- Hey, smart people, Joe here, and this is a chicken.

This humble bird and some of her friends are gonna help us solve one of evolution's greatest mysteries: how fluffy feathered dinosaurs became today's masters of the sky.

At least, that's what we hope is gonna happen.

Okay?

(playful bright music) (chickens squawking) Today, we are gonna use chickens to remake a genius and rather hilarious experiment that helped unlock the answer to how the first feathered dinosaurs started to fly.

Why chickens?

Because today's birds are the only surviving lineage of the dinosaurs, the incredible creatures that dominated our planet for tens of millions of years.

More than a century of fossil discoveries have shown us that, like today's birds, many extinct dinosaurs had feathers.

But when and how did feather-covered arms become fully functional wings capable of flight?

This is a puzzle that's captivated and confused scientists for more than a century.

In fact, that question was one of the first great challenges Charles Darwin himself faced after publishing his theory of natural selection.

The evolutionary transition from feathers to flight is one of the most important and amazing mysteries in evolution.

And these chickens, well, hopefully they're gonna help us solve it today.

Aren't you?

(light upbeat music) Bird flight is, without a doubt, one of the most incredible feats of any animal on Earth.

Flight has created blazingly fast and agile hunters.

Others can fly standing perfectly still.

Some can soar as high as jet airliners over the Himalayas or glide over the sea for months without landing.

And the secret to flight are these feather-covered arms we call wings.

Inside a bird's wing are the same bones you have in your arm with fused fingers near the tip.

Extending from the back of that hand and arm are the flight feathers, the incredibly light but stiff nature of these lets them not only provide thrust through the air.

They also turn the shape of the arm into an air foil, capable of creating lift.

The arms and flight feathers we find in different birds today are specialized and adapted to the different ways that birds fly, from soaring to hovering to aerobatics.

But all of the incredible and varied forms that flight takes today can trace their origin to some ancient bird ancestor; the moment a feather dinosaur first used its wings to fly.

Until 1861, the only animals with feathers we knew of were birds.

For that year, a single isolated fossil feather was found in Germany.

Its discoverer, Hermann Von Meyer, noted it was, "indistinguishable from a bird's feather."

Later that year, a near complete skeleton of that feathered animal was found and given the name Archaeopteryx.

The rocks in which Archaeopteryx was found date to the Jurassic, around 150 million years ago.

Yet this mysterious creature possessed features of both dinosaurs and modern birds.

Just two years before Archaeopteryx's discovery, a British naturalist named Charles Darwin had published a revolutionary new theory about how species change over time.

Darwin's theory states that evolution's creativity plays out in tiny doses of change with natural selection as the great filter deciding which changes let an organism survive and reproduce better in its environment.

One of the predictions of Darwin's theory is that between some ancestral trait and its modern form, we'd expect to find an intermediate form, what some people call a transitional fossil.

Archaeopteryx appeared to be exactly the transitional fossil that Darwin's theory predicted.

And as early as 1868, Thomas Huxley, one of Darwin's most vocal supporters, was the first to claim dinosaurs like Archaeopteryx were ancestors of modern birds, but not everyone was convinced.

In 1871, one English biologist reportedly challenged Darwin with a simple question: What good is half a wing?

In other words, it's easy to understand why a normal arm with claws for grasping would be good for ancestral dinosaurs.

And how later, fully feathered bird-like wings would be an evolutionary advantage for flying dinosaurs, AKA birds.

But complex structures like wings don't just pop into existence fully formed.

In between, we'd have some not quite bird dino with a feathered limb that's not exactly an arm, but less than a wing.

What possible evolutionary advantage could that provide?

You can't fly with a fraction of a wing, so how can natural selection explain those in-between forms?

Well, Darwin himself came up with the solution.

What if that half a wing is an advantage for something other than flying?

This is precisely the question I'm hoping to answer with the chicken experiment I'm going to do later.

(chicken gurgling) We're doing some science here.

Even though the first fossil evidence of Jurassic Era feathers was found more than 150 years ago, the link between birds and dinosaurs was largely ignored for a century.

That's because early on, most known dinosaurs were gigantic sora pods and carnivores.

These supposedly slow, cold-blooded, clumsy branches of the dinosaur family tree couldn't have led to birds.

Other than Archaeopteryx, small, bird-like dinosaur fossils were still hidden in the rocks.

And in the 1920s, influential scientists declared that since no one had found fossil wishbones, one of the key skeletal characteristics of birds, dinosaurs couldn't be their ancestors.

But decades later, researchers realized that many fossils they already had were in fact dinosaur wishbones mislabeled as other bones.

And beginning in the 1960s, the link between birds and dinosaurs was revived.

Paleontologist John Ostrom discovered Deinonychus.

- [Announcer] The evil robot shackled the Deinonychus.

- Gray fox on attack!

- [Joe] A fast moving, warm blooded dinosaur with bird-like feet that Steven Spielberg renamed Velociraptor in Jurassic Park, even though there's already a different dinosaur named Velociraptor.

- Clever girl.

- Anyway, Ostrom later described many more bird-like traits in other fossil species.

His work proved, once and for all, that modern birds are a living subgroup of theropod dinosaurs that survived the giant space rock mass extinction, and diversified into the forms we see today, even chickens.

(chickens squawking) But we still have to solve Darwin's dilemma.

What mechanism could explain how a perfectly good arm evolved into a feathered flying wing?

What was half a wing actually useful for?

New fossil discoveries began to fill in the blanks.

In the 1990s, Sinosauropteryx was unearthed in China.

It was the first non-avian dinosaur found to have feathers.

The color pattern of those feathers was even preserved, showing stripes on its tail.

But Sinosauropteryx' feathers were primitive, more like short filaments or bristles.

Since feathers had now been found in two different branches of the dinosaur family tree, it's a good hint that they evolved in some ancestor of both branches.

In other words, feathers showed up long before flight, and other feathered fossils were found with more complex feathers, like those of modern birds, arranged on wing-like arms.

But mathematical analyses showed that the shape and size of these feathered arms couldn't create lift.

But since these species were also in branches outside of birds, it was more evidence that a bird-like wing structure first arose in dinosaurs that weren't flying.

So what was the purpose of these non-flying half wings?

Feathers located far away from the body, the tips of early wings or tails could have been used for communication or attracting mates.

You know, think of birds with mohawks or those guys at the airport, waving planes in.

There are plenty of examples today of bird feather structures and patterns that evolved thanks to sexual selection and communication.

Also, right around when feathered wings first appear in the fossil record, there's evidence many dinosaurs began to sit on their eggs rather than bury them.

Feathered arms would be helpful in covering or keeping eggs warm.

It's also likely that theropod dinosaurs, the group of dinosaurs that contains birds and their ancestors as well as others, they were warm blooded, and small, warm blooded animals have to have insulation to hold in their heat or else it's too energetically costly to be warm blooded.

Feathers could help with that too.

These were all examples of how half a wing could be useful in the evolutionary journey toward birds, but none of these put wings on a pathway toward flight.

What if these feathered proto wing structures first evolved for communication and thermal regulation, but then just happened to also have some small benefit in helping bird-like dinosaurs move?

Well, it wasn't until the 21st century that researchers began to finish this story.

Now, one campus scientists noticed that the theropod dinosaurs, birds are closely related to, they look like they run on the ground.

So maybe flight began as a series of running jumps with longer and longer hang time as wings and feathers grew more complex.

It's called the Ground-Up Theory of Flight.

Another group of scientists said, "No, you'd never want to evolve flying going against gravity.

It's way too hard!"

So the half wing ancestors of birds must have been living in trees already.

They were climbers and they started gliding down: a Trees-Down Theory of Flight.

Neither camp could assemble definitive evidence to prove their case, but then came a third model, and that is what our chicken experiment set out to demonstrate.

In the early 2000s, a biologist named Ken Dial noticed that young birds, before their full feathers come in, they basically have half a wing.

They could be a living stand-in for those extinct transitional dinosaurs.

So what do young birds do with their half wings?

I found a farm with some young chickens so we could put them to the test.

The birds are going to make their way up a ramp placed at various inclines, so that we can observe if they're non-flying wings are able to help them move.

Not exactly a calculus exam, but hey, they're chickens.

- The enormity of of their flat brain, the enormity of their stupidity is just overwhelming.

- We're gonna see what this bird does, ready?

Yeah!

For an easy incline, the birds just walk, because why fly if you don't have to?

That was easy mode.

I think it's time to level up.

Chicken didn't have to use its wings that time, but we made this a little bit steeper and we're gonna see what happens.

Okay, go to your friend.

Do some science.

(dramatic suspenseful music) As the incline gets steeper, the bird is still mostly walking.

Then it's starts to engage its wings to help it get up.

But is it creating lift or are those wings doing something else?

Let's make it even steeper and then see what the chicken does.

This is your gold medal moment.

Ready?

(dramatic suspenseful music) As the incline gets nearly vertical, the chicken has to flap hard to make the climb, but what's really cool is it's flapping its wings in a different way than birds do when they're flying.

Don't tell the chickens, but they're not really great flyers.

That's why there's such a great model for that transition from feathered wings to flight.

Now, you might think that a bird would try to push itself up with its wings, but what shots like this and experiments like this have shown scientists is that instead of flapping themselves up, they're pushing themselves toward what they're trying to climb.

When a bird is in the air, it flaps its wings almost perpendicular to align between the beak and tail in order to create lift.

But up the incline, the young chicken's wing is instead flapping down and behind it, pushing its body toward the surface to increase climbing traction, not simply pushing its body up.

What I've done here on the farm is basically a very silly and non-scientific replication of some very rigorous and very scientific research by Ken Dial.

He found that young birds with immature wings will climb steep or even vertical surfaces by using the force of their wings to increase traction.

He called it Wing-Assisted Incline Running or W.A.I.R.

If you wanna avoid predators on the ground, wing-assisted incline running is a great way to get yourself up to safety.

And we still see many birds climb this way today.

We know from fossil skeletons that transitional bird-like dinosaurs wouldn't have been able to flap their wings with the full range of motion that modern birds can, but they would have been able to move their wings the way young birds do when using W.A.I.R.

Today, W.A.I.R.

is one of the most accepted theories for how those transitional half wings, those intermediate forms between fluffy dinosaur arms and fully developed flying wings on modern birds, could have provided their own evolutionary advantage along that journey from feathers to flight.

Darwin's dilemma solved with a chicken.

(chicken squawks) Later on, it was shown that baby birds will also flap to slow down a fall.

So when it comes to the trees down versus ground up versus W.A.I.R.

camps, it's not all or nothing.

More than one answer can be correct.

This is still new science in just the last 10 to 15 years, which is basically brand new as paleontology is concerned, and a more definitive answer may lie in future fossil discoveries that continue to fill in the blanks.

This whole process is known as exaptation, when evolution takes an existing structure and hijacks it for some new purpose.

And when you think about it, it's a pretty logical way for change to occur in nature because every organism has to be fully functional as it is in the moment it's alive on its own evolutionary journey.

It has to be able to feed and move and get away from predators and reproduce, all of those things.

So any organism that survives is already pretty darn advanced and specialized for its environment.

Really, almost the only way for new structures and abilities to arise is for them to evolve from something that already exists.

Flight has evolved in different organisms on earth many different times, but the way that birds took to the sky is one of the most fascinating stories in the history of life.

Thankfully, we still got a few dinosaurs left over that helped us solve that mystery.

High five.

High wing?

Stay curious.

Good chicken.

Good job.

Up here, up here.

Ready?

Go.

You're startin' to get a little feisty.

Go!

(chicken thuds) You're done.