Out of the darkness emerges a 12-tonne giant.
She is an Indricothere,
and she is about to give birth.
She seeks out a secluded spot in a canyon
where she hopes the newborn will be safe from predators.
It is now just before dawn,
and at last, her two-year pregnancy is coming to an end.
The calf is a male and he is already in trouble.
The noise and smell of the mother's labour
has attracted the predators Hyaenodon.
Despite the name, these are not relatives of hyenas.
These are more dangerous by far.
Next to the Indricothere, they may look small,
but these carnivores are as big as rhinos,
and easily capable of killing the calf
if they can get past his mother.
She desperately tries to keep the baby between her legs
where she can defend him with powerful kicks.
But the Hyaenodon keep coming and coming.
This calf will be lucky to see his first sunrise.
It is a time called the Oligocene.
The world has just been through years of catastrophic climate change
that led to the extinction of one species in five.
Earth has since bounced back, but in the process,
it's become a changed world -
different weather, different plants and different animals.
Here in Mongolia, some animals have grown enormous.
This programme will follow the fate of just one of these giants -
a baby Indricothere.
During the night, this canyon was the scene of his dramatic birth,
but it seems his mother saw off the Hyaenodon. He is a very lucky calf.
As a newborn, he already weighs a quarter of a tonne,
and his legs aren't yet used to bearing any weight at all,
so he'll spend the rest of the morning learning how to walk.
Around him, the other residents of the canyon are starting to stir -
strange beasts like the Chalicotheres.
Almost three metres tall,
they walk on their knuckles to protect their long claws.
These powerful limbs are very effective defensive weapons,
but more often, these herbivores use them to hook down branches
to get at the softest leaves.
They walk like gorillas and eat like pandas,
but their closest modern relatives are actually horses.
The newborn calf also has food on his mind.
This is the beginning of the most vulnerable period of his life.
He'll depend on his mother for at least three years.
He needs her protection, and for the first year he relies on her milk.
This is an astonishingly long commitment for any mother,
but if he can just survive that long,
his size will mean that no predator on Earth can touch him.
In the Oligocene, large parts of the world have become highly seasonal.
Here in Mongolia, long dry seasons
are broken by short violent wet seasons.
The climate has shaped not only the environment, but also the animals.
It is survival of the meanest,
and they don't come much meaner than this -
a giant Entelodont,
the bully of the plains.
These are distant relatives of the pig.
Two metres tall, aggressive,
and built like tanks,
but with a brain no bigger than an orange.
The Bear Dog mother and cubs retreat to a safer distance,
but it isn't going to be their day.
Although these relatives of both bears and dogs
are vicious little predators, at the waterhole it's size that matters.
The mother Indricothere has also come here to drink,
but her calf is nowhere to be seen.
A second male Entelodont arrives at the waterhole.
During the mating season, they get very aggressive towards each other
as they try to establish who is top hog.
Entelodonts are their own worst enemy.
(GROWLING ROAR)
It is enough to put off even an Indricothere.
The Entelodont's fearsome set of teeth have inflicted deep wounds
on the loser.
Back in the canyon, the calf lies motionless.
He is playing dead -
until disturbed, that is.
(TWIG SNAPS)
This is the young calf's survival strategy.
In his mother's absence, he instinctively lies still
to avoid the attentions of predators.
Only when she eventually returns does he come out of hiding.
He remains within the comparative safety of the canyon another week,
but then his mother decides it is time to leave.
The calf is now strong and sure-footed enough
to venture out onto the open plains
where he will spend the rest of his life.
Not since the dinosaurs has nature got this big.
Indricotheres are related to rhinos,
but their size is in a different league.
A fully-grown male stands over seven metres tall
and weighs in at 15 tonnes. That's equivalent to eight modern rhinos.
No other land animal even comes close.
Being big not only protects them from predators,
but in this dry land, allows them to go without food or water for days.
The calf is eager to explore this big new world...
but his first encounter is a shock for him.
An aggressive Bear Dog has no time for his curiosity.
Her cubs are nearby,
and to her the gawky calf is a threat.
It doesn't take much to send him running back to his mother.
Right from the beginning, the calf starts copying his mother
to learn how to deal with the world.
So when she arrives in her usual feeding area
and sniffs a pile of dung, he does the same.
One day he will understand
that since every Indricothere has a unique odour,
this is how his mother tells which other animals are in the area.
His lessons in survival have begun.
Nearby is a juvenile Indricothere, just over three years old.
This is no stranger, but his mother's previous calf.
However, she has no time for him now and violently repels him.
He doesn't know it, but our calf is looking at his own future.
The long dry season is just starting,
but there is still plenty of food.
In another month or so, the situation will be different,
and so animals are gorging themselves while the going is good.
As the mother browses, she has the tops of the trees to herself.
It's another advantage of being big.
Only two months old, the calf doesn't need to eat -
he is still living off his mother's milk -
but it doesn't stop him trying to imitate her.
This is how calves learn which parts are safe to eat.
However, this is a bad time for him to wander off.
Just 20 metres away, a Hyaenodon is stalking.
Hyaenodon's jaws have a bone-shattering force
of over 1,000 pounds per square inch.
One bite breaks the Chalicothere's neck.
They are capable of eating everything on a carcass,
even the teeth.
But he isn't going to get the chance.
The local bullies have arrived and he is outnumbered.
In a futile attempt to hide the smell of the carcass,
the Hyaenodon defecates on it,
but it will take more than that to put off the Entelodonts.
The hogs from hell have won the prize.
The dry season has transformed the landscape.
It's as if the whole life-blood has drained out of the plains.
This is what it means to be a seasonal world -
it hasn't rained for months, there is no greenery left to eat,
and precious little to drink.
Here in Mongolia, mammals have evolved to cope
with even these kinds of conditions,
but at this time of year they are put under their greatest stress.
The mother Indricothere has not found water for several days,
and in her dehydrated state she is not producing milk.
The calf is now 9 months old,
and it is his first taste of true hardship.
Despite his repeated attempts to suckle, the mother rejects him.
Because of their size, adults can survive without water for days,
but the calf, being smaller, has fewer reserves to draw on
and is getting dangerously weak.
The nights are cooler and a far better time for them to move around,
so every evening they set off in search of water.
Other Indricotheres are on the same journey,
and tonight the pair is joined by an older female.
Indricotheres can easily live into their eighties,
and this longevity gives them
a unique knowledge of their environment.
If there is any water nearby,
it is a good bet the old female knows where it is.
At last they find the muddy remnant of a lake,
and just in time, because the calf is close to collapse.
He will have to wait a while for his milk,
but at least he can now quench his thirst.
It is the first night for some time
that these giants can lie down and get a proper sleep.
The next morning, the mother receives an early call.
The calf is still starving.
But this time she responds
and he is rewarded with milk.
When the rains eventually come, they come down hard.
From parched plains...
to a slippery quagmire -
everything changes...
in an instant.
But the sudden downpours bring disaster for some.
This Bear Dog's den
has been undermined by the flood waters and collapsed.
An anxious mother is forced to dig to find her cubs.
She finds only one,
and it is already dead.
There is a river now where none existed before.
In their search for food, the pair must cross it.
This is a whole new ordeal for the calf,
and at first he is very reluctant to follow his mum into the water.
In the end though,
it is obvious that he is more frightened of being left behind
than he is of crossing the river.
This is the first swim of his life.
Unfortunately, it turns out to be the easy bit.
When the calf gets to the other side,
he still has to get to the top of the opposite bank.
It is steep and slippery,
and, unlike his mum, he isn't going to make it in one small step.
But there is nothing his mother can do to help him.
He is on his own...
and he doesn't like it.
Not an easy task for such a heavy baby.
The drought might have been a tough time, but in many ways,
the coming of the rains marks the end of the good life for the calf.
It is time for him to feed himself.
Indricothere calves are weaned off milk at about a year old,
but having known little else but milk for his whole life,
he needs quite a lot of persuading to give it up.
Try as he might to get his favourite tipple,
Mum is having none of it. He will have to eat his greens
or go without.
After the long dry season,
colour slowly returns to the plains
as the cycle of life starts over again.
And for the Indricotheres, it is time for mating.
A young male has caught the scent of a receptive female.
It is the calf's mother, who is fertile again
now she has stopped suckling her calf.
The calf plainly has no idea what to make of this approaching colossus,
and at this point his mother shows no interest either.
Then a second, older male appears on the scene.
He emphasises his status by spraying the ground liberally with urine.
The younger male isn't so easily intimidated,
and the two rivals size each other up before starting a brutal contest.
When 30 tonnes' worth of Indricothere fight,
the forces involved are bone-shattering.
The skulls of males are specially built
to withstand these contests.
It takes half an hour of abuse
before the older male backs down.
But with the fight over,
the calf is now at serious risk -
the biggest threat to a young Indricothere is an adult male.
During mating, calves can get trampled to death.
It is the first time that another individual
has come between him and his mother.
In his distress, he tries desperately to get in the way,
to stop the mating.
Over the next 36 hours,
the male will mate with the calf's mother several times,
preventing other males from doing so.
Our calf is in for a tough time.
It's the very beginning of the wet season,
and the Chalicotheres have started to search out new growth.
The mother Indricothere is expecting.
She is, in fact, only days from giving birth.
Standing by her is the calf,
now three years old and weighing over a tonne.
He has learnt many of the tricks of Indricothere life from his mother,
like using trees to scratch the parasites off his back.
However, nothing he has learnt so far
will prepare him for what is about to happen.
In the morning, the calf will suffer the biggest upset of his life.
The mother is behaving very oddly indeed towards her calf...
but she is only doing what she has to do.
She is chasing him away.
To give her unborn calf the best chance in life,
she must sever the 3-year-old bond she has had with the male calf.
It is time for him to make his own way in the world.
This is the hardest lesson of all.
For some time, the calf disappears,
only to turn up a couple of weeks later
alone on a river bank.
The cause of his limp is unclear,
but the most likely explanation
is that he fought with a young adult Indricothere,
one of the few creatures that can threaten him.
He heads back to the only safety he knows.
His mother has a new calf in tow
and her maternal instincts have been transferred to the newborn.
To her he is now just a threat.
He is never again going to have the protection of his mother.
The rest of his life he will spend alone.
One month...two months,
three months go by,
and there is no sign of the calf.
Then suddenly,
one day he reappears, back in his old territory.
His injury has healed and he seems in good health.
But he is dangerously close to an Entelodont.
Any fears about his safety suddenly seemed to be misplaced,
because this Indricothere is now big enough to look after himself.
Next episode, prepare to come face to face with mankind's beginnings
when we go back to Africa three million years ago
to meet a species of ape that walked upright.
Our ancestors lived in a dangerous world.
Unlike us, they weren't predators, they were prey.