl.

FARISH JENKINS: There is anatomically, simply put, no way that it could elevate the hind limb into a horizontal position.

NARRATOR: The question is, "Could it fly any other way?"

One way to find out is to create a model that can be flown in a wind tunnel to see how it performs. And for that they'll need more than a skeleton.

Jason Brougham builds up the body form one muscle at a time, guided by published science on dinosaur anatomy. To estimate the animal's mass and center of gravity, the sculpture is CAT-scanned and computer-modeled with internal organs.

Mick Ellison reconstructs the feathering of the wings and tail. He traces the feather impressions from eight different specimens, scales them to the same size, then makes a composite that combines all the information from all the fossils and reveals the shape of the wings.

Then model-builders John Allen and Hall Train take the knowledge accumulated by the science team–the arrangement of feathers on the wings, the range of motion in the limbs, the body shape, its mass and center of gravity–and build a jointed, feathered model that can be posed in the various postures it might have used in flight.

It all comes together at the Massachusetts Institute of Technology in the Wright Brothers wind tunnel. It's been the scene of some unusual experiments, but never anything quite like this.

Some of the world's leading figures in paleontology, biomechanics, aerodynamics and scientific reconstruction have come together for a rare collaboration. The aerodynamics crew from Brown University, led by Kenny Breuer, specializes in natural flight and gliding.

This is their first experience with a dinosaur. And it's the first experience anyone's had with four wings.

STEVE GATESY: I have colleagues that work only on living animals that say, "I can't even understand the living animal sitting in front of me. What makes you think you could make sense of this fossil?" And yet, you know, how could not want to try.

NARRATOR: Xu Xing has the most at stake. He's claimed that Microraptor was a link between dinosaurs and flying birds. But he needs to show how four wings could work without splaying the legs.

XU XING: Those feathers must be related to flight in some ways. But some people disagree.

NARRATOR: Norell, for one, is reserving judgment until he sees some hard evidence.

MARK NORELL: I'm fairly conservative in the way that I interpret behaviors of extinct animals. I wouldn't say that we know that Microraptor even was a glider, let alone a flyer. I mean, I don't think we know that.

NARRATOR: Birds fly with a complex wing beat that propels them through the air and also produces lift. But gliders, like flying squirrels, get the energy to generate lift from falling. The more lift they produce, the longer the glide path, and the farther they can travel before they have to land.

Assuming that Microraptor was a glider, they'll test the hind limbs in different postures to see what, if anything, works best. They decide to start with something simple: legs down and spread as wide as they go.

FARISH JENKINS: This is what we're here for, right?

MICK ELLISON: This is the one. This is the one. This is one my money's on.

FARISH JENKINS: You're money's on this one?

MICK ELLISON: Yeah, like a betting game, yeah.

FARISH JENKINS: And I bet you're a big bettor too.

KENNY BREUER: Is that ready?

ARNOLD SONG: Yeah, we're ready.

KENNY BREUER: Yep, okay. This was one kilogram, so the lifting force is about 10 newtons, right?

HALL TRAIN (Model Maker): The legs are really stable.

FARISH JENKINS: Yeah, they're very stable.

DICK PERDICHIZZI: That's 10 meters a second.

FARISH JENKINS: That looks good. That looks like a natural glider.

NARRATOR: In the wind tunnel, the model is mounted on a sensor that measures the forces produced by air flowing over the wings, including the lift that keeps the flier aloft and the drag that slows it down.

During each test, they'll gradually raise the angle of the wing into the airflow, which produces more lift but also more drag.

KENNY BREUER: So the blue dots on the left curve is the lift.

NARRATOR: The data shows up in the control room on a graphic display. The blue dots indicate the amount of lift being generated, and the red triangles are the drag. The higher the lift climbs, relative to drag, the farther Microraptor could glide in that position.

FARISH JENKINS: I like the way these data come in. They're fun. It's kind of like watching the horses.

KENNY BREUER: I was going to say it's like slow-motion horseracing.

FARISH JENKINS: Kiss three, baby, kiss three. Get up. Get up.

KENNY BREUER: Now, I think this is significantly plummeting to the ground.

FARISH JENKINS: Oooh!

JOE BAHLMAN (Biologist, Brown University): That's a lot of drag. That's going to give it a nice...it's still going to slow its fall.

FARISH JENKINS: Smarty pants!

NARRATOR: With too much drag and not enough lift, the best it could do in this position is not much better than parachuting. A variation with the legs tucked up under the body isn't any better.

It could produce more lift if it flew faster, forcing more air over the wing. But gliders can't generate their own power.

KENNY BREUER: Okay, so we're ready to try a new posture, right?

NARRATOR: The front wing doesn't provide enough lift on its own, so the hind limbs will have to contribute somehow.

One possibility is to form a second set of wings with the leg feathers straight back and the foot feathers spread horizontally, something like an old fashioned biplane.

JOE BAHLMAN: If you look at it like this, I mean, it's clearly a second lifting surface.

NARRATOR: It could be the only way to get more wing area without splaying the legs. If it doesn't work, Xu's dinosaur might not be much of a flier after all.

KENNY BREUER: Right, but the wing is pushed forward.

FARISH JENKINS: That next one, at 2.90, will be almost at 3.

NARRATOR: But the early returns are good.

FARISH JENKINS: Look at that, huh? Almost at 3 already. We're coming out of the gate fast.

NARRATOR: It looks like four wings are better than two.

JOE BAHLMAN: We may have a winner then.

FARISH JENKINS: That's it.

KENNY BREUER: So this is actually a fairly stable configuration. I mean this would be a pretty good way to travel.

NARRATOR: The biplane generates enough additional lift to produce a longer, slower, more gradual descent, but still not enough to travel very far. Moving the legs forward is worse, and causes the body to pitch, nose up.

If Microraptor were a full-time glider, it would need a way to extend its range through the air.

XU XING: I'm not sure, but this is just a kind of a possibility.

NARRATOR: Xu Xing has been holding back so far, but now he pitches an idea of his own. It's a bit unorthodox, and takes a while to sink in.

XU XING: It goes towards the tail.

NARRATOR: But eventually they come around.

FARISH JENKINS: Well, why didn't you say this earlier? This is a very bright idea.

NARRATOR: Xu's idea is to extend the legs almost straight back, allowing the leg and foot feathers to form a canopy over the tail.

FARISH JENKINS: That's your hypothesis?

XU XING: It's a possibility.

KENNY BREUER: It's possible. But that would not be a lifting surface.

FARISH JENKINS: Boy that's an interesting one. This is it. Let's see. This is it.

NARRATOR: Once again, all eyes are on the lift numbers.

FARISH JENKINS: Starting low, 1.12.

KENNY BREUER: That canopy's not necessarily a lifting surface. I mean you really have to have an airfoil to make a lifting surface.

FARISH JENKINS: Get up!

I like the drag staying low.

KENNY BREUER: Yeah. If it did leap out of a tree, that would be its initial posture, right? It would push off with its hind feet, and they'd be behind. So it would sort of dive and that would be a diving maneuver.

FARISH JENKINS: 3.42.

NARRATOR: Now the lift starts to climb.

FARISH JENKINS: 3.51.

XU XING: Wow.

FARISH JENKINS: Almost close to 4.

KENNY BREUER: That's amazing. So I take back everything I said about it not acting like an airfoil. It's acting like an airfoil in the back.

JOE BAHLMAN: This is the highest lift and the lowest drag.

FARISH JENKINS: I realized this. I kept telling you this.

KENNY BREUER: I know you did.

FARISH JENKINS: You win. Congratulations! You're all right! Nice going! Beautiful, beautiful glide, huh? It worked! It works in that position. That's the off-the-branch position. That's fantastic! You called it.

NARRATOR: Xu's posture makes for a long, fast glide. But when it's time to stop, it needs help. And that could be where the biplane comes in.

JOE BAHLMAN: So, if this animal jumps off of a tree, as it jumps, its legs are already behind it. It's able to dive; it's got a nice glide ratio going. And then when it gets ready to land, it can start bringing its legs gradually forward, through those biplane configurations. And then, as it brings its legs all the way forward, it's able to pitch up and land on a tree. So, the ultimate glide story is going to be this transition from the legs all the way back, to the legs all the way forward, which gets you very nicely from the top of the tree to the bottom of the next tree.

NARRATOR: The experiment says that Microraptor could glide very well without splaying its legs. But does that mean bird-flight evolved through gliding from the trees down?

Microraptor belongs to the clan of dinosaurs that includes Deinonychus, and it's close to the branching point that leads to birds, which means it might represent the body plan of the common ancestor from which both lines evolved. If so, then bird flight probably did arise from four-winged dinosaurs gliding from the treetops. And Deinonychus could be the dinosaur version of an ostrich, the flightless descendant of a flying ancestor.

But if another dinosaur is found to be even closer to the branching point, and it was not a four-winged glider, then flight could have evolved twice, one line leading to birds and another to Microraptor, making four wings a side branch on the tree of life.

KEN DIAL: This is not a ladder; this is a bush with twigs that branch out in different directions. And some of them are, in fact, terminal buds.

NARRATOR: Over millions of years of evolution, feathered dinosaurs could have experimented with flight in different ways, with some becoming flightless again and some going extinct.

JACQUES GAUTHIER: And they all might have been perfectly adequate. This Microraptor may have been a better flyer than an Archaeopteryx, but for whatever reason they went extinct, either by chance or because they weren't as good a flyer.

When you look at all these efforts to make the early flying machines, there are some pretty weird-looking contraptions in there. But that same sort of thing went on, you know, in the evolution of the biological flight in dinosaurs.

NARRATOR: If Microraptor proves to be a dead-end, then all bets are off on whether flight evolved from the ground, up or the trees, down.

JULIA CLARKE: We don't have a lot of other fossils that can speak to that question right now. Microraptor is a fine candidate. It's also one of our only candidates.

NARRATOR: Ken Dial may be right that young birds show how it all began, scrambling to safety with their proto-wings. Or more fossils like Microraptor could prove Martin and Burnham right about flight evolving from arboreal gliders, though they'll need more evidence than that to persuade other scientists that those gliders were not dinosaurs.

JACQUES GAUTHIER: This is a question about the origin of flight, not the origin of birds. Those are separate issues. We should have the dinosaurs getting in trees first or running off the ground first, but they're still dinosaurs that are doing this, feathered dinosaurs.

NARRATOR: We still don't know exactly how it happened, but Microraptor is changing the way people think about the origin of flight.

JULIA CLARKE: Microraptor has thrown our understanding into a new and productive chaos. It doesn't solve the problem. It doesn't give us an answer. But it gives us another way of thinking about the data. And, I think, eventually, we are going to get to some answers.

NARRATOR: Microraptor is a creature no one expected, but we know of only a tiny fraction of the dinosaurs that once lived. More than 400,000 species may have come and gone in their 175-million-year history. To date, we've found only 1,100, not counting the ones that are still with us today, long after the extinction of their ancestors.

JACQUES GAUTHIER: After the asteroid hit, 65 million years ago, the only dinosaurs that came out of it are today's birds. They seem to have survived it well enough. There are 10,000 living species, only about 4,000 mammals, so still the age of dinosaurs.

NARRATOR: On NOVA's Four-Winged Dinosaur Web site, compare the skeletons of ancient feathered animals and learn more about the evolution of flight. Find it at PBS.org.

NEIL DEGRASSE TYSON: I'm Neil DeGrasse Tyson, host of NOVA scienceNOW. What if you dug a hole through the center of the Earth, out the other side and then jumped in? What if you did that? You'll find out in a moment.

Major funding for NOVA is provided by David H. Koch. And...

Discover new knowledge: HHMI.

And by the Corporation for Public Broadcasting, and by contributions to your PBS station from viewers like you. Thank you.

NEIL DEGRASSE TYSON: Hi, it's Neil again. You ever wonder what would happen if you dug a hole from one side of the Earth, through the center, out the other side and then jumped in?

Before we show you, a few disclaimers: if there was any air in the hole, air resistance would slow me down, so let's ignore that. Earth's molten core is 11,000 degrees Fahrenheit. On the way past, you'd simply be vaporized. So let's ignore that, too. We would also have to ignore Earth's spin, which would make me ricochet from side to side down the hole. And please don't try this experiment on the actual Earth.

All right, here we go!

I fall, gaining speed as Earth's mass pulls me towards the center. Fourteen minutes into my fall, halfway to the center, I've accelerated to more than 15,000 miles an hour. Here, there's only half the force of gravity than on the surface, so I'm still gaining speed, but at a slower rate than when I first jumped in. Twenty-one minutes into my fall, and I'm at the center of the Earth, going my fastest, about 18,000 miles an hour.

As I pass the center, gravity now works against me, slowing me down. And by the time I make it halfway between the core and the other side of the Earth, I'm back down to about 15,000 miles an hour. It'll take only 42 minutes to make the entire trip to the other side. At which point, I'll slow to a full stop.

Just like when I started, all of Earth's mass will pull me back towards the core. Unless somebody catches me, I'll fall down the hole again and yo-yo back and forth forever.

Thank you!

MAN: No problem.

NEIL DEGRASSE TYSON: I'm Neil DeGrasse Tyson. NOVA scienceNOW returns soon.

NOVA is a product of WGBH Boston.