Original subtitles

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.

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.

Subtitles by Red Bee Media Ltd