When the dinosaurs disappeared,
so, too, did the gigantic marine reptiles
that once terrorised the oceans.
For almost 25 million years,
there was nothing around to eat the sharks.
But there are now awesome new monsters of the deep.
The giant whales have arrived.
Forget the gentle filter feeders of the 21st century.
These days,
every whale is a killer.
It is the late Eocene, and the world is still a hot one.
However, it is dryer than before,
and so, where once the land was covered in lush rain forest,
there are now more open spaces.
Freed from the constraints of living in dense forest,
some mammals have started to get larger.
Here on the scrub plains,
big is beautiful.
These changes have not suited everyone.
The large killer birds are no longer much in evidence,
replaced on most continents by fearsome new mammal predators.
Like dinosaurs before them,
mammals dominate the planet,
but they are about to undergo their severest challenge.
The climate change the world has seen so far
is mild compared to what is coming.
This programme is about the beginning of climate chaos.
The problem starts not on land but in the ocean,
and affects even the mightiest of mammals.
This is the legendary Tethys,
a tropical sea that stretches almost halfway round the world,
and has connected Asia with the Atlantic
since the time of the dinosaurs.
It is now home to one of the mammals' biggest success stories.
Basilosaurus.
18 metres of predatory whale.
Four times the length of a great white shark,
this female weighs 60 tonnes.
Incredible to think, then,
their ancestors were tiny, furry, shrew-like animals
that lived in trees.
At this time of year, off the coast of the northern Tethys,
Basilosaurus gather to mate.
A female is eagerly pursued by several males,
but it is the eldest and biggest male that she chooses to mate with.
Mating is not an easy task for two such huge, free-floating animals.
They need a little extra help.
As the successful male manoeuvres into position,
he calls upon one small legacy of their distant land ancestors.
Basilosaurus retain two tiny back legs.
These are useless for walking or even swimming,
but they help lock their narrow bodies together during mating.
The whales of the future will lose these legs altogether.
The courtship is over,
but as it will turn out, the natural disaster that is looming
is about to make things very tough indeed for marine life.
This female's fate
is being influenced by events far, far away in the Antarctic.
For the first time in hundreds of millions of years,
the sea is freezing at the poles,
throwing ocean currents into turmoil.
For a whale that needs on average 80 kilograms of food a day,
the slightest change in fish stocks is bad news.
She is at the top of a food chain that is about to collapse,
and she has just become pregnant.
The ocean currents are also starting to disrupt the climate,
and have already affected weather patterns along the Tethys coastline.
Many areas used to high rainfall
have suffered prolonged drought this year.
Despite these problems,
life ploughs on as best it can.
But the drought can have some pretty nasty side effects.
This is Andrewsarchus,
a huge carnivore as tall as a horse and weighing close to a tonne.
Normally, he wanders inland in search of food,
but the drought has driven him onto the beach.
Bad news for the exhausted turtle
struggling back to the water after a night laying eggs.
At first, the Andrewsarchus seems unsure of what to do
with these curious shelled creatures.
He is more used to picking over the carcasses of giant herbivores.
Despite appearances,
Andrewsarchus is not related to modern scavengers
like dogs or hyenas.
Bizarrely, he has hooves instead of claws.
In fact, his nearest modern relatives are hoofed animals
like sheep and goats.
He is, in a sense, a sheep in wolf's clothing.
He is also the largest mammal carnivore ever to walk the Earth.
His huge one-metre-long jaws are designed to crush anything,
so the turtle's main defence is of little use.
During droughts, scavengers often do well, but in the long run,
the hoofed predators will not adapt quickly enough
to changes in the climate.
He's the last of a dying breed.
Months have passed, and the erratic ocean currents
have disturbed fish stocks so much
that the female Basilosaurus is searching for food
hundreds of miles from her usual hunting grounds.
Four months pregnant, her situation has become critical.
She is now forced to hunt in the most unlikely places.
Lining the southern edges of the Tethys
are endless expanses of mangrove swamp.
In the Eocene,
it's a vast, thriving network of waterways,
but believe it or not,
this will become one of the driest areas on the planet -
the Sahara Desert.
It's hardly a classic hunting ground for an ocean-going whale,
but she is desperate,
and there is prey here of a sort.
In the labyrinth of tidal channels,
her size is a real handicap,
but hunger draws her in.
Watching from the branches above are primates.
These are Apidium
that live in highly social groups,
and word quickly gets around when a threat is spotted.
(APIDIUM SCREECH)
Other creatures here are oblivious to the new danger.
The amphibious mammal, Moeritherium,
is too large to be bothered by predators such as crocodiles,
so they ignore the chattering primates
and return to the business of eating.
The Apidium move away from the channel,
and continue through the mangroves looking for fruiting trees.
Because different trees fruit at different times,
they often have to cross the waterways - a very risky activity.
There are sharks and crocodiles to worry about,
and now there is a whale as well.
It's a leap of faith, but leaping is one thing Apidium are very good at.
The whale is frustrated,
for the moment.
With the rising tide, however,
some waterways become too wide to jump,
and the Apidium have to find more hazardous ways across,
closer to the water.
(THEY SCREECH)
Today, their nightmare came true.
It was a shark.
They certainly won't cross now.
They'll have to wait for the tide to go down.
By contrast, the water isn't usually dangerous for the Moeritherium.
They spend most of their day here.
Although they are shaped like hippos,
and look a bit like pigs,
Moeritherium are related to neither.
The Moeritherium's nose betrays its true family connection.
The nostrils and lip have joined together
to become one dexterous muscular unit
which helps them forage for food.
This is in fact a type of trunk.
These benign herbivores are early relatives of the elephant.
At around 200 kilos,
they are too big for the sharks.
One Moeritherium heads off for pastures new...
but he is heading straight for the jaws of the female Basilosaurus.
The Moeritherium scrambles onto dry land
just in time.
But the hungry Basilosaurus isn't about to give up yet.
Dry land and safety are only temporary things in the mangroves.
The water still has some way to rise,
and most solid ground will become seabed in just a few hours' time
as high tide sweeps in.
The Basilosaurus will soon be able to reach the stranded Moeritherium.
In her desperation, she has attacked too early and run aground.
By the time the whale has worked herself free,
the Moeritherium has escaped to the shallower channels
where even the starving Basilosaurus won't follow.
For the whale, the mangroves are no better than the open sea.
The Eocene El Niņo continues to wreak havoc.
The weather patterns that animals rely on are confused.
In the northern Tethys, the rains have come,
but six weeks later than usual.
And then, instead of lasting months, they are over in a few weeks.
In the scrubland, the rains have finally provided new growth,
but the damage caused by the prolonged drought
has already been done.
These Brontothere herds in particular have been hit hard.
Though distantly related to horses and rhinos,
they're not much like either.
They are twice as big as modern rhinos,
with brains just one third of the size.
They are not the brightest of beasts.
Still, they are one of the most successful groups of mammals around,
found across the northern hemisphere in herds of hundreds.
This year, though, some herds are in a sorry state.
This should be a time for calving,
but a high proportion are being stillborn.
The few youngsters that have survived the drought
behave as youngsters always do.
These two adolescent males practise for adulthood,
challenging one another over as yet non-existent females.
In another difference to rhinos,
the outgrowths on their noses are not horn or hair but bone.
These are more for show than for head-butting.
The bone crests are far too brittle for that.
The scavengers are having a better time of it.
An Andrewsarchus tries to get at a Brontothere's dead calf.
The female, though, is being fiercely protective.
It is impossible to say
whether this mother understands that her calf is dead.
Like most mammals, she has a very strong bond with her offspring,
and will defend it for as long as she can.
Now a second Andrewsarchus has arrived,
and it is going to be difficult for the mother to keep them both away.
The first Andrewsarchus seizes its chance.
After all day defending a lost cause,
the mother appears to be giving up.
(SNARLING)
The scavengers aren't used to having to share the spoils.
As they wrestle for the calf,
the mother mistakes its movement for signs of life.
She is spurred into action again.
She returns to her lonely and fruitless vigil.
The Andrewsarchus will be back.
(PLAINTIVE LOWING)
As the weeks pass,
the environmental crisis at sea shows no sign of easing.
It can only be described as an ocean famine.
For the female Basilosaurus, who is now heavily pregnant,
things are desperate.
She is eating barely enough for herself, never mind her unborn calf.
With her limited fat reserves all but gone,
her body will soon abort her offspring
in order to preserve her own life.
Suddenly, the sea around her is filled with the bustling activity
of a group of smaller whales called Dorudon.
When Dorudon gather in numbers,
it means that the females are about to give birth.
This might look like a friendly greeting.
It is anything but.
The smaller whales are mobbing the giant,
using their numbers to force her away.
Basilosaurus preys on their young,
and somewhere nearby must be the Dorudon's calving ground.
It appears as though she has been driven to the seabed,
but there is method in her madness.
She has to do something that is more important than ever
if her unborn calf is to survive.
She must scratch.
By dragging herself over a convenient sand bar,
she can slough off the outer layer of her skin
and any parasites or barnacles that may have got a foothold.
For an animal that relies on speed to hunt,
keeping her body sleek and streamlined
is the difference between success and hunger.
She is ready to go hunting whales.
Every year, Dorudon gather in this calm, protected lagoon
to give birth.
The first calves are appearing, just a few days old...
and already in mortal danger.
The Basilosaurus has found the lagoon.
She has been spotted,
and the calves are quickly shepherded away from the threat.
Defending together, the adult Dorudon launch aggressive attacks
against the Basilosaurus in an attempt to drive her off.
Their efforts appear surprisingly effective.
The Basilosaurus moves away.
Calm is restored,
but it won't last for long.
This is a hungry mother.
Over the course of the next few days,
the Basilosaurus returns time after time.
The lagoon is transformed from a sanctuary
into a bloodbath.
This is what it means to be top of the food chain.
It has taken the death of several Dorudon calves
to further the survival of the unborn Basilosaurus.
It has been a year since the female Basilosaurus mated.
She is once again in the open seas.
But this time, she is not alone.
Against all the odds,
she has finally given birth.
Against the odds,
she and her calf have survived.
Sadly, what they have been through
is merely the start of the climate chaos to follow
which will end the Eocene period
and cause the largest extinction
since the death of the dinosaurs.
As devastating as any meteor
will be the catastrophic disruption of the ocean's currents
caused by the gradual freezing of Antarctica.
Like a gigantic El Niņo, only much, much worse,
20 per cent of living things on Earth will die out.
Whales as a group will survive,
but Basilosaurus and her kind
will not be among them.
Next time, mammals on land go from big to bigger.
We will walk with the Indricotheres,
mammals that rival in size the dinosaurs of old.
It is a world of the big,
the bad...
and the ugly.