KEN DIAL: The evolution of flight in birds
has long been a great scientific puzzle.
Fossil evidence from the past 40 years
has established that birds descended from theropods--
a lineage of two legged dinosaurs
that included species with feathers on their bodies
and arms.
But those early animals could not fly.
So how did birds take flight?
A new hypothesis came from my studies of young birds
that are learning to fly.
And it all started with a bale of hay.
I've been studying the mechanics of bird flight for over three
decades at the University of Montana's Flight Lab.
I became interested in an intriguing question
that critics of evolutionary theory had posed to Darwin.
So there's a gentleman by the name of Sir George Jackson
Mivart.
He challenged Charles Darwin when Charles
wrote "The Origin of Species."
Darwin had proposed that structures
evolve through intermediates.
So a wing would have evolved from four-limb in stages.
But some of those early stages were clearly not capable
of flight.
Mivart confronted Darwin.
He said to Darwin, how do you explain
in the evolution of birds from reptiles,
the function of half a wing?
So to answer that question I look to a living example.
Bird chicks with small, immature wings.
I wanted to make careful observations
of how chicks use their wings.
As they learn to fly.
I was keeping the birds on these shiny floor
clean lab conditions and trying to have
them go up a wall that was slick to fly up to their siblings.
As soon as a rancher came in once and said,
what are these birds doing on the ground?
They hate being on the ground.
Giving them a bale of hay, give him something to get up on.
As soon as I got some hay bales for the chicks,
we made an interesting observation.
I came back one day, and my son who had been helping me,
I asked him how are they doing?
How was the data today?
And he says, it was horrible.
I say, why?
He says, they were cheating.
And that moment-- a watershed moment in my life.
What do you mean they were cheating?
They ran straight up vertically.
I said, that's impossible.
To better understand this behavior,
my son Terry and I decided to carefully measure
how young birds use their legs and wings together to travel up
ramps of different angles.
But here's our little experimental animal here and--
My friend Julia Clark, an expert on the evolutionary origin
of birds, joined us on our experiments.
The first angle is not steep.
It's wild.
JULIA CLARK: Right.
KEN DIAL: This is not a--
JULIA CLARK: Not a trained animal.
KEN DIAL: This is not a trained animal.
JULIA CLARK: That was easy.
KEN DIAL: Just walked.
No problem.
JULIA CLARK: Strolled.
KEN DIAL: Nonchalant.
No wing necessary.
Next, we try a sharper angle.
JULIA CLARK: OK.
So this is much deeper than before.
KEN DIAL: It is.
JULIA CLARK: Let's see what happens.
KEN DIAL: There we go.
JULIA CLARK: You're not sure what you see.
KEN DIAL: Exactly.
JULIA CLARK: Because it's so fast.
KEN DIAL: And until you see it in slow motion,
you don't know how beautiful it actually is.
This time the bird used its wings as it ran.
Every time, it felt like it was falling backwards it used
its wings, not to pull it up.
And this is what we discovered.
It wasn't to lift it up and take it like a bird flying straight
up to this refuge.
It was using its wings to pull it forward on to this log.
And now a really steep ascent.
That is going to be a challenge, right.
JULIA CLARK: So this is now nearly vertical.
KEN DIAL: This is vertical.
Standing tall, vertical like any tree.
Let's see what the animal can do to negotiate this.
It can fly, but--
JULIA CLARK: But it's still using its legs.
Still using his legs.
KEN DIAL: It climbed with its legs,
using its wings to pull it towards the tree.
But not to fly it up this.
Pretty cool.
This has turned out to be not just representative of the bird
we looked at, but every flight capable bird
that we've looked at in the 15 years since.
Dozens and dozens of birds.
I have also observed the same behavior in the wild,
and I've even seen young birds use their wings
to assist their hind limbs to paddle across a body of water.
JULIA CLARK: So, Ken, how did this change the way
we think about how dinosaurs get in the air.
KEN DIAL: Well, we now know that there
are a lot of dinosaurs-- little feathered theropods
that have little wings.
And the explanation for their existences
is really difficult to resolve.
And I think that a reasonable explanation
is to look at what the young birds with similar wings
can do today.
Birds show us the possibility of what these dinosaurs could
have done.
Scientists have long debated two main possibilities
for how flight evolved.
Dinosaurs could have used their clawed hands
to climb up trees and then glide down.
With this gliding behavior eventually evolving
into flight.
Or dinosaurs could have run faster and faster
on the ground, flapping their wings
and some species then evolved the ability to fly.
But our research suggests a third possibility.
So every bird that we've looked at-- dozens of different
species--
do this behavior.
They exhibit this behavior of flap and running.
No gliding.
They don't jump off of a bale of hay and glide down
and they don't fly up.
They flap run up and they flap down.
Juvenile theropods might have used their four limbs
similarly.
In the adults the legs alone were probably
sufficient to escape predators or to hunt down prey.
But in growing therapods, small wings provided an advantage.
Just imagine the selective pressure
at the time of therapy dinosaurs.
Everything's trying to eat everybody-- chasing everybody.
If you could have moved up to an elevated refuge
with the use of these little wings
you would have lived to see tomorrow.
Those young theropods would have run up to escape predators,
and then flapped back down when it was safe.
Over time, small wings evolved into larger ones,
until these feathered dinosaurs were able to take to the air.