'Of all the animals that live on our planet,
'one extraordinary group dominates.
'It has produced the largest...'
The blue whale!
'..the fastest,
'and the most intelligent creatures
'that have ever lived.
'They're known as the vertebrates,
'and they all share one vital feature.
'A backbone.
'Now, I want to travel back in time to explore their ancient origins.
'And investigate the key advances that led to their amazing success.'
Advances that can also reveal
how we came to acquire the characteristic features
of our own vertebrate bodies.
Jaws that bite, lungs that breathe, ears that can hear.
Because the story of the rise of animals
is also the story of how you and I came to be as we are.
'I will find evidence in a series of spectacular fossil discoveries
'around the world and within living animals.'
That's it.
'With the latest scientific analysis,
'we can bring our ancient ancestors back to life.'
And understand how, over 500 million years,
they developed the bodily features needed to master the seas...
..colonise the land, and take to the skies.
This is the story of the rise of animals.
The history of life on Earth
has been known in outline for many years,
but there were a number of tantalising gaps in it,
particularly in the history of animals with backbones.
When, for example, did the first signs of a backbone appear?
And is it really true that dinosaurs developed feathers
and turned into birds?
Well, in recent decades,
answers have been found to those extraordinary questions,
here, in China, and I'm here to look at them.
China is the new frontier for fossil discoveries.
Excavations here are unearthing links in the story
of the vertebrates that have so far eluded us.
I have long wanted to see this sensational evidence for myself.
I will be travelling to the frozen north of the country,
and to the capital, Beijing.
But to search for the first step in our journey,
I'm heading south, to Yunnan Province.
This is the site of a thrilling discovery that has given us
new evidence for the very first vertebrates.
Excavators here are exposing a rich seam of rocks
known as the Chengjiang fossil beds.
Remarkably, they contain the remains of creatures that once swam
in the ancient seas 525 million years ago.
'Palaeontologist, Hou Xianguang, was the first to discover
'the unique features of these beds,
'an astonishing perfection of preservation.'
Are these mouth parts?
Yeah. That's very beautiful.
You can see it's got striations on it.
'To find complete bodies like this is extremely rare.'
When an animal dies in the sea,
normally bacteria destroy the soft parts very quickly
so that all we can find afterwards are the hard parts, bone or shell.
Why that didn't happen here in this particular part
of this particular sea is something of a mystery.
It may be something to do with the lack of oxygen,
but whatever it was, it has given us a privileged view
into one of the most exciting chapters
in the whole history of life.
The beds have so far yielded over 200 separate species.
This was a time period known as the Cambrian.
The land was still bare and lifeless,
but, underwater, it was exploding into a multitude of forms.
The major animal groups we know today were appearing on the planet
for the very first time.
They built their bodies entirely of soft tissue.
Some protected and supported it with a hard outer casing.
But none had anything that resembled a backbone.
These were the invertebrates.
'Then, Professor Hou and his team found one intriguing exception.'
Oh, yes, yes, yes.
It's a fossil called Myllokunmingia.
But to examine it in detail,
you've got to look at it under the microscope.
Its features reveal evidence of a new type of support,
not outside the body, but inside.
This is one of about 30 specimens that have already been found
of this tiny little creature.
Under the microscope, it contains an extraordinary amount of detail.
Those marks are marks that have been made by the excavator's needle.
This is the animal itself. This is its head, the top of its back.
And nearly every one of them have these two little black spots
at the front, eye spots.
Looking farther down the animal,
there are just some striations here,
little bars which are thought to have been the gill bars,
the little constructions that carry blood vessels
which enabled the animal to extract oxygen
from the waters it flowed over and breathe.
And behind them, farther down the animal, there are these bars...
..bands of muscle, and they were probably attached to a gristly rod
somewhere in the middle there.
This is called the notochord,
which was the forerunner of the backbone.
Myllokunmingia is the earliest creature we know of
that we can truly call a vertebrate.
And it seems clear that it used its strong inner rod
to move in an entirely new way.
As the muscles contract, they bend the rod from side to side.
This movement pushes against the water and creates forward thrust.
Here was a revolutionary new way to get around.
It allowed Myllokunmingia to roam far and wide and escape
the dangerous invertebrate predators that were prowling the seas.
The vertebrates would diversify over millions of years
to create the spectacular variety of backboned creatures we see today
in every environment on the planet.
Fish dominate the seas, lakes and rivers.
The amphibians live in both water and land.
The reptiles can survive in the driest places on Earth.
The birds rule the skies...
..and the mammals have insulated their bodies
to adapt to every climate.
We humans have used our greater intelligence to overrun the planet.
This astonishing journey
was built on a series of key evolutionary steps
that helped our ancient ancestors to exploit their environments
and overcome huge challenges.
The first of these advances was the development
of that inner support - the notochord.
Back in Europe, you can find a creature that represents
the next critical step in our story.
It lives unobtrusively and often ignored in British rivers.
And it sheds light on the challenges those first vertebrates faced.
Ah, there it is!
This is a lamprey.
You might think at first sight that it was a kind of fish, but it's not.
It's something much, much more primitive.
It has no fins, and even its tail
is nothing more than a flattened blade.
But what is most remarkable about it
is that it doesn't really have a true mouth.
Its mouth is just a simple hole
with little bristles about it.
And it feeds by sucking in water through that mouth
and then filtering out little particles of food.
So this little animal takes us right back to the time
when the first animals with backbones appeared on Earth.
It's a true living fossil.
The first vertebrates seem to have had the same kind of mouth
and they were almost certainly limited
to the same kind of simple food.
Over time, other forms evolved with different shapes and sizes,
many of them rather larger than Myllokunmingia,
but all of them had that very simple mouth,
an opening at the front of the body as the lamprey has today.
If the early vertebrates were going to really take advantage
of the variety of food that was available in those early seas,
they were going to have to develop a much more complex
and powerful form of eating machinery.
Scientists on the east coast of the United States are seeing evidence
of this evolutionary advance, not in fossils but in living creatures.
Maine, New England.
Marine biologists at the University of New England
are studying a group of fish with a very ancient ancestry.
They build their skeletons with the same strong material
that formed the gristly rod of the first vertebrates - cartilage.
They're the sharks, skates and rays.
This group appeared among the vertebrates
over 420 million years ago.
And that means we can use them
to examine the development before that split of a remarkable piece
of engineering that changed the course of evolutionary history.
The jaw.
If you look back on the evolutionary tree,
you'd find that a jaw is a really important feature to have
and it's one of the features that have made skates
and sharks apex predators in the environments in which they live.
A jaw hinged to the skull brought the new ability to grab food,
then rip or grind it into digestible pieces.
But where did this amazing piece of equipment come from?
Scientists have found an answer
by studying the way living vertebrates develop as embryos.
Skates lay their fertilized eggs on the sea bed
inside leathery cases called mermaid's purses.
Scientists can open these up and observe them as they develop,
fed by a generous supply of egg yolk.
The skate embryo has a simple structure
shared by all embryonic vertebrates
that served as the basis of the first jaw.
What we see are these folds...
..and what's really interesting about this,
is that this skate is in about...
four months of its development.
If we take a close look at another vertebrate,
we can see it looks very similar.
Here we have the head, as you can follow it down to the body.
You also see the folds.
Now, this is actually a human being.
It's thought that the embryos of the earliest vertebrates looked
much like this and that each fold developed into a gill.
In a skate embryo, the folds furthest from the head
keep to their original purpose and form the rigid arches of its gills.
But the nearest fold has been adapted to form
an upper and lower jaw.
In a human embryo, the lower folds develop into structures that
include the larynx and the throat.
But the top fold, once again,
constructs the jaw.
The development of the jaw improved the ability to collect food,
and those that lacked it, with a few exceptions like the lamprey,
died out.
But in order to collect food, you have to find it
and that led to an improvement in swimming.
In the Chinese capital, Beijing,
I've been given special access
to a newly identified missing link.
'This tiny fossil holds clues that are just fragments and hard to spot.
'But it's the earliest example yet found of a creature
'with two pairs of fins.'
It's called Parayunnanolepis and
it's about 410 million years old.
Its front part, like many other fish at the time,
was covered by armour plating
to protect it from predators.
Underneath, you can see that it has a mouth,
and although the lower jaw is missing,
you can tell from marks on the upper jaw that it was once there.
But what is most important about this is its fins.
Just their stumps are visible.
It had two fins at the front -
petrol fins.
They were shaped rather like the wings of an aeroplane,
and they had the same effect,
creating upwards lift through the water.
Front fins have been found on older fish.
But what's interesting is this is the earliest example which
has another pair of fins at the back -
the pelvic fins.
This second smaller pair brought much more stability,
helping the fish to hold its course through the water.
This system was hugely successful
and it made the sharks the skilful swimmers that they are today.
So now, the vertebrates had jaws and four fins.
To see evidence of the next crucial development,
I'm heading out onto Lake Fuxian, in southern China.
These waters are home to living descendants of a group
that developed a new kind of inner support,
a support that would have huge significance for later life.
Here they get a lot of fish
like this.
It's a carp and it's very different
from the sharks
we've been looking at,
because instead of having cartilage skeletons,
carp and others like it, have skeletons that are strengthened
with calcium phosphate.
They're bony and most fish today have bony skeletons.
Bone contains the main material found in cartilage,
a long stringy protein called collagen.
Hard crystals of calcium phosphate add strength.
But the collagen can still flex slightly
and prevent the bone snapping under pressure.
These bony fish could subject their skeletons to the far greater
forces that come from increases in speed and agility.
They added mobile fan-shaped fins
and assumed a multitude of different forms.
From their simple origins over 500 million years ago,
the sharks and bony fish
diversified to dominate
every underwater environment on Earth.
There are over 35,000 species alive today.
The strong inner bony support to the body had evolved in water,
but it would prove most spectacularly successful,
in a completely new environment.
For most of the Earth's history until now,
the land had been empty and barren.
But around 450 million years ago, first plants, then worms
and then the ancestors of insects began to colonize it.
Here were rich pickings for any vertebrate that could reach them.
The stage was set for one of the most astonishing
leaps in evolutionary history.
The vertebrates move onto land.
But to achieve this remarkable feat, they would need to make
a major modification.
To move around on land without the support of water,
these fish needed a way to lift their bodies
up from the surface of the ground.
They needed limbs.
Scientists have recently found the earliest evidence for this
key moment in Eastern Europe.
Zachelmie, Poland.
Once a quarry for building stone,
today this is a hugely significant fossil site.
But palaeontologist Per Ahlberg and his team aren't looking for bodies,
they're looking for the marks the bodies left behind.
393 million years ago,
this was the soft muddy floor
of a tropical lagoon.
You can still see mud cracks here from an episode
when the lagoon dried out and the mud all flaked up.
Over millions of years, the mud solidified into layers of rock,
which were then tilted by movements in the Earth's crust.
By carefully exposing each layer, Per and his team have been
able to uncover a series of intriguing tracks.
There are three big dimples in the rock.
There's one here, one here and one down by my feet.
These are not erosional hollows,
it's not like rock has been scooped away, something's been
pressed into the surface of the mud while it was still soft.
You can see that from the internal texture here,
but also from the fact that you've got a slightly raised rim
round the edge where the mud has been displaced.
So a large heavy animal, presumably a vertebrate of some sort,
pushed an appendage into the mud here,
once, twice, three times in succession.
The marks suggest a creature floating
and pushing itself around in the shallows.
But Per and his team have found a more detailed set of prints
that show an animal doing something even more radical.
This is one of the most important specimens from the entire site,
and the reason for that is the pattern that these prints make.
You can see, easily I think, that they make pairs,
one in front of another, in this kind of diagonal arrangement.
In order to be able to produce this,
you need to have limbs that stick out to the side
and which can be swung forwards and backwards rather freely
while you're flexing your body from side to side.
Then, you can generate this kind of pattern.
A fish crawling trace would not look like this.
Another extraordinary slab has even preserved the imprint
of what Per believes is a fully-formed foot
complete with toes.
So, how did the vertebrates make this astonishing transition
from fish swimming to animals with four legs walking on land?
In search of clues, I'm heading to London
and the Natural History Museum, home to the largest
collection of plant and animal specimens in the world.
I'm here to see the remains of an ancient creature,
once hailed as a missing link that would answer such questions.
This is a type of bony fish called a Coelacanth.
Its fossilized skeletons have been found in rocks even older
than those in Poland.
Its fins have an intriguing feature not seen in other kinds of fish.
Their base is a rounded fleshy stump that looks
tantalizingly like the beginnings of a leg.
So scientists thought that this might well be
the ancestor of all land-living vertebrates.
And then, a sensation.
A living coelacanth was hauled up from the depths of the Indian Ocean
and the museum has acquired several of them.
Here is the body of a baby coelacanth.
The coelacanth female retains the egg in her body
until it's fully developed.
There's its yolk sack,
and here's its fin,
and you can see this fleshy base to it here and then its fin rays.
The question is, was that strong enough to enable a fish like this
to haul itself out of the water and up onto land?
Now, living coelacanths have been
filmed in the depths of the sea.
Its fleshy, muscular fins
do certainly help it to
manoeuvre its five-foot-long body.
There is even the hint of a walking pattern,
but detailed analysis has revealed that their fins
are still a long way from being legs.
The ancient coelacanth marked a crucial early stage in that
transition, but some characteristics ruled it out as a direct
ancestor of the land vertebrates.
All land-living backboned animals have limbs which have a basic
similar bone structure.
There is one bone at the top,
then there are two bones
and a group of bones, followed with digits.
And the coelacanths didn't have that structure.
And then, recently, another fossil discovery was made.
Ellesmere Island lies in the icy waters
between northern Canada and Greenland.
A team of American palaeontologists, who shot this footage,
believed that the rocks here were deposited in the right
sort of environment for the vertebrates move to land.
We learned about a sequence of rocks
that formed in ancient stream systems.
Our hypothesis was that it was in those sorts of environments,
where limbs were being favoured over fins.
The arrival of plants on land had stimulated a surge in life
in and around fresh water swamps
and this created new opportunities for the fish that lived here.
One of the nitches that was being developed at the time,
was for shallow-water predators.
You know, which fish could find other fish that were living
in the shallows, the swamps, the productive eco systems
that were just starting to appear on Earth at that time?
Ted Daeschler and his colleagues believed that limb-like fins
could have helped a fish to hunt in this kind of environment.
And then, on the slopes of a barren valley,
they made a thrilling discovery.
This was the fossil that got us really excited.
We couldn't have dreamed actually that we would find
something as well preserved as this one.
It's about the front
two-thirds or half of the body,
as you can see,
a very complete skull,
and a large piece of the body,
including parts of the fin.
The team found features that matched the profile
of a shallow-water predator.
Eyes placed on the top of a flattened head...
..and ranks of sharp teeth.
They gave it a local Inuit name -
Tiktaalik.
We can now work out from its bones how Tiktaalik
moved around in those swamps and shallows.
In deep water, it must have swum like any other fish.
But further examination of its bones showed that it could also
move its body in a far more radical way.
One of the really amazing aspects
of Tiktaalik that we've noticed
is this evolution of the neck.
There was not a rigid connection between the skull
and the rest of the body.
Tiktaalik is the first vertebrate
we see that has freed up the neck. And when you think about it,
all limbed animals, including ourselves, would not be able
to move our head independently of our shoulders if it were not
for these innovations that were occurring in a form like Tiktaalik.
A flexible neck allowed Tiktaalik to point its jaws at its prey
when space was too cramped to manoeuvre its whole body.
But it was the fins that provided the team
with the most exciting evidence.
Behind the spiny rays, there were lobe-like stumps,
like those of the coelacanth.
But Tiktaalik's bones revealed a pattern that was much
closer to the basic structure of limbs.
We learned a lot
about the fin of Tiktaalik
from this specimen.
Now, this is a cast of all the different bones that we found
in association, including the shoulder girdle here.
But that is the complete fin skeleton
from the front fin so...
I'm a lobe-fin fish, here is my front fin, we call it a limb now,
but here is Tiktaalik's front fin.
We've got a shoulder joint
and it's very important that there's a shoulder joint
which is oriented a little bit laterally,
a little bit down in Tiktaalik.
Very different from an animal that's just swimming with its fin
and paddling along, this fin seemed to be oriented beneath the body.
So this is the humerus.
We all have a humerus, that's the first bone in the front appendage.
We have an ulna and a radius.
So you and I, all limbed animals,
have an ulna and radius.
We have some wrist bones
and we actually then have something
which, like a wrist, could
bend together and allow this fin
to sit down and to contact
a surface with a surface area.
And so, when we see all of these features,
we see a structure which is very much like our limbs.
So here is a fish using its fin in a very limb-like way.
Tiktaalik's heavy-duty fin still helped it to swim.
But if it hit the shallows,
the bones and joints would help to
push itself up and punt around.
But this new limb didn't just help mobility in the water.
It became the driving force behind one of the most spectacular
events in evolutionary history...
..the arrival of the first vertebrate animals on land.
Over time, creatures evolved
that spent most of their time out of water.
They formed a new group we call amphibians.
And to survive on land, they had to solve a new challenge.
They had to be able to extract oxygen
not from water, like their fish ancestors, but from the air.
Fish use gills to absorb oxygen into the body.
In air, gills quickly dry out and stop working.
China is the home of a rare and fascinating creature
that can show us how the ancient amphibians overcame this problem.
Today, the biggest amphibian alive is this creature -
the Chinese giant salamander.
It breathes partly through its skin which has these long flaps on it,
and that absorbs oxygen from the water...
..but it also breathes air.
It's going to come up, and as it does, it snatches a gulp of air,
blows a few bubbles...
..and sinks down again.
Its jaw acts as a pump, forcing air down into the body.
Here, oxygen is absorbed into the bloodstream
from two inflatable sacks with permeable walls - lungs.
Because they're enclosed inside the body, they don't dry out.
The lungs it uses are just simple pouches
coming from the back of the throat.
But nonetheless, they were the first kind of lungs that animals had.
The forerunners of the air-breathing organs that all of us
land-living vertebrates now have.
From their origins,
around 365 million years ago,
the amphibians took on many different forms.
Over 7,000 species now live in a variety of habitats
on land and in water.
They include salamanders...
..frogs...
..and newts.
But two things tie the amphibians to water.
First - their skins are moist and if they dry out, they die.
And secondly - their eggs, like this frogspawn,
are covered in nothing more than jelly.
And they have to be laid in water or at the very least,
in moist conditions.
And until the vertebrates could solve those two problems,
they would not be able to colonise the dry parts of the land.
Then, a group of pioneers appeared
with an amazing new feature to their bodies.
We can find the evidence for this next step
by looking at animals that can survive far from water today.
This little creature is a lizard.
They call it in these parts a tree dragon.
And its body is very much the same shape as an amphibian.
Long body with a backbone and two pairs of limbs.
But there's one crucial difference
between an animal like this and an amphibian.
Its skin is not moist, it's dry.
We can see what has changed by putting the two types of skin
under the microscope.
The skin of an amphibian
is smooth with living cells
visible on the surface.
A lizard's skin is much rougher
because it contains large amounts of keratin -
a protein similar to that from which our own fingernails are formed.
Keratin-filled cells dry out and layer up to form scales.
This creates a barrier, sealing water inside the body.
We humans have inherited this keratin barrier in our skin,
allowing us to maintain up to 70% of our bodies as water.
Animals with this body plan became a huge success.
They evolved into a great number of species big and small.
We call them reptiles.
But the reptiles still had to overcome a second challenge -
how to lay their eggs out of water.
'I have come to Lufeng, in southern China, to see evidence
'gathered by local scientists of the ingenious solution.'
Thank you very much.
These eggs were laid by a reptile,
and as you might imagine,
a pretty big one at that.
The first reptilian eggs almost certainly had a leathery covering,
rather like those a turtle lays today.
But these eggs are different, they have a hard covering - a shell.
And you can see where the weight
of the sand that eventually
covered them and fossilized them
bore down upon them, they crushed that shell,
but the pieces are still in place.
From examining modern reptile eggs,
we know that this shell must have been made of hard calcium carbonate
and it must have supported an inner fibrous membrane.
Together, they made the egg water-tight.
And that meant that the animals that laid them no longer had to
go back to the water to lay their eggs, as all amphibians had to do.
Instead, they could go to the driest part of the land and breed
and nest and lay their eggs.
So all the dry land was open to them.
The amphibians had spearheaded the move to land.
Now, their descendants, the reptiles,
were able to establish themselves
in its driest parts.
Over 9,500 species now inhabit our planet.
But the limbs that helped the vertebrates
emerge from the water began to present problems
when it came to walking efficiently on dry land.
Because they projected sideways,
it took a lot of effort to hold their bodies off the ground.
Then, around 230 million years ago,
one set of reptiles developed an amazing solution.
These eggs were laid by an animal belonging to the most
successful of all reptile groups,
a group that dominated the world for 100 million years -
the dinosaurs.
More than 150 different species of dinosaur have been
found in the rocks of China alone, and over 1,000 worldwide.
And they too depended on a crucial advance.
A radical modification of the bone that connects the leg to the body -
the hip.
This is Lufengosaurus, a plant-eater.
The early reptiles had legs which splayed out from either side
of the body and left the body very close to the ground.
But a change in the shape of the hips of the dinosaurs
enabled them to bring their hind legs underneath the body
and so, lift them up and give them greater freedom of movement.
And some of them, including Lufengosaurus,
were able to support the entire weight of the body on the hind legs.
This new hip, along with sturdier leg joints, allowed the dinosaurs
to take longer strides...
..and support heavier bodies.
They became the largest animals that have ever lived on land.
But this new way of walking was also the first step on the road to
an even more radical evolutionary advance.
It was from this group of two-legged dinosaurs that there came a truly
astonishing development that we are only just beginning to understand,
and that was to lift the vertebrates to a completely new level.
The backboned animals had colonized the seas and invaded the land.
But there was one final habitat to explore - the skies.
Another extraordinary Chinese fossil bed is providing the missing
evidence for one of the great mysteries in evolutionary science -
the intriguing link between dinosaurs and birds.
I'm heading for Liaoning province to fulfil a long-held dream
and see the site of these discoveries for myself.
These rocks are about 125 million years old.
At that time, this part of China was tropical
and the land was covered with a lot of freshwater lakes.
And in those lakes was washed sediment
which formed these bands here.
But every now and again, the sediment changes colour.
And that is ash that was spewed out from a nearby volcano
so that about that level there,
there were a lot of skeletons waiting to be discovered.
And when they were discovered, they revealed some sensational facts
about dinosaurs, the most sensational for a very long time.
This fossil was one of
the most remarkable to emerge.
A two-legged dinosaur
about the size of a cat.
It's been named Sinosauropteryx.
Its discovery revealed an intriguing feature
never seen before on a dinosaur.
Up its tail and down its back,
a covering of what looks like fur.
Fresh finds have revealed that a wide range of two-legged dinosaurs
had skin covered by very similar hair-like filaments.
But what were they for?
In Beijing, there are the crucial specimens
that answered those questions.
This is one of the world's leading institutions in the study
of dinosaur evolution.
Professor Xu Xing and his colleagues have been analysing another
larger specimen of Sinosauropteryx.
It too retains traces,
just fragments of the mysterious filaments.
If you look near the tail, the dark things there near the tail,
they are single filaments, just like our hair,
which are very, very simple.
Xu Xing has been puzzling over their function.
Together, these filaments create a covering like fur,
so the most likely answer is that they served
to keep these dinosaurs warm.
But detailed examination has suggested an additional
and very different function.
Experts at the institute have taken minute samples
and examined them under powerful magnification.
They contain intriguing structures.
Some are lozenge-shaped,
some spherical.
Investigators identified them as melanosomes -
microscopic capsules that contain pigment.
They would have given the filaments on Sinosauropteryx's tail colour.
Based on our analysis, you see stripes.
One like white, brown, white, brown. Really?
Yes, definitely. It's a beautiful pattern. Of course you can't see all,
that's maybe for display or communication or...
Do we know how it held its tail?
Uh, tails definitely can move in different directions.
In most cases, I would guess is up or horizontal.
So it's like a ring-tailed lemur waving its tail around as a display.
Dinosaurs may have used
these coloured furry bands to signal
to other members of the species
or to act as camouflage.
But then came a discovery that suggested another far more
significant function.
I've been granted privileged access to the underground vaults
of the Beijing Museum of Natural History, to look at one of the most
important creatures yet to be found in the fossil beds of Liaoning.
This is Anchiornis, a creature that's clearly a dinosaur.
It's got powerful legs here ending with toes with sharp claws
on them, and its head,
which has been detached, lies here
upside down but you can see the jaw,
which has teeth in them.
But what is spectacular about this particular specimen
is the perfection of the preservation of these structures.
They show that the simple filaments have developed into something
far more complex.
The central stalk has tiny strands
branching out on either side.
The filaments have become feathers.
Analysis of them has shown that the crest here on the head
was a rufous red colour
and the body feathers
were striped black and white.
There are feathers all down the legs.
And looking at the density of them on the forearms here,
it does look very like a wing.
So the question is,
could this animal fly?
Could this be the moment when a dinosaur became a bird?
A clue to the answer could come from the environment in which it lived.
At this time, this area of northern China was covered in lush forests.
Animals that could climb trees would be able to collect food
that was not available on the ground.
They could also find safety from ground-living predators.
Xu Xing and his colleagues see evidence that Anchiornis adapted
to a tree-living way of life, by putting its feathers to a new use.
Anchiornis has some features suggesting a tree-living lifestyle.
For example, you look at the Anchiornis' toe,
they have very curved claws.
And also, they have big feathers attached to their feet.
If Anchiornis is a tree-living animal, then I have good reason
to believe that flight started from tree down.
Which means that the birds' ancestor can take advantage of gravity
and then start their journey to the sky.
Because Anchiornis lived high up,
it could use its feathers to glide.
It must have needed all the feathers growing along its front limbs,
hind limbs and tail to create a large enough surface to catch
the air and slow its descent.
It wasn't capable of flapping flight but, at 160 million years old,
it's now the earliest creature we know to have used feathers to fly.
The gliding dinosaurs would eventually give rise to
a whole new group of vertebrates...
..the birds.
Over 9,000 species crowd our skies today.
An astonishing evolutionary journey had enabled the vertebrates
to dominate every corner of the planet.
It was a journey that began in the Cambrian seas
over 500 million years ago,
and that led to the development of
a set of body parts that we ourselves would ultimately inherit.
Jaws and a bony skeleton from the early fish...
..limbs and lungs from the amphibians...
..water-tight skin from the reptiles.
By the time the birds appeared on the planet,
the early pioneers of another major vertebrate group had also evolved.
At first, they were tiny but they were destined eventually to
dominate the Earth -
they were the mammals.
Most, I dare say, were little better than snack food
for the dinosaurs, but all that was about to change.
A devastating meteor strike,
that many believe triggered a mass extinction.
We don't know exactly what happened,
but certainly, 65 million years ago, all the dinosaurs disappeared.
But some of the birds and mammals survived
and with the bigger dinosaurs gone, the world was up for grabs.
Next time, I'll be investigating the extraordinary rise of the mammals
to discover how they developed a remarkable set of new bodily
features to become the most complex and successful vertebrates yet.
Powerful senses,
a radical new way of producing their young,
and monstrous bodies.
We will also see how we humans finally
arrived on the tree of life
with hugely advanced brains that would allow us to out-compete
all other species on the planet.
Subtitles by Red Bee Media Ltd