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.
Of all the animals that live on our planet,
one extraordinary group dominates.
It has produced the largest...
The blue whale!
..the fastest,
the most intelligent creatures that have ever lived.
They're known as the vertebrates.
And they all share one vital feature - a backbone.
I'm travelling back in time to look for the key advances
that drove their remarkable success.
So far, I have seen the vertebrates grow from tiny origins
to dominate the oceans,
colonise the land...
..and take to the skies.
In this programme, I'm going to track the rise of a whole new branch
of vertebrate life.
The most complex animals yet to appear on Earth.
They started as a group of tiny little creatures
scarcely bigger than my little finger.
Nocturnal animals.
But they were to develop into some of the biggest creatures
the planet has ever seen.
It's a group that also contains us.
This is the story of the mammals.
I want to investigate how the mammals acquired a new set of key
features that allowed them to thrive in every corner of our planet.
Features we also have inherited.
We'll find the evidence in a series of thrilling fossil discoveries
and in living animals.
With the latest scientific analysis,
we'll be able to bring our ancient ancestors back to life.
Today, animals with backbones dominate our planet
on land, in the air and at sea.
But how did that evolutionary takeover come about?
There've been lots of gaps in the story.
But in recent decades,
exciting new discoveries have been made here in China,
and I'm here to look at them.
The rocks of China are yielding up the elusive missing links
in the vertebrate story.
Ancient creatures preserved as fossils.
To find new evidence from the very start of the mammals' story,
I'm travelling to the south of China,
and the province of Yunnan.
Fossils found here can reveal the kind of world
those first mammals encountered, and the kind of animals
they had to compete with to gain a foothold and survive.
This area of southern China is known as the Lufeng Basin,
and 180 million years ago, it was a vast natural hollow
into which waters from all the surrounding hills flowed.
And with those streams came sediment, which is now this,
and they also brought the bodies of the animals that lived
in those hills, including creatures like this one -
a dinosaur.
Excavators have uncovered hundreds of specimens like this one
in the surrounding countryside.
The local museum is crowded with one of the largest collections
of complete dinosaur skeletons in the world.
But a unique discovery here
has revealed some of the earliest evidence
for the origins of the animal group that would eventually succeed them.
At the same time the dinosaurs were roaming in this area,
there was another very different creature evolving in their shadow.
One that was on a much, much smaller scale.
Palaeontologist Wang Tao has spent his life exploring these hills.
He's used to finding the remains of large dinosaurs.
But on this hilltop site, he and his colleagues discovered
something that didn't match the usual profile.
TRANSLATION: I came to collect fossils with my colleagues
in this area here.
At the time, it was not like this.
There were no crops growing here.
After looking around, we followed this little slope.
And finally we found a small fossil about two centimetres long.
We thought it might be something special,
so we sent it to the lab in Beijing to clean it up.
I have travelled north to Beijing to see Wang Tao's discovery for myself.
It's now stored in one of the world's leading institutes
for the study of fossils.
And this is it.
And what seems extraordinary, near miraculous to me,
is that anybody should notice that a tiny, tiny little thing like this
is actually a fossil.
But a fossil it is.
It's the head of the tiny animal.
There's the tip of its nose.
That's the back of its neck.
And you can also see it's got an eye socket.
It's called Hadrocodium.
If I turn it upside down
you can see the bottom of its jaw.
It might be the skull of a really minute little reptile.
But it's not.
Because reptiles have simple cone-shaped teeth,
and this one has a tooth that is rather different.
That has the shape of a little insect-eating mammal's tooth.
So, this is one of the earliest mammal fossils we know of.
And to that extent, it's the ancestor of all mammals alive today,
including ourselves.
As such, Hadrocodium holds a key position in the evolutionary story
of the backboned animals, the vertebrates.
The first creature with the beginnings of a backbone
lived over 500 million years ago.
Then fish,
amphibians
and reptiles evolved.
It's from the reptile line that the first mammals emerge.
The Hadrocodium fossil dates to 195 million years ago.
These simple origins led to the vast diversity of mammals
we see around us today.
Over 5,700 living species have adapted to survive
in every corner of the planet.
We humans dominate and are the most numerous of the large mammals.
This astonishing journey was built on a series
of key evolutionary advances that began in very early forms
like Hadrocodium.
We only have its skull, but we can work out from modern mammals
what the rest of its skeleton was like.
So, how did this minute animal gain a foothold
in the age of the dinosaurs?
Kunming city in southern China.
I've come to this late-night market
to observe one of the first crucial steps in the mammals' story.
The development of an amazing feature
that gave them a key advantage.
But only after dark.
The mammals found a niche for themselves
not so much in space as in time - at night,
when the reptiles are not active.
A simple experiment with two pets
that happened to be for sale in the market tonight
can demonstrate why this is so.
This is a thermal camera,
and it will show a cold body as a black or very dark.
So, this lizard which is on the table is cold-blooded,
and it appears to be very much the same temperature as the table.
Reptiles get much of their energy directly from the sun as warmth.
But there is no sun at night.
As a consequence, it's scarcely got the energy to move.
This puppy, on the other hand,
is very active.
And when you look at him with the camera,
you can see that his body is very warm indeed.
And you mustn't eat the lizard!
The mammals, very early in their history, developed
the remarkable ability to generate heat within their bodies.
They became warm-blooded,
and they achieved this by driving their metabolism
at a much higher rate.
But to do that, you need extra fuel, extra food.
A reptile like a lizard can go for many days without eating.
But if a mammal is denied its food for several days, it will die.
So, in order to keep their fuel bills down,
the mammals used a technique familiar to any householder -
insulation.
They coated their bodies, as this puppy has,
with fur.
With warm blood and a covering of hair,
Hadrocodium was free to hunt for insects in the cool of the night.
But now came a new challenge -
to find its way around in pitch darkness.
Detailed analysis of Hadrocodium's skull is revealing remarkable
new evidence of a set of ingenious solutions to this problem.
The clues are tiny and invisible to outside scrutiny.
But professor Zhe-Xi Luo, an expert on early mammals,
is using a micro CT scanner to unlock the skull's inner secrets.
X-rays penetrate the rock
and pick out detailed fossil structures within.
A computer then builds a 3D model of the bones,
and, in particular, the cavity that once held the brain.
Professor Luo is able to identify an area that is clearly
much larger than its equivalent in a reptile.
If you look at the CT scan here,
you can tell that, despite a tiny little skull, the brain is enormous.
But one of the most striking features of this particular fossil
is that it has very large olfactory bulbs.
When you say olfactory bulbs,
those are the part of the brain that detects smell. Correct.
This mammal must have had very refined sensory detection
of all kinds of smell, allowing it to be active in the dark of the night.
This powerful sense of smell would have helped Hadrocodium pick out
the scent of the worms and insects it fed on.
The scanners have also revealed a radical advance in a second sense
that's vital in the dark.
Hearing.
The tell-tale clue lies, surprisingly, in Hadrocodium's jaw.
One very interesting feature
that's so unique about this fossil mammal is...
very flat jaw.
The surface on the inside of the jaw is perfectly flat.
In the primitive,
pre-mammalian forms, there are big grooves.
Grooves like these indicate the presence of two key bones
that are attached to the jaw of a reptile.
Seen here in green and red.
A third bone, coloured blue, transmits sound waves in its ear.
In a mammal there has been a truly amazing evolutionary development.
The two jawbones have shifted to form, with the third...
..the middle ear.
This three-bone arrangement
opens up a range of higher-pitched frequencies
that a reptile cannot hear.
It's the system we have inherited inside our ears.
So, in Hadrocodium, we get the earliest indication
that the three ear bones so important for our hearing
have already originated with this fossil.
Now ears could pick up the faintest rustle in the undergrowth
and guide Hadrocodium to any insects moving nearby.
Professor Luo's analysis has also identified
a spectacular advance in a third key sense.
It also has very large areas
responsible for skin touch.
For touch? That's right. Mammals have hairs.
One of the most important functions of the hair is actually
to give us the sensory touch,
and this animal has already developed that.
The use of hairs as touch sensors is perhaps most obvious from the way
modern mammals use their whiskers.
This brown rat relies on them for finding its way around at night,
or underground.
At the base of each of those long hairs on its nose,
there is a nerve receptor.
And whenever the hair is touched,
a message is sent up to the rat's brain.
It's not just the whiskers, though.
Hairs all over its body are wired up to its nervous system.
This creates a sensory bubble,
allowing the rat to map the world around it just by using its hairs.
195 million years ago,
the hairs on Hadrocodium must have been wired up in the same way.
This remarkable little creature now had a whole array
of new powers with which to meet the challenges of the night.
A heightening of the senses powered by a growing brain
had enabled the early mammals to survive in the shadow
of the dinosaurs. And then, they also developed a radical new way
of nourishing their young.
We can look for clues to this next crucial step
in our evolutionary story in Australia.
Not in fossils,
but in the bodies of two highly unusual creatures that live here.
The first is the platypus,
which uses its rubbery beak like a radar transmitter
to hunt for shrimp or molluscs underwater.
And the second is the echidna,
which forages for ants and termites on land.
The platypus and echidna are the only two survivors
of a group of mammals called the "monotremes".
Trace their genetic line back,
and we discover they split from all other mammals
around 200 million years ago.
Because they retain traits from that distant time,
they give us a remarkable insight into very early mammals
like Hadrocodium.
The most extraordinary feature of all is one that no other
modern mammal has retained.
They lay eggs.
This echidna egg is tiny,
only about the size of a marble.
The hatching process itself has only rarely been captured on film.
These are newly-hatched platypus young,
filmed in their mother's burrow.
They're only about the size of jelly beans.
The early mammals must have laid eggs in the same way,
and they inherited this trait from their reptile ancestors.
This is a view inside a reptile egg.
The embryo feeds on a supply of highly nutritious yolk.
By the time reptiles hatch,
they're sufficiently well-developed to go looking for their own food.
But the platypus and echidna are very different.
Their smaller eggs contain only a small amount of yolk,
so their young hatch in a far less-developed state.
They need a lot more nourishment if they're going to grow and survive.
But at Healesville Sanctuary near Melbourne,
we can find delightful evidence that platypus young do develop
with great success without having to leave their mother's burrow.
Four months after it hatched,
a youngster is emerging for the first time.
It has grown from a tiny hatchling to near adult size.
And that is thanks to an amazing form of nourishment
that is a defining feature of all mammals.
Milk.
This rich mixture of proteins, fats, carbohydrates and minerals
oozes from the bellies of female platypus and echidna
rather like sweat,
and provides their young with everything they need to grow.
It's likely that early mammals like Hadrocodium
nourished their young in the same way.
First with a reduced amount of yolk, and then with milk.
So, what could explain this hugely significant step?
New genetic analysis is providing the answer.
Dr Henrik Kaessmann has been using the platypus to investigate
the DNA of the early mammals.
The platypus is really an amazing creature.
It's really this crossover of a mammal and a reptile, right.
And so it has a key position in the evolutionary analysis
of all mammals.
First, he looked at the reduction in egg yolk.
Reptiles have at least three genes that together
manufacture their large yolk.
Dr Kaessmann has found that the platypus DNA
records a dramatic change taking place in the early mammals.
We found only one egg yolk gene in the platypus genome
that really was functional and was producing the egg yolk protein.
Presumably the fact that there was only one gene
which was producing yolk accounts for the fact
that the platypus egg is so small?
Exactly.
The early mammals must have started to switch off their yolk genes.
And Dr Kaessmann has made a second key discovery.
The trigger for this shutdown was the arrival of the genes
that produce milk.
So, you have the milk genes appearing that then allow
for the subsequent loss of the egg yolk genes.
The mammals began to favour milk over egg yolk as a way
to nourish their young.
And that is because milk has one key advantage.
It's on tap, and that means that none of it need go to waste.
And there's no limit on how much
and for how long a mother can feed her young.
Warm bodies, powerful senses,
and now, milk, had allowed the early mammals like Hadrocodium
to gain a foothold while the reptiles still ruled.
But combining egg-laying with milk-feeding
brought a new challenge.
A mammal mother could not leave the eggs to hatch by themselves
as most reptiles do today.
She had to stay with them.
Then came a truly astonishing solution.
The egg, instead of being laid,
was retained inside the body and started its development there,
so that the young was born alive.
Apart from the monotremes,
there are two other major groups of modern mammals around today.
Marsupials and placentals.
It's thought that they first appeared
around 160 million years ago.
Both give birth to live young.
But they do so in two very different ways.
Spectacular fossil beds in the north of China have, in recent years,
produced the earliest ancestors yet found of these two groups.
This is Liaoning province.
125 million years ago,
volcanoes were erupting in this region.
They left layer upon layer of yellow ash in these rocks.
Excavations have revealed the fossilised remains of animals
trapped in these layers and preserved in extraordinary detail.
This is a fossil that's been called Sinodelphys.
Its skeleton is very easily seen.
But around its skeleton there are dark marks,
and close examination shows that they are fur.
So, we can be pretty sure that this is the fossil of a mammal.
But its skeleton, and in particular, its teeth,
make it clear that it was a marsupial.
Marsupials were once distributed throughout the globe.
But most are found today in Australia.
And they allow us to see
how their ancestors began to bring their young into the world alive.
This is a sanctuary for breeding endangered species of wallaby
through the use of foster mothers.
Running the conservation project
is Dr David Taggart of the University of Adelaide.
Today, he and his team are conducting a health check
on a newly arrived baby wallaby, known as a "joey".
This joey looks like it's about two grams, so about 16 days old.
So, 16 days ago, this young would have been born.
All marsupial young are born very immature,
so its ears are folded and the eyes are closed.
Instead of being enclosed in an egg when leaving its mother
like a baby echidna, this joey emerged
directly from its mother's birth canal just 30 days after conception.
Its front legs are more developed
and strong enough for it to pull itself up through the fur
and wriggle inside a feature that is unique to marsupials - a pouch.
Here, there's a highly developed milk delivery system.
The milk is channelled through long, fleshy tubes, teats.
A wallaby mother has four of them,
and can even feed young of different ages at the same time.
She might have a young, just newly born,
attached to one teat, and she'll have a young
with its head in the pouch feeding from another teat.
And those two teats will be producing a milk
that is of different consistency.
So, one will be to nourish a new-born young
and the other's to nourish a young that's almost ready to wean.
It's a great system.
The long teats also give the young a way to cling
onto their mother as she moves around.
This opossum is a marsupial that lives in South America
and it has no pouch.
Its young seal their mouths so tightly round the teats,
they stay firmly attached.
This may well be how the early marsupials, like Sinodelphys,
carried their young around.
They were now no longer tied to a nest or a burrow
like the egg-laying mammals.
But this method had one obvious drawback.
Outside their mother's body,
the newborn young were vulnerable to accident and exposed to disease.
In China, new evidence is emerging for the pioneers
of an even more radical solution.
At the same time as the marsupials appeared,
another branch developed on the family tree of the mammals,
a branch that we belong to.
And it had way of nurturing their young before birth.
I'm travelling to Beijing and its museum of natural history,
to see remarkably early evidence for this group.
This is it.
It's been called Juramaia,
which means "Jurassic mother".
Its bones, and in particular, its teeth,
identify it as a member of the mammal group to which we belong.
But the key thing about it is its date.
It's Jurassic - 160 million years old.
And this makes Juramaia the earliest creature we know of
that could have nurtured its young in a revolutionary new way.
Juramaia lived and hunted in a world still dominated by the dinosaurs.
But it may have had a powerful advantage -
the ability for a mother to carry her young,
not outside her body like the marsupials...
..but inside, in a womb.
To understand how Juramaia could have achieved this, we can look at
one of its living descendants, the one that carries its young inside
for the longest period of all mammals, the elephant.
This is Dokkoon.
She is part of a breeding programme at Melbourne Zoo in Australia,
and she is pregnant.
Dr Thomas Hildebrandt, one of the world's leading experts
in mammal birth, is monitoring progress with an ultrasound scanner.
We study the longest pregnancy on the planet,
which the elephant has with 22 months.
And so ultrasound allows us
non-invasively to see all the differences
during the foetal development,
which is quite exciting and was never done before.
More detailed 3D scans give us
a spectacular view inside her womb.
Even at an early stage of development,
the baby's trunk is visible and moving.
But we can also see the presence of a remarkable organ
that evolved to make it possible to feed a developing baby before birth.
The placenta.
This baby elephant was born in the zoo just three weeks ago
and its placenta has been saved for analysis.
Here we have the elephant placenta of the baby which is running
outside the yard.
These blood vessels form the umbilical cord,
allowing to move all the nutrients to the baby
and take all the waste material away.
On the underside is a ring of sponge-like tissue
that attaches to the lining of the mother's womb
and allows nutriment to flow in and waste to flow out.
But it also operates as a life-saving barrier.
Because half of the unborn baby's genes are from its father,
it was under threat in the womb from its mother's immune system.
The baby is foreign materials and alien to the mother,
and would be rejected if there's not this very specific system engaged
which protects the baby against the maternal immune system.
Because the tissues of the placenta are composed of cells
from both mother and baby, and the two blood supplies never mix,
the baby is protected.
This allows it to remain inside the womb
until it's ready to survive in the outside world.
Mammals equipped with this miracle of evolutionary engineering
are known as "placentals".
It's likely that their earliest ancestors, like Juramaia,
were the first to rear their young inside their bodies
160 million years ago.
By now, the mammals had acquired all the key characteristics
that define them as a group.
Hairy bodies,
milk
and live birth.
And this combination would eventually provide them
with the platform for an astonishing explosion in diversity.
For millions of years, they remained the small,
shrew-like creatures that we've encountered so far,
skittering about around the feet of the dinosaurs.
But then came a sudden global catastrophe
that threatened to bring the whole history of the vertebrates
to a sudden end.
A meteor impact that sent shock waves around the world,
and coincided with the extinction of the dinosaurs.
We're still not exactly sure WHY the dinosaurs disappeared,
but certainly 65 million years ago,
they disappear from the fossil record.
But many other vertebrates survived,
and for them, the dominance of the world was now up for grabs.
Scientists are unearthing stunning evidence in Germany
for how the mammals seized this opportunity.
This natural hollow is known as the Messel Pit.
An entire community of animals was entombed here
by an extraordinary freak of nature.
47 million years ago,
this was a lake fringed by a subtropical rainforest.
But its waters held a dark secret.
The lake was in fact a flooded volcanic crater.
It's thought that lethal carbon dioxide gas
released from its depths periodically bubbled to the surface,
killing the creatures that drank at its shore or flew over its waters.
Their bodies drifted down to the bottom
to be entombed in the muddy sediment.
It's now one of the most remarkable
fossil excavation sites in the world.
Painstaking work is uncovering creatures sealed inside layers
of the ancient lake bed.
They're preserved in extraordinary detail.
It's a unique snapshot of life after the dinosaurs.
There are reptiles, like lizards and snakes.
Here, too, are ancient birds,
the vertebrate group that evolved from the dinosaurs.
But the biggest changes are amongst the mammals.
They have started to specialise.
This, perhaps, is the least specialised of them.
It's an insect-eater, a creature like a large shrew,
and its teeth are still relatively simple.
But then there are also animals like this.
And this has very big, gnawing front teeth.
This is an early rodent, a creature like a rat.
And then bigger still...
..is this animal.
This has grinding molar teeth at the back,
and long legs.
It's beginning to stand up on its toes.
This is an early horse.
And perhaps the most specialised and remarkable of all
at this still very early date
is this extraordinary specimen.
This, as you can see, is a bat.
And the preservation is so remarkable
that the skin can be easily seen,
not only on its forelegs,
which turns them into wings,
but even you can see this large ear
on the side of its head,
which suggests that already it was beginning to echo-locate,
to hear its own calls so it navigates during flying.
The mammals were displaying an extraordinary ability
to rapidly adapt their bodies to fill the range of niches left vacant
by the death of the dinosaurs.
They had new opportunities,
but they also faced a new evolutionary pressure.
Climate change.
Ten million years of gradual global-warming
had triggered a surge in plant life.
The land became covered in forests that grew ever denser and darker.
New mammals emerged with new features that helped them
to thrive in this changed environment.
Features that would have huge significance for humans.
This is an early member of the group of mammals
that was going to produce us.
This is an early primate.
And you can see that on its front legs, its hands,
they have an opposable thumb,
so it could grasp.
And the same on the back legs - the big toe is also opposable.
So, this animal was a climber.
The primates could now reach food that was high up in trees.
And it's thought that it was a new type of food that triggered
another astonishing advance in their bodies.
A major improvement in sight.
Dr Sandra Engels is part of a team investigating
the diet of the fossilised primate from the Messel Pit.
Remarkably, she's able to examine the preserved contents of its gut.
We have particles of the last meal of this primate,
and we analysed it with very high magnification
and we found the oval outline of a seed
which is part of a fruit.
And because we found it in the gut of this primate,
we know that it fed on fruit.
3D scans of its teeth make it clear that fruit was a major part
of its diet. This animal was a specialised fruit-eater.
If we take a closer look to the shape of the teeth,
we have structures as deep basins or rounder cusps
that are the right tools to break up fruit.
47 million years ago, large, fleshy fruit like this
had only recently been developed by plants.
It was one of the ways in which they had adapted
to the new dense forest environments.
Many early plants relied on the wind to distribute their seeds.
But in the forest, there is little or no wind, so they had a problem.
They solved it by recruiting the help of birds,
and they did that by wrapping their seeds
in an edible, sweet flesh, fruit.
Birds carried the seeds in their stomachs
and eventually deposited them elsewhere in the forest.
The primates had clearly begun to exploit this cosy arrangement,
but to take full advantage, they needed to improve their vision.
During the age of the dinosaurs,
when the mammals were largely nocturnal,
they had developed better night vision,
but sacrificed a feature not needed in the dark.
The ability to see colour.
Today, most mammals still see the world largely in black and white.
But the reptiles and their cousins, the birds,
retained excellent colour vision.
And the fruit-bearing plants
had evolved a signalling arrangement to match.
There's no point in having your seeds distributed
before they're fully formed.
So, the plants evolved a colour-coding system
to show when that was.
This plant, for example, here is a young fruit still growing.
Its flesh is hard and bitter, and it's green.
But this fruit is fully formed.
Its flesh is good to eat, soft,
and the seed within is ready to go.
And it's red.
To spot a flash of red colour in amongst the green foliage
is easy for a bird or a reptile.
But for a mammal, with their night-time vision,
red and green are indistinguishable.
Then, remarkably,
some of the primates managed a feat no other mammal has achieved.
They put evolution into reverse and re-acquired colour vision.
The common ancestor of this monkey, and of me,
lived up in the trees in the daylight.
And they quickly evolved the ability to see colour,
and therefore, to know which was ripe and which was unripe fruit,
and so take advantage of the system
that had already been worked out
between the birds and the plants.
Let's just see what she thinks about that.
Which of those do you like?
There's it.
After the dinosaur extinctions of 65 million years ago,
the mammals were using their spectacular adaptability
to evolve and diversify at an astonishing rate.
In the process, they laid the foundations
for the major mammal groups we see today.
But then, around 47 million years ago, came a new set of problems.
The Earth's climate changed yet again.
Many places became drier, and where that happened,
the forest thinned out and was replaced
by low, scattered bushes and grass.
And those new environments presented new challenges to animals
and ushered in the age of the mammal monsters.
Scientists are finding stunning evidence of this change
in the Great Plains of North America.
This dramatic country in South Dakota is known as the Badlands.
Streams and rivers have eroded the rocks into fantastic shapes.
But 40 million years ago,
these were layers of sediment laid down across an open flood plain.
Palaeontologist Clint Boyd is looking here
for the fossilised remains of creatures from that ancient time.
And he's finding mammals that are giants.
This is part of the bone we call the femur or the upper-thigh bone,
and this round surface right here is for the hip socket.
And so you can see it's very large.
We'd be talking about a very large animal.
And not only do we have the thigh bone but we've got ankle bones
spread out over here, and then cascading down from that spot,
we've got some of the tail bones coming down.
So, if we add all this up together, based on the size,
we're looking at an animal that's probably
about two metres tall at the hips.
The creature is known as a Titanothere.
It was a herbivore.
It fed on the lush vegetation that once covered this area
of the United States.
A range of different specimens have been collected
at Denver Museum of Nature and Science.
And they reveal that the first Titanotheres
were built on a much smaller scale.
When Titanotheres first appear on the scene, they look like this.
This is the lower jaw of one of the first Titanotheres,
and it's one of these sheep-sized animals.
In only five million years,
members of the group go from sheep-sized...
..to about the size of a small horse.
Within only 15 million years of their first appearance,
Titanotheres look like this.
Here you can see the skull of one of these Titanotheres.
In evolutionary terms, the size increase is astonishingly quick.
But what drove this remarkable change?
Another fossil could provide an explanation.
It dates back to the time of the first and smallest Titanotheres,
but it's a very different type of mammal.
This is the skull of Malfelis Badwaterensis,
the "bad cat from Badwater".
This was the largest predator at the time.
This is the skull. This large crest is for large jaw muscles
which would've given a powerful shearing bite that ran these
blade-like teeth, perfect for chopping up a Titanothere.
And what's interesting is that Malfelis was exactly
the same size as the top herbivores of the time, like Titanotheres.
The earliest Titanotheres could hide from these bad cats
in the dense forest environments.
But as those forests began to thin out,
the Titanotheres were more vulnerable to attack.
One way to improve their chances was to grow bigger.
An herbivore is much more likely to survive an encounter
with a predator if it's a little bit larger.
And so there was a bit of an arms race
between the predators and the prey.
And animals like Titanotheres were able to escape this predator pressure
by becoming the super-sized giants we see 35 million years ago.
Fossilised remains of Titanotheres from the Badlands of South Dakota
and elsewhere across the Great Plains
allow us to reconstruct its rapid growth spurt.
From modest beginnings,
they increased their bulk ten times over...
..till the largest stood over eight feet tall.
On the open grasslands that increasingly covered the Earth,
many other giant mammals emerged.
Together, they're known as the "Megafauna".
This giant sloth was found in California.
In China, I've come to see the remains of mammoths.
And a remarkable creature that was the largest land mammal
to walk this Earth.
This great beast is called Paraceratherium.
It stood five metres tall and nearly eight metres long.
Those furry little mammals
scampering about in the shadows had produced descendants
that could stare the biggest dinosaur in the eye.
Today, the elephant is one of the few species of Megafauna
to have survived.
But those outsized versions
have otherwise disappeared from the planet.
So, what happened to them?
Their eventual extinction coincides with another key event
in the history of the Earth.
From around two and a half million years ago,
ice sheets spread down from the North and up from the South
to cover vast areas of the continents.
But it was only when the ice finally retreated,
just 10,000 years ago, that the Megafauna vanished.
Some have blamed that on the rise and falls of the temperature
as the Ice Age finally came to a close.
But others have sought the culprit amongst the mammals themselves.
A newly-evolved super predator.
To see some of the earliest evidence for its arrival in China,
I've returned to Beijing.
These fossilised remains belong to a primate.
It's been dated to around 68,000 years ago.
This primate had two new evolutionary features.
First, its pelvis.
An animal with a pelvis like this
would have been able to walk upright.
Secondly, the skull.
Its brain case is enormous.
In proportion to the size of its body,
it's six times the average mammal size.
And that would have brought great intelligence.
And this creature, of course, was a human being.
The early humans put their new intelligence to deadly use.
They worked out how to make weapons.
These stones, carefully chipped to form sharp blades,
were found alongside human remains.
And they developed new powers of communication
that enabled them to join forces and hunt in teams.
This was a new kind of predator.
It first appeared in Africa
and then spread to all the other continents,
and each time its appearance in that continent
coincided more or less with the disappearance of the Megafauna.
Which suggests, at the very least,
that this creature had something to do with that event.
To conclude my journey in China,
and find the last step in our evolutionary story,
I'm back in Kunming city to visit one of its busiest maternity wards.
An enlarged brain brought us huge advantages,
but its size also presented a basic design problem at birth.
The bony skull encasing the brain
still had to make it out through the mother's birth canal.
A new addition to our species, just 12 hours old,
can reveal how this is possible.
This little boy's name is Shao Bao.
It means "little treasure".
He was born because of a special feature in his skull.
Mammal skulls are made up of separate bones.
And in most species those are fused together at the time of birth
to form a hard, bony box to protect that most special organ, the brain.
But not so with Shao Bao and other human beings.
They remain separate,
and that allowed his head to slightly change shape
and squeeze through the aperture of his mother's pelvis.
This also allows the brain to continue to grow and develop
after birth.
In fact, the plates won't start to fuse
until Shao Bao is around two years old.
It's one of the most recent
in a long line of remarkable evolutionary developments
that allowed the vertebrates, animals with a backbone,
to create the dazzling diversity we see around us today.
Shao Bao's ancestry, like that of all of us,
stretches back over 500 million years
to a tiny little wormlike creature swimming in the bottom of the sea.
His backbone and jaw came from the early fish.
His limbs and lungs from amphibians.
The reptiles gave him his watertight skin.
Tiny nocturnal mammals donated a bigger brain...
..sharper senses...
..and the manner in which he was born.
His hands and colour vision came from the fruit-eating primates.
And his larger brain and greater intelligence, from the first humans.
So, all our features of our body can be traced back
to our ancient ancestors,
and there's much more we have yet to learn about them.
But one thing is certain -
the evolution of the vertebrates has not yet come to an end.