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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.
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