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We human beings are very latecomers to the skies,

and although we might think that we're now pretty good at it,

the natural world, with the help of several million years of evolution,

has produced a dazzling range of aeronauts whose talents are far beyond ours.

The story of how animals managed to colonise the air is truly astonishing.

First into the skies were insects.

They initially had two pairs of wings

which in due course, were modified in many different ways.

But after having had the skies to themselves for about 100 million years,

a new group of animals took to the air:

Vertebrates, creatures with backbones.

They faced a different challenge, for their bodies were much bigger and heavier.

But eventually they evolved several ways of solving that problem.

We will travel the globe to trace the details

of the extraordinary skills, of the backbone flyers.

This is Borneo.

And here there are still great tracts of pristine rainforest,

forest that is wonderfully rich in animals of all kinds.

I am being winched-up into one of the tallest trees here,

in search of a creature that can give us a hint,

of how backboned animals first took to the air.

Hidden among these leaves, of this fern, high up here, in the canopy,

is a very remarkable, little frog.

It's a Harlequin Tree Frog, and it's a very, very good climber.

It spends most of its life up here, clambering around in the branches.

Here it's away from the numerous predators there are

that might attack it down on the forest floor.

But if in fact, a predator were able to get up here, to hunt it,

a snake perhaps, well the Tree Frog has a remarkable trick for defence:

It glides.

It has membranes between greatly elongated toes,

so that each foot becomes a parachute which slows the frog's descent,

and so enables it to make a relatively safe landing.

The vertebrates made their first foreys into the air around 260 million years ago,

and it's very likely that some of these pioneers used skinny membranes

to control their falls, in much the same way as this little frog does.

It has to be said, that it's not a very good aerial navigator,

it seems as though it just jumps and hopes for the best.

But there are animals up here, that glide around from tree to tree,

which are very good navigators indeed,

so good in fact, that they can go from one tree to another,

and never go down to the ground in their entire lives.

One of them is a little lizard called Draco.

Each male has his own little territory in the branches,

and tries to attract females and warn off rivals, by flashing his dewlap.

He also spread coloured flaps of skin from his flanks,

that when fully extended, do more or less the same thing.

But there are predators among the branches.

Snakes also live up here, and they hunt lizards.

But Draco's side flaps now serve another purpose.

He uses them to glide, by hidging forward his specially elongated ribs.

And he is so skilled in the air, that he can steer and land

on the trunk of his choice.

So, if you live up in the branches, it's less laborious,

and indeed safer, to travel by air, than to come down to the ground.

But if you want to be a true flyer, you have to be able to fly

not only downwards but upwards, you have to have powered flight.

This is another reptile,

and one with even greater flying abilities than that little gliding lizard.

Today, sadly, it's extinct.

This is Dimorphodon. We can deduce from its fossils

that it had the muscles needed to beat its wings,

and computer imagery can show us what it must have looked like.

Dimorphodon was one of the first large animals ever to travel by air,

200 million years ago.

It belonged to a group called the Pterosaurs, the winged reptiles.

It was probably a forest dweller and a descendant of a tree living glider.

This gliding ancestor might have had wings like those of Draco's

that were made of skin, and perhaps extended from its fingers down to its ankles.

But Pterosaurs had evolved larger wings with a hugely elongated fourth finger.

The wing membrane was strengthened internally, by thin rods of a stiffer tissue.

They were muscles fibres too, that enabled it to modify its contours as it flew.

Looking at the wings in section, reveals a secret of their efficiency.

They have a rounded front edge and a sharp back edge,

a shape known as an aerofoil.

It works by forcing the air flowing above the wing, to speed up.

This faster air has a lower pressure, and the wing is sucked upwards.

The larger the surface area of the wing, the greater lift it can produce.

So it seem certain that Pterosaurs were very competent flyers.

And judging from their teeth, it seems likely, that many fed on the great variety of insects

that had preceded them into the air.

Insects have had the skies to themselves for around 100 million years.

Now, bigger creatures had arrived. Reptiles.

The Pterosaur design for flight proved hugely successful.

They used their new powers to spread beyond the forests,

and colonize whole new environments.

A great number of them lived and fed near water.

We know this because fossils of many species

occur in rocks that were once mud at the bottom of lakes and shallow seas.

This one shows the skeleton of an animal that 150 million years ago,

fell to the bottom of a shallow lagoon.

This is its head, here's its backbone,

tail, hind legs,

and here, stretching from these long extended finger-bones, are its wings.

And this fossil is particularly remarkable,

because it shows an impression of the membrane in extraordinary detail.

You can see every little tiny fold.

You can judge how an animal lived, by its skull.

And this one, had these long jaws,

with forward pointing teeth, and we think that this indicates

that it lived by skimming across the surface of the lagoon,

and snatching up fish which impaled on those teeth.

This, very different one, it's just the head.

As you can see it has very long jaws,

and at the tip of the lower one is this little tuft of very fine filaments.

And we know from other specimens that those filaments

originally stretched right along the length of the jaw.

This bristely fringe enabled the creature to filter-feed, taking in a beak full of water,

expelling it through the bristles with the beak half closed,

and then swallowing what the bristles retained.

And here is a skull of a very much bigger species from Brazil.

And it had neither teeth nor bristles in its jaws,

but microscopic examination of the surface of the bone here,

reveals very tiny little blood vessels

and that suggests that these jaws were once covered with a horny beak.

So that maybe this animal used it's beak like a pair of forceps

to pick up small little reptiles, or maybe catch dragonflies in the air.

And this particular skull reveals something else about the lifestyle of this specimen,

because at the back of the skull it has this great flange.

And Pterosaur skeletons from other species

have been found, some with such flanges but others without.

So it's thought that maybe this was the difference between the sexes.

Maybe was the male that had these big flanges at the back which it displayed them,

and maybe it was covered with skin, we can only guess.

Many different Pterosaur species evolved these headcrests,

and seems very likely, that they were coloured.

This spectacular example, is known as Tapejara.

And it made its home beside inland lakes.

But Pterosaurs diversified in other ways too. Some evolved much larger bodies.

This species had a wingspan of over 20 feet, 7 metres.

But not all Pterosaurs lived in the forests or near water.

An open, arid landscape like this one, was the likely home

of one of the most extraordinary.

Around 70 million years ago, a Pterosaur appeared

that was of truly colossal proportions.

That was one of the largest creatures that has ever flown,

it was in the size of a small aeroplane, and it was called, Quetzalcoatlus.

Its immense wingspan allowed it to ride on the currents of warm air

that rise up from sun-heated land.

It could then glide great distances, searching for food.

Small creatures like lizards, or the dead bodies of much larger ones, dinosaurs.

But the Pterosaurs, with their wings of toughened skin,

weren't the only group of reptiles to make it into those ancient skies.

About 150 million years ago, another reptilian group appeared

on the planet that also flew.

Like most reptiles, including Pterosaurs,

these creatures began their lives inside an egg.

But they had evolved a revolutionary new design for flight,

one that would usher in a remarkable fresh chapter, in our story.

And unlike the Pterosaurs, they're still with us today.

There are of course, the birds.

Some today can provide clues, about how their ancestors

managed to get into the air.

This is the chick of a bird found in farmyards everywhere:

A Bantam Hen.

And at this very early stage in its life, it can show us something very interesting

about the origin of that crucial piece of flying equipment, a feather.

Its feathers are downy, that's to say, they're made up of simple filaments,

and their function is not for flight, but insulation,

to keep this little creature warm.

And back in the Jurassic period, long before the arrival of true birds,

very similar looking feathers appeared on very different animals,

reptiles, dinosaurs to be precise.

To find evidence for that astonishing statement,

which not so long ago was highly controversial, we're heading for China.

Northeast of China's Great Wall, near the borders of Mongolia,

lies the chilly province of Liaoning.

Here, there are great areas of rocks that were laid down as mud,

in the bottom of immense fresh water lakes.

The bodies of animals that were swept down into these lakes,

were slowly entombed by the fine-grained sediment

that preserved them entire and in exquisite detail.

And from these rocks have come specimens

that solve one of the most hotly debated of evolutionary arguments:

The origin of the birds.

The key specimens are now in Beijing,

where they've been delicately prepared, under the microscope.

They have been studied here, by one of the world greatest dinosaur experts,

Professor Xing Xu.

First, he showed me one of his oldest specimens, part of a dinosaur's arm.

But thanks to the fineness of the mud of those ancient lakes,

there is more here than just bones.

You see here, this species is called a Beipiaosaurus,

So because this is an animal like two or three metres long,

so quite a big animal. And here is an arm, hand, you see here...

dark filamentous structures... - Yes.

along that arms and hand, they're actually primitive feathers.

And those feathers are very simple, very very simple,

so we believe they represent the very primitive stage for feather evolution.

These simple strands were made of the same material, as the feathers of today birds.

They were relatively thick, and must have been quite stiff,

so they would have stuck out beyond the dinosaur's arm.

Behind them, were shorter strands that covered its whole body.

Like the down on the chick, these might have kept the dinosaur warm.

But those long strands most likely had a different function.

Clues to what that might have been can be found on an even more extraordinary fossil.

These claws and finger bones belong to a creature called Caudipteryx.

The long dark shapes around them, are the remains of feathers.

The single strands are here rather more complex.

They had barbs, thin filaments attached to either side of a central rod.

This looks more like a bird's feather.

Caudipteryx had around 26 of them, along each arm.

This may look like a wing, but the feathers were not very long.

And when you compare them to the size of this creature's body, and its long legs, it's clear

that they weren't big enough to enable Caudipteryx to fly.

So, what were these feathers for?

Microscopic examination has revealed that they were coloured, and patterned.

So, maybe they were used for display,

perhaps to wave around during courtship, to attract a mate.

But then is seems that they also helped the dinosaur, in a different way.

We can find a hint of how they might have done this,

by watching the way some young birds use their first feathers today.

These are ten day old Pheasant chicks.

Their feathers are not yet fully developed.

At this stage they're similar in structure, to the feathers on that dinosaur, Caudipteryx,

and grow in a line along each arm, in much the same way.

But these early feathers are also too short, to enable these creatures to fly.

Nevertheless, they're very helpful.

Pheasant chicks hatch in nests on the ground,

but they soon need to roost high up, where they'll be safe from predators.

Flapping these simple wings gives the chicks a little extra lift,

to help them climb into a tree.

And when the time comes to return to the ground,

those first feathers again, are a help.

They don't provide a large air-catching surface,

but they're enough to slow a chick's fall,

and make that landing, just a little softer.

Maybe the feathers that had initially kept the dinosaurs warm,

now also helped them to get into the air.

And then, only a few years ago,

the mudstones of Liaoning produced yet another extraordinary fossil.

It's been named Microraptor, and it's clearly a small dinosaur.

But this specimen is particularly exciting, because of its feathers.

Feathers on the forearms there.

Feathers on its hind limbs.

And even feathers right at the end of its very long tail.

But there is something that makes these feathers

different from any other feathers we've seen on dinosaurs before.

They are narrower on one side of the quill than on the other,

just like bird feathers.

Microscopic structures within them, suggest that they had flashes of iridescence.

So these feathers were probably used for display.

But their asymmetric shape is characteristic of flight feathers.

The air flowing over the narrow front of the feather, can produce lift.

So could this strange looking dinosaur with feathers all over it, actually fly?

Some people think that those feathers on its hind legs

would have made it rather difficult for it to walk around on the ground,

and that it would have been more at home, climbing.

And those claws on the fingers and toes

are obviously very helpful, in climbing up tree trunks.

But those aerodynamically shaped feathers certainly suggest

that its arms were been used as wings.

This four-winged dinosaur must have been a really extraordinary animal.

Its front wings were broad enough to enable it to glide,

and its muscles on the chest were sufficiently strong

to enable it to flap every now and then, and help it on its way.

But the wings on the hind legs were probably not held spread out,

but kept beneath the body to help the animal to steer.

Now clearly, these dinosaurs were on their way

to join the Pterosaurs in the sky.

And then, discovered once again in the rocks of China,

came creatures that are recognizable as birds.

This is Confuciusornis. There are two of them here.

They no longer have heavy bony jaws studded with teeth.

Instead, they have short beaks made of horn,

without teeth, lightweight.

And the tail is no longer supported by a whole chain of small bones.

These bones have been reduced to this tiny little stump here.

These are true birds.

But the long feathers attached to the tail of one of these specimens,

can reveal something intriguing about these early birds.

To find out what they were for, we can look for a bird here in Borneo

that has very similar tail feathers.

This is the racket-tailed Drongo,

and it's tail feathers bare an astonishing resemblance,

to those of its distant ancestor, Confuciusornis.

They don't seem to help its flight in any way.

So the Drongo must be using them for something else:

Display.

And so, while the birds continued to improve their flight,

they also continued to use their feathers in courtship,

as their dinosaur ancestors had probably done.

But birds use, not just the shape of their feathers for display,

but also their colour, and there's some really lovely examples of that,

here in Borneo.

These birds are colourful enough, but one is particularly spectacular.

This is the Bornean Peacock-Pheasant.

This is the male. His feathers are emblazoned with colourful iridescent patterns.

And that's because they're used to attract the attention of a female.

Her feathers are comparatively drab.

First, the male lures the female into his courtship arena, with the promise of food.

A worm.

He begins to shake his magnificent feathers.

He clears the ground of anything that might interfere with his performance.

As the female dives in after the worm, he raises all of his feathers in a huge fan.

If she approves of his display, she may choose him as a mate,

over other rival males.

Eventually she makes off with the offering of food,

and it seems she was not as impressed as she might have been.

So feathers, so lightweight, and so easily erected,

can serve as billboards on which to advertise for a mate, or warn off rivals.

But to see how the early birds used their feathers to achieve fully powered flight,

we are returning to Britain.

Here, on a Loch in Scotland, we can watch some of the most majestic flyers around today:

Whooper Swans.

These particular birds were in contact with human beings

from the very first moment that they hatched, so they allow me to get really close to them.

The small feathers on their bodies are still essential for keeping their owners warm.

But this one is a wing feather.

It extremely strong, but very light,

and the filaments on either side of the quill, the barbs,

zip together, to form a continuous surface which is strong enough to hold the air.

But if the air is to support a big bird as it flies,

it has to move over the wing very fast.

And in order for that to happen, these Swans will move at speed

across the surface of the water, like an aircraft taxiing before take-off.

When you're close up to a flying bird like this,

you can see how a wonderful piece of complex engineering their wings are,

able to change their shape and their beat,

to respond to every little change in the currents of the air around them,

and so propel them forward and lift them upwards.

So, how do bird wings actually work?

If we slow them down, we can watch in detail the many subtle changes

they make, as they move up and down.

The feathers overlap to form a smooth, contoured surface

that extends far beyond the bones within.

With a curved leading edge at the front, and a sharp trailing edge at the back,

they have the classic aerodynamic shape, that produces lift.

They are aerofoils.

With each downward beat, the air pressure above is reduced,

so that the bird is sucked upwards.

Wings like these, consisting of jointed bones covered with closely fitting feathers,

can make very subtle, delicate movements.

The feathers slide over one another, so that when the wing changes shape,

there is no loss of smoothness on the contour.

When the swan slightly retracts its wings in between beats,

the sliding feathers ensure that the aerofoil still produces lift.

As well as lightweight beaks and shortened tails,

some of the bones of its body have become hollow.

The result is an extremely efficient, light-weight flyer.

We are traveling around 30 miles an hour now,

and yet these birds could easily accelerate, and leave us behind if they wanted to.

So feathers, since they're first appearance on the bodies of dinosaurs,

have acquired several different functions.

Initially, they served to keep their owners warm.

Then, some grew large and acquired colour, and were probably used in courtship displays.

And only then, after millions of years,

were they used to help their owners get into the air.

So around 150 million years ago, birds joined the Pterosaurs and insects in the skies.

Then, around 66 million years ago, came the global catastrophe

that triggered the disappearance of a vast proportion of the animal life of this planet.

An asteroid hitting the Earth, was the most likely cause of this mass extinction.

In the devastation that followed, the dominant creatures of that age,

the dinosaurs, disappeared.

The Pterosaurs were completely wiped-out.

And only a few of the birds survived.

The skies for a short period, must have been relatively empty.

But then, a new kind of flying animal appeared.

Now is a chance for a group of furry warm-blooded little creatures,

that had been scampering around the feet of the dinosaurs for several million years.

They were the mammals.

The first of them to take to the air were doubtless gliders.

And one mysterious creature still alive today,

can give us an idea of what they were like.

It lives in the rainforests of Borneo, and its called the Cobego.

It has an enormous blanket of furry skin, that stretches

from the side of its head, right down to the very tip of its tail.

But to see how it travels through the air, we must wait until nightfall.

As soon as it lands, it regains the height it's inevitably lost, by clambering up the trunk.

It's by far the most skilful of the forest gliders,

and can travel over a hundred metres in one leap.

It's undoubtedly a very ancient animal, and some believe that it may well

have survived virtually unchanged from that time long ago,

when mammals first took to the skies as gliders.

But soon, the mammals did better than that.

This is a fossil that dates from about 52� million years ago.

Here's its head, with very well-developed teeth, backbone and ribs,

and long tail, hind legs, and most important of all,

from our point of view, hands with enormously elongated fingers.

And there was skin between those fingers.

These were wings, and they could flap.

This is the earliest fossil yet discovered, of a bat.

We have new evidence to show exactly how a bat's fingers

first began to lengthen, to support their wings.

But we can understand how those early bats flew, by looking at their modern descendants.

These are some of the largest.

They're so big, that they're often called Flying Foxes.

And they have a wingspan of over a metre.

When you slow a bat's flight down like this, you can see that its four fingers

are spread wide on the down-stroke, keeping the membrane wide and taut,

and then clump together on the up-stroke, with just a thumb at the top free.

This folding of the wings reduces the bat's air resistance, between each beat.

To maximise the size of its wing, the back edge of the wing membrane

is attached to the ankles.

Bats roost by hanging upside down.

And this is how they tend to spend their days.

It's thought that the first mammals were nocturnal,

that doubtless was the best thing to be, out of the way of the dinosaurs

that were rampaging around during the day.

So the bats continued the nocturnal habit of their ancestors,

and they had also inherited the acute sensors, needed to move around at night.

Eyes specially adapted to operating well in low light,

and an acute sense of smell that enables them to find food in the dark.

In any case, birds already dominated the daytime skies.

With their wings of skin and nocturnal senses, the bats became a huge global success.

Today, there are over 1,100 species of them, that's over a fifth of all mammals.

So, by 50 million years ago, three groups of large backboned animals,

had joined the insects in the air.

The pioneers were reptiles, Pterosaurs,

with membranes of skin, stretched from elongated fingers.

Then, came a group of dinosaurs, that acquired feathers and became birds.

But when the Pterosaurs and Dinosaurs were swept away in a global extinction event,

the stage was set for the birds, and the newly emerge bats

between them, to take command of the skies.

Each of these two groups had evolved its own techniques for getting into the air,

and each was destined to bring their skills to astonishing extremes.

Next time, we see how birds adapted and diversified

to become the remarkable creatures we see in our skies today.

Lethal hunters.

Formation flyers.

And aerial acrobats.

We explore how the bats developed a new super sense,

that enabled them to hunt in the pitch-blackness of the night.

And we visit one spectacular place, where the battle for the skies,

between insects, bats and birds, still continues.

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