is made up of major groups, recognized by key traits. Fish have fins. Some land animals
have four legs, others six, and several different groups have wings. Biologists have long sought
to discover how groups of animals, and their key features, evolved.
And one of the greatest mysteries
has been the origin of birds. Our world has more than 10,000 species
of birds with feathered wings. Where did birds come from, and how did wings
and feathers first arise? To find out, scientists
have scoured the fossil record... And they have uncovered
surprising twists in the evolution of birds
from their flightless ancestors.
[CLARKE (to camera):] In the past 30
years we've found a treasure trove of new fossil discoveries. They've made the origin of birds one of the best-documented transitions
in the history of life. [Music plays, birds call] [walking through grass] [CLARKE (narration):] I am fascinated
by birds. And as a paleontologist,
I've spent my career chasing their evolutionary origins
in the fossil record.
[CLARKE (to camera):] Above all else,
what makes birds unique are their wings. They're made of feathers
that are stiff, yet flexible. And bird wings are even more remarkable
than airplane wings, because they can flap, which allows them to maneuver rapidly
and ultimately defy gravity. [CLARKE (narration):] The quest
to understand the origin of birds and other animals began in earnest
over 150 years ago.
When Charles Darwin
wrote "The Origin of Species," he argued that every major group of animals
evolved from a pre-existing one. He predicted that we would find fossils with features
that linked one major group to another. In fact,
he staked his theory of evolution on the existence of these intermediates. But no fossils were yet known
that revealed these transitions.
Then, just two years later, a marvelous creature was unearthed
from a limestone quarry in Germany. The 150-million-year-old fossil,
named Archaeopteryx, rocked the scientific world. [CLARKE (to camera):] This Archaeopteryx
fossil is truly remarkable. It preserves in fine detail feathers
along the wing-- just like those we see in living birds-- and feathers along the tail.
But the bony features
tell a very different story. We look closely,
we'll see see teeth in the jaw, tiny claws preserved in a hand, and a long bony tail,
lacking in living birds, but present in things
we think of as traditionally reptilian. For Darwin, it must have been
an incredible vindication. He predicted that we would find forms
like these.
[CLARKE (narration):] Archaeopteryx
pointed to a close link between birds and reptiles. But which group of reptiles? Flying pterosaurs had been discovered
with light hollow bones. But their wings are constructed
very differently than the wings of Archaeopteryx and birds. [CLARKE (to camera):]
Here is a tiny pterosaur, and if we take a closer look at its arm,
we'll make out 3 small digits, and a fourth,
which is really, really long.
[CLARKE (narration):] The membrane
of a pterosaur's wing attaches to this fourth digit
and along its body and hind limb. In contrast, the wings of Archaeopteryx
and birds have only three digits. And their feathers attach individually
along their arm and hand bones. These differences tell us
that pterosaurs and Archaeopteryx
evolved flight independently.
Archaeopteryx must have descended
from different reptiles. Thomas Huxley, Darwin's champion, was astonished
by Archaeopteryx's resemblance to a turkey-sized dinosaur
called Compsognathus. Compsognathus' hand
also had three digits. It had hollow bones
and stood on two legs.
Similarities like these led Huxley
to propose that birds are related to the branch
of reptiles called dinosaurs. But other scientists
questioned this conclusion. Birds appeared so different
from dinosaurs, and some characteristic features
of birds--like wishbones-- seemed to be missing from dinosaurs,
but were present in other reptiles. [HORNER:]
We found an articulated foot...
[CLARKE (narration):]
When paleontologist Jack Horner began his career, few thought that birds
could have come from dinosaurs. [CLARKE:] So Jack, why was it so hard to believe that birds
and dinosaurs were related? [HORNER:] Most of the dinosaurs that the public
knew about were really big. Like, you know,
this is a shoulder blade of a Sauropod. And Sauropods were gigantic.
[CLARKE (narration):] Scientists thought
that dinosaurs were cold-blooded and slow-moving, like other reptiles. [HORNER:] People
couldn't imagine dinosaurs being agile and hopping around. They look at these big giant things
and they lumber. There's no way to relate them to birds.
[Music plays] [CLARKE (narration):] Then,
in 1963, John Ostrom discovered a fossil in the badlands
of Montana that challenged that view. [HORNER:] What John Ostrom
first found was was this claw. Obviously goes to a foot. It was not a claw for walking on.
This dinosaur actually used that claw
for slashing. [CLARKE (narration):] Deinonychus
was small with a delicate build. It ran upright on two legs. It had a long, stiff tail for balance.
Not all dinosaurs
were big and lumbering. [HORNER:] Ostrom hypothesized
that the animal would scale its prey and start using its slashing claw and probably eating the animal
while it was alive. [CLARKE (to camera):] Ostrom's discovery
set off a revolution. What if dinosaurs weren't slow, but warm-blooded and fast-moving,
like birds? [CLARKE (narration):] When Ostrom
compared Deinonychus to Archaeopteryx, he saw that they both had
lightly-built, hollow bones.
And they shared even more features, including long arms
and similar hip and shoulder bones. Ostrom concluded that birds
did descend from dinosaurs as Huxley had argued. Not from lumbering sauropods, but from another lineage called
therapods that walked on two legs, and included T. Rex, and agile predators like Deinonychus.
While some scientists
did not accept this idea at first, supporting evidence
continued to accumulate, including the discovery
that theropods had a feature of birds not previously found: a wishbone. [HORNER:] People
had sort of looked for them, and really didn't know
what it was going to look like. And then all of a sudden
we started finding them. Here is the wishbone
of Tyrannosaurus rex.
[CLARKE (narration):] When scientists
analyzed the skeletons of theropods and birds, they found too many similarities
for any explanation but common ancestry. Jack's collection
at the Museum of the Rockies offers an opportunity
to compare their features. [HORNER:] Here is an Albertosaur tibia, and as you can see, it's hollow,
just like a modern bird. [CLARKE (to camera):]
This is a T.
Rex foot. What we see here
are three forward-facing digits that bear the weight of the animal, and in the back, a much smaller digit. If we take a look at this chicken foot,
we'll see the same pattern. We've got three forward-facing digits
and on the back, a much smaller one.
All dinosaurs share an S-shaped neck. You can see it here and in living birds
like this chicken. [CLARKE (narration):] New kinds
of evidence also emerged. In 1978, Jack made the surprising discovery
of a vast dinosaur nesting ground.
[HORNER:] We discovered
that dinosaurs nested in colonies, cared for their young,
brought food to their babies. We also had evidence that they
came back, probably over and over again, for many years to the same site. [CLARKE (narration):] In a radical
shift, by the 1980s, a consensus was finally building that birds
descended from theropod dinosaurs-- from active predators
that walked on two legs. But scientists were about to discover
the most startling evidence of all.
In the mid-1990s,
farmers in northeast China began unearthing dinosaurs
120 million years old. And these fossils
preserved astonishing detail. [CLARKE (to camera):] In 1996,
I was a first year graduate student at my first scientific meeting. They were passing around pictures
of this dinosaur.
[CLARKE (narration):] This chicken-sized
theropod, named Sinosauropteryx, did not have scales. It was covered
in some primitive kind of feather. [CLARKE (to camera):] To see
those photos of a tiny, fuzzy dinosaur... It just blew everybody's minds.
[CLARKE (narration):] This dinosaur
was just the first of many fuzzy and feathered theropods to be uncovered. Another, called Caudipteryx, had feathers identical to living birds
on its tail and hands, but lacked wings. With the discovery
of these spectacular feathered finds, there was no longer any doubt
that birds were related to theropods. But while feathered dinosaurs
settled one question, they raised a new one: These animals could not fly.
Why were they feathered? [CLARKE (to camera):] It was long
assumed that feathers evolved for flight. But what we found
was that clearly feathers predate flight and arose for some other purpose. [CLARKE (narration):] So why did
the first feathers evolve? That's hard to tell
from just the fossil evidence. But living birds may offer answers.
Feathers provide insulation. So the first feathers
might have helped keep dinosaurs warm. Birds also use colorful feathers
in communication, in courtship and in territorial displays. Dinosaurs may have used feathers
in the same way.
Feathers likely played different roles
at first, and then were modified for flight. The modification
of an existing structure for a new use is called co-option. It is a common way that new structures
and abilities evolve. Bird wings are modified forelimbs
once used for grabbing and feeding.
Just as the walking limbs
of land animals are modified fins. And the turtle's shell
is a modified ribcage. So the co-option of feathers for flight enabled Archaeopteryx
and its relatives to take to the air. And other features also evolved.
[CLARKE (to camera):] When we look
at evolution after the origin of flight, we see a lot of characteristics
of living birds gradually accruing. [CLARKE (narration):]
But not in a simple linear sequence. Like other dinosaurs, this crow-sized
bird had large claws on its hand, but like living birds, it had a toothless beak
and a short bony tail. While this species had teeth, its hand bones were partially fused
to form a stronger wing.
And this bird had a large breastbone
for well-developed flight muscles, like living birds. But it also had teeth. [CLARKE (to camera):]
We don't find forms that are somehow lock-step intermediate
between Archaeopteryx and living birds... We find a diversity of forms, forms
we could not have predicted.
[CLARKE (narration):] For tens
of millions of years, an assortment
of scaly dinosaurs, feathered dinosaurs, and many types of birds lived together. Then, 66 million years ago, almost all of these creatures died out. [Rumble] A six-mile wide asteroid
slammed into the planet... [Explosion] ...And triggered a global mass
extinction.
[Music plays] Only a small group
of toothless birds survived... And they evolved into the 10,000 species
of birds we see today. [Bird calls, music] We once might have said
the dinosaurs all died out, but now we know that living birds are a lineage of theropod dinosaurs
in the same way that we are a lineage of primates. [HORNER:] Have dinosaurs gone extinct? Absolutely not.
We separate dinosaurs
into two groups now: the non-avian dinosaurs
fortunately have gone extinct, and the avian dinosaurs are still alive,
making it a beautiful world. [Music plays] [CLARKE (to camera):] Dinosaurs
are still with us. We just call them birds. [Music plays] [bird calls] [music plays].
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