Original subtitles

In the four billion years of life on our planet,

countless species have risen...

and fallen.

But one remarkable dynasty has had a greater impact than any other.

The mammals.

This is the story of their epic journey...

...to become rulers of the world.

Hidden in the ancient Jurassic forests...

mammals came from humble beginnings...

ruled over by the greatest dynasty that ever lived...

...the dinosaurs.

They dominated so totally...

...that early mammals were confined to the shadows.

For the most part, they were small, keeping to the trees...

away from the giants.

They became nocturnal,

developing a range of heightened senses that helped them in the dark.

Under the dinosaurs' rule,

there was a limit to how far they could rise.

But in time, everything changed.

One of the most violent events in our planet's history

heralded a new era.

Despite the devastation,

the planet did recover.

Within a few million years,

forests had regrown and carpeted much of the land.

The climate became hot and wet.

The perfect conditions for life to flourish.

A world primed for a new wave of evolution.

The asteroid had inflicted devastating losses on the mammals...

yet some of the smaller ones,

able to hide in holes, had survived.

Free of the dinosaurs,

they could finally venture out of the shadows

and come into the open.

Like their ancestors,

coatis have many adaptations

that help them thrive in the rich rainforest.

Extremely social, they find safety in numbers.

The mother's flexible nose can smell food hidden beneath the leaf litter.

A supersense honed over millions of years of foraging in the darkness.

Like all mammals, coatis are warm-blooded,

so can be active at all times.

That means these babies need plenty of energy.

Fortunately, mammals have evolved an extraordinary solution.

Milk.

They can feed their young whenever they choose,

even when food is scarce.

But milk wasn't the early mammals' only secret weapon.

They also had brain power.

And it's still just as important today.

In the forests of Costa Rica,

this capuchin monkey is trying to solve a puzzle.

How to get the meat from inside a closed clam.

The monkey isn't strong enough to crack the shell...

but it's discovered that repeated hammering will tire the clam...

until it finally opens.

Learning this lesson takes a large brain...

...and coordination.

This unique intelligence allowed their primate ancestors

to survive in the forest of 50 million years ago.

Clever animals tend to be inquisitive...

and that can pay huge dividends.

This female has learned

and remembered

where she can find water.

But it's too deep to reach.

Luckily, she has an inspired solution.

She turns her tail into a sponge.

But not all their challenges are to do with finding food and water.

Some fruit is collected not to eat...

but to rub over their bodies.

This fruit contains citronella,

an oil that mosquitoes can't stand.

The monkeys have discovered their very own insect repellent.

Free from pesky biting insects,

the clever capuchins can finally relax.

But just as their ancestors were beginning to make progress,

the warm, wet conditions in which they had thrived began to change.

The Antarctic landmass started to split away from South America...

creating the space for new ocean currents...

that helped drive a global cooling.

The climate was switching into a new, colder state.

But that was just the beginning.

As Antarctica became colder and more isolated,

the huge continent froze,

locking up much of the planet's fresh water.

As the world further north dried out,

it became a harsher, more difficult place to live.

Food became more spread out,

harder to find.

And as the Earth changed,

so did the mammals.

Some became enormous.

A male Megacerops.

Two and a half meters tall...

...four meters long...

and three tons in weight.

Its huge size allows it to travel large distances in search of food...

...and ensures it has reserves to get through leaner times.

Mammals were now big enough to rival the long-lost dinosaurs.

But being big has other implications...

especially when it comes to finding a mate.

This female is already closely guarded by a male,

much larger than the first.

If challenged,

he could seriously injure his opponent.

But the smaller male may be in luck.

Another female,

and she seems to be alone.

It's the chance he's been waiting for.

He has a rival.

This time, they're evenly matched.

With these mammals...

...size matters.

But even the mightiest Megacerops

couldn't overcome the planet's next great challenge.

The formation of Antarctica was creating global climate chaos.

As ocean currents shifted

and the planet dried...

it became a tinderbox.

Wildfires ravaged the land...

clearing the way for something completely different...

something that would challenge mammals as never before.

As the smoke cleared,

there rose from the ashes

a life-form that had previously only survived in the margins.

Grass.

Taking root during the great drying...

it spread so quickly,

it would soon cover one-fifth of the land.

But the endless plains were no paradise for the mammals.

Many couldn't digest grass

and soon went extinct.

Twenty million years later,

and today's grazing mammals remain locked in an arms race

with this green invader.

They have evolved a whole swathe of game-changing adaptations.

Specialized teeth,

strengthened to prevent silica in the grass

from wearing them out...

and huge bellies to process it all.

Their young are born fully formed...

...able to walk within hours of birth...

and follow their herd in search of fresh growth.

Mammals have truly conquered the grassy plains...

their huge numbers the most obvious sign of this exceptional success.

But just as some mammals evolved to feed on the grass,

others evolved to feed on the grazers.

Hunters.

The herd has its own defense against the cheetahs' blistering speed.

Many eyes...

...spot the danger early.

Running bodies create confusion for the young to hide in.

But the big cat has a plan...

forged by its clever mammalian brain.

Panic the herd,

split them up,

and target a lone calf.

Desperate to defend her only calf,

the female wildebeest counters.

The chase is far from over.

The cheetah has lost a lot of ground...

but the calf is tiring.

As the youngster turns,

it's the hunter who must now run for its life.

The calf is safe once more...

...but the cheetah will be back...

and the war between the grassland mammals will continue

as it has done for millions of years.

In prehistoric South America,

the grasslands played host to the same battle...

but with very different animals.

This giant armadillo...

is also a mammal...

though one the size of a car.

Doedicurus may not be fast,

but what they lack in speed...

...they make up for with armor and a lethal tail...

...adaptations that protect them and their young

from any would-be predators.

A Smilodon...

but a mere youngster.

He's not a serious threat...

...even with his dagger-like teeth.

He's never seen a Doedicurus up close,

and their defenses are intriguing.

His curiosity gets the better of him.

He explores every angle.

For the young Doedicurus, the only option is to stay put.

The giant armadillos' armor is completely impregnable...

...and they know it.

But there is one obvious opportunity.

The young Doedicurus is so much smaller.

If he can get a hold on it...

...he might be able to turn it over.

But not with the adults around.

Another stalemate.

Battles between the mammals have always driven their evolution,

and they don't just happen in the grasslands.

Fifty million years ago,

what are now India and Asia collided on an epic scale.

The crash forced the land into the sky,

creating the towering Himalayas.

Today, they are 2,500 kilometers long,

almost nine kilometers high,

and still growing.

With their freezing temperatures and thin air,

these mountains are uninhabitable for most animals.

But not all.

Deep in a sunless valley...

a male snow leopard...

and an ibex that has fallen to its death.

The leopard is very old,

worn and tattered by his harsh mountain life.

At these altitudes, mammals must feed regularly to keep warm.

But the Himalayas present the ultimate hunting challenge.

High above the valley, a female...

and her cubs, fathered by the male.

Three of them in total,

all amazingly camouflaged.

Snow leopards have thick fur,

large paws,

and long tails...

perfect adaptations for the coldest,

most precipitous nursery on Earth.

Away from the dangerous cliffs...

...the mother initiates a hunting lesson.

Teaching and caring for one's young

is a key mammalian trait.

These cubs will stay with their mother for nearly two years...

learning the skills needed to survive on the very edge of life.

At these altitudes, every opportunity must be taken.

The old male is tracking a herd.

In this vertical world, he must get above his prey...

...and close enough...

for an ambush.

This time

death has claimed them both.

But the leopard's legacy lives on.

In a few months' time,

his cubs will leave their mother and start to hunt alone,

having learned all the skills they need

to survive at the top of the world.

Although they had conquered the land,

the mammals didn't stop there.

One branched out and went in an entirely different direction.

Fifty million years ago, bats opened up a whole new realm...

through flight.

Flying allows animals to search much further for food...

and gather in their millions.

Today, one in five of all mammal species is a bat.

But bats were not the first animals to take to the skies.

Having spent the night foraging,

these fruit bats are in a race to return to the roost...

...for with daylight comes danger.

Birds.

They evolved flight long before the mammals.

Faster and stronger than bats...

eagles are armed with deadly talons.

But as dusk falls,

the skies belong to the bats once more.

Millions take to the wing,

knowing that safety lies in numbers

and in the coming darkness.

At the same time as bats were first moving into the air,

other mammals were moving into the sea...

...attracted by the abundance of food hidden beneath the waves.

The mammals' journey into the oceans was a gradual one.

These are Maiacetus,

two-and-a-half-meter-long ancestors of the whales.

But unlike whales, they still spend much of their time lazing on land.

It's in the water that they come alive...

chasing schools of fish.

His time down here is limited by how long he can hold his breath.

He seems caught between two worlds,

especially when facing danger.

The mammal's intelligence is his only defense...

but he can't stay down here forever.

Otodus.

Twice the length of a great white shark.

With finely-tuned senses, it can pinpoint his exact location,

and it's in no hurry to make its move.

The Maiacetus is running out of air.

In open water, there's no escape.

An overhang offers refuge.

But time is running out.

A gully provides cover.

Safe

and able to breathe once more.

Mammals had a long way to go to become masters of the world's oceans,

but it was only a matter of time.

As the planet continued to cool,

it fed an explosion of life...

allowing the giants of today to evolve.

The mammals had gone from tiny land animals...

to being bigger than dinosaurs.

With each giant breath,

they use oxygen far more effectively than Maiacetus.

And being warm-blooded,

their muscles are always at a temperature for maximum performance...

...allowing the whales to generate sustained bursts of energy.

When competing for a mate,

humpback males exploit this ability to the full...

pursuing a single female.

This is much more than about speed.

The males stay as close to the female as possible

so she can judge their strength and stamina.

Bubble streaming intimidates rivals.

The males plow into each other.

Some get left behind,

still fighting.

The main group keeps up the chase...

hot on the tail of the female.

Then suddenly,

she stops.

Hanging in the water...

she has called off the race.

She has made her decision...

and allows one male...

to approach.

The mammals have conquered every habitat on Earth.

Land, air, and sea.

The asteroid strike had allowed the mammals to go from lowly underdogs...

to become a global power.

Such is the role of chance in the history of life on our planet.

But there is one more chapter in the mammal story still to be told.

Two and a half million years ago,

the mammals were faced with a new challenge.

What had been a slow cooling of the planet intensified.

The start of an ice age.

From its frozen wastes would come a new force...

a mammal armed with more intelligence and adaptability than any before.

The most dangerous animal the world had ever seen.