[SOUND EFFECT] [MUSIC PLAYING] JULIA CLARKE: The animal
kingdom 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 arrive? To find out, scientists have
scoured the fossil record. And they have uncovered
surprising twists in the evolution of birds from
their flightless ancestors. 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. [BIRD CRIES] I'm fascinated by birds. And as a paleontologist,
I've spent my career chasing their evolutionary
origins in the fossil record. 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. 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. 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. If we look closely, we'll
see teeth in the jaw, tiny claws preserved
in the 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. Archaeopteryx pointed
to a close link between birds and reptiles. But which group of reptiles? Flying pterosaurs
have been discovered with light, hollow bones. But their wings are
constructed very differently than the wings of
Archaeopteryx and birds. Here is a tiny pterosaur. And if we take a
closer look at its arm, we'll make out three small
digits, and a fourth, which is really, really long. The membrane of a pterosaur 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's 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
in dinosaurs, but were present
in other reptiles. JACK HORNER: We found
an articulated foot. JULIA CLARKE: When
paleontologist Jack Horner began his career, few
thought that birds could have come from dinosaurs. So Jack, why was it so
hard to believe that birds and dinosaurs were related? JACK HORNER: Most
of the dinosaurs that the public knew
about were really big. Like, you know, this was a
shoulder blade of a sauropod. And sauropods were gigantic. JULIA CLARKE: Scientists
thought that dinosaurs were cold blooded and slow
moving, like other reptiles. JACK HORNER: People
couldn't imagine dinosaurs being agile, hopping around. They look at these big,
giant things and they lumber. There's no way to
relate them to birds. JULIA CLARKE: Then,
in 1963, John Ostrom discovered a fossil in
the badlands of Montana that challenged that view. JACK HORNER: What John Ostrom
first found was this claw. Obviously, it goes to a foot. It was not a claw
for walking on. This dinosaur actually used
that claw for slashing. JULIA CLARKE: 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. JACK 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. JULIA CLARKE: Ostrom's
discovery set off a revolution. What if dinosaurs weren't
slow, but warm-blooded and fast-moving, like birds? When Ostrom 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 theropods 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 wish bone. JACK 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. JULIA CLARKE: 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. JACK HORNER: Here's
an Albertosaur tibia. And as you can see, it's
hollow, just like a modern bird. JULIA CLARKE: 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. New kinds of evidence
also emerged. In 1978, Jack made the
surprising discovery of a vast dinosaur
nesting ground. JACK HORNER: We discovered that
dinosaurs nested in colonies and 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. JULIA CLARKE: 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. In 1996, I was a first
year graduate student at my first scientific meeting. They were passing around
pictures of this dinosaur. This chicken-sized theropod,
named Sinosauropteryx, did not have scales. It was covered in some
primitive kind of feather. To see those photos of
a tiny, fuzzy dinosaur-- it just blew everybody's minds. 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? 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. 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 a turtle shell is a modified rib cage. So the co-option of
feathers for flight enabled Archaeopteryx and its
relatives to take to the air. And other features also evolved. When we look at evolution
after the origin of flight, we see a lot of characteristics
of living birds gradually accruing. 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. We don't find forms
that are somehow lockstep intermediate between
Archaeopteryx and living birds. We find a diversity
of forms, forms we could not have predicted. 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. A six-mile-wide asteroid
slammed into the planet. [EXPLOSION] And triggered a global
mass extinction. Only a small group of
toothless birds survived. And they evolved into the 10,000
species of birds we see today. We once might have said that
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. JACK 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. JULIA CLARKE: Dinosaurs
are still with us. We just call them birds. [MUSIC PLAYING]
Great stuff!