For the last 50 million years, the world has been getting slowly colder.
But something else has now helped tip it over the edge -
a change in the Earth's orbit is taking it further from the sun.
This has brought on an ice age.
Conditions for all life become extremely hazardous -
even for the mighty mammoths.
The early autumn snowfall has concealed a barely frozen pond.
This young female has fallen through the ice and is trapped.
Such are the bonds between mammoths
that the herd do not leave their stranded sister.
They stay nearby, distressed but powerless except to comfort her.
Before long, the scavengers have started to gather.
It could take days for the female to die and the herd cannot afford to wait.
Eventually, they are forced to abandon her.
They must keep moving to escape the coming ice age winter.
For the few months of summer,
there is little here to suggest that the world is in the middle of an ice age.
An endless sea of grass and flowers supports a variety of animals,
but just a few metres down, the ground is frozen solid all year round.
There is now so much water frozen at the poles that sea levels have fallen dramatically.
If it wasn't an ice age, this grassland would be the bottom of the North Sea.
As it is, it's a magnet for millions of grazing animals during the summer.
The biggest grass eaters of them all are the mammoths.
This programme will be following one particular herd
in their annual struggle against the elements.
Like all mammoth herds, this one is mostly female,
since males leave at adolescence.
There are five adult females, a ten-year-old male
and smallest of all, a male calf just six months old.
Born in the spring, he has yet to experience how bad a winter can be.
The eldest member is the matriarch.
She is a veteran of some 40 winters
and it is her experience that ensures the survival of the herd
when the going gets tough.
Mammoths are just one of many herbivores to enjoy the summer bounty.
They are all animals in their own way built for the cold.
The mammoths and bison have their characteristic shaggy coats...
while the Saiga antelopes have another adaptation -
broad noses help them warm up the air before they breathe it.
With so much game, it is not surprising there are also predators.
Humans hunt on these plains in summer.
They have no physical adaptation to cope with the cold
but instead wear animal hides and weave cloth to protect themselves.
These predators specialise in small prey
and seldom hunt anything as big and dangerous as the mammoths,
but mammoths are nonetheless very important to them.
With so little wood on parts of the plain,
they collect the tusks of dead animals to build huts,
and even use their bones for fuel.
These most inventive of creatures
are capable of making the most of every resource available.
Of all the summer visitors, the most numerous are the flies,
which are a constant source of irritation for the mammoths.
The humans have learnt to do something about them.
They grind up a paste with the mineral ochre which helps keep the insects at bay.
Come the autumn, this plain will become a freezing hell-hole,
so the mammoths make the most of the summer while they can.
They eat up to 180 kilograms of grass a day,
while the males mate with as many females as possible.
Early autumn, and the big freeze has begun.
The mammoths are still on the northern plains.
These creatures evolved from hairless elephants in Africa,
but have become living fortresses against the cold.
Under their skin is a layer of fat ten centimetres thick for insulation.
Their small ears and short tails prevent them losing too much heat.
On top of all this they have their characteristic carpet of hair,
which has now grown to a metre in length
as their bodies respond to the drop in temperatures.
As the weeks pass, the true savagery of the climate begins to bite.
The humans have already left the plains
to seek shelter and milder weather further south.
Soon the mammoths, too, will go.
With little grass available
and no protection from a windchill that will average minus 50 degrees centigrade,
even THEY have to move south.
This ice age is in fact only one of many.
Two and a half million years ago,
the Earth's climate started on a rollercoaster of warmer and colder periods.
There have been almost 50 ice ages so far, but this is the coldest yet.
(TRUMPETING ROAR)
The matriarch decides it is time to head off towards the less exposed valleys of the Alps.
It is a 400 kilometre journey that they are driven to every year.
For the Megaloceros, autumn is not a time to travel, it is a time to rut.
These males are fighting for the harem of females near by.
Winner takes all.
Megaloceros are a giant form of deer
and the males sport an impressive set of antlers.
Each one is as long as a person.
It's a wonder they can even hold their heads up.
Exhausted from fighting, neither has noticed they are in a trap.
Humans are predators unlike any before.
They use not strength but strategy, pushing the Megaloceros into the trees,
where their antlers prevent escape.
Only one of them has the energy to break out.
The humans will have to take what they want as quickly as possible
before ice age scavengers like wolves, lions and hyenas
make this a dangerous place to be.
Halfway through their journey and the mammoths are progressing well,
but for one member of the herd it is a constant struggle - the youngest.
This is his first winter.
Many yearlings never see their second.
They soon catch up with other migrating animals.
Humans are also heading for the Alps,
where the extensive cave networks make perfect winter retreats.
The matriarch keeps a wary eye on them
and the humans know better than to get too close.
The days pass and the calf is starting to get into trouble.
He is having difficulty keeping up.
His mother stays with him
but the herd cannot wait and the pair are starting to fall behind.
On this snowscape, a lion might seem out of place,
but they are common in Europe at this time,
and a baby mammoth in trouble
is just the sort of thing to bring this cave lion out of its den.
Defended by a full-grown adult, the baby is relatively safe,
but the lion continues to stalk, hoping for the mother's guard to slip.
The pair have now completely lost the herd.
Across the landscape, other mammoths are making the same journey.
This is an adult male and he travels alone.
There are lions in his way, feasting on a straggler.
In this case, not a mammoth calf but a human.
(TRUMPETING ROAR)
The lions will have to let him pass.
Nothing gets in the way of a mammoth migrating.
Mammoths are sociable animals
and, as the forests around them become denser,
so the small herds start to join up.
Soon, hundreds of mammoths are winding their way
through the wintry lowlands of Europe.
As the mammoths near the Alsace region,
the terrain changes dramatically.
Flat plains give way to hills and valleys.
It is here that lives another species of human - the Neanderthals.
They are much shorter than the other humans,
and, like other ice age animals,
they have big noses to warm the air they breathe.
Rather than migrate, they stay within the same valleys all year long,
weathering out the worst of the winter in shallow caves.
But with this recent ice age, their numbers have plummeted.
This is now an endangered species.
Another animal that doesn't travel south in winter is the woolly rhino.
They are very short-sighted,
and this huge male has not even noticed the Neanderthal 50 metres away.
For a while, the Neanderthal doesn't notice the rhino either.
(RHINO GRUNTS)
Woolly rhinos are easily startled
and will charge at anything that surprises them.
Even at this distance he won't see the Neanderthal,
but his sense of smell is better than his sight.
The rhino has picked up the scent.
Aagh!
It is fortunate for the Neanderthal
that his species is stronger and more heavily built than the other humans.
Thick bones and a barrel chest help him deal with a very tough lifestyle.
Having travelled almost 400 kilometres in two weeks,
the herd at last arrives at the foothills of the Alps.
Here in the sheltered forests and valleys, they will spend their winter.
Here there is protection from the wind as well as food.
Thanks to their highly dextrous trunks,
these grassland animals can sustain themselves on trees like these spruce
when their usual food is buried under thick snow.
(DISTANT TRUMPETING CALL)
A week later, and the matriarch hears a familiar call.
The calf and his mother have at last caught up with the rest of the herd.
Touching each other with their trunks,
members of the herd renew their bonds with the pair.
The most hazardous part of the winter is over and the calf has survived.
In an ice age, spring comes late.
It is May before things have warmed up enough
for the snow to melt and a carpet of flowers to spring up.
With the increase in temperature,
the woolly rhinos have moulted and are not as woolly as they were.
The mammoths too have shed their long winter coats,
to leave a lighter underwool only a few centimetres long.
The calf has come through the winter in good health
and, now he has been weaned, spends more time away from his mother.
He is also no longer the youngest.
The herd's latest member arrived just a few weeks ago.
It's a little female.
The rhinos meanwhile are getting feisty again.
These two males are fighting for territory.
But these confrontations aren't always violent.
They scrape their horns on the ground until one of them backs down
and leaves the area to the victor.
But while winter may be over here,
the northern plains are still defrosting
and so the herd will spend a couple of weeks in the valleys yet.
The newest arrival will get some strength in her legs
before the journey back north.
There is a downside to all this warmer weather -
insects are becoming a problem again.
But where there's mud...
there's a way.
A good shower of mud can help prevent the insects biting.
Or maybe mammoths just do this for the fun of it.
Like autumn in reverse,
the great mammoth caravans form once more as it's time to head north.
Soon the mountains are behind them,
but they are coming into dangerous territory.
The Neanderthals are waiting.
For them, the mammoths coming north
is one of the most important times of the year.
They are about to risk life and limb to hunt them.
To take on a mammoth, they need every advantage they can get.
They wait for dusk, when the cliff will become less visible
and they can make effective use of their secret weapon...
fire.
The Neanderthals descend to the bottom of the cliff to butcher their kills.
One of the mammoths is still alive.
(MAMMOTH BELLOWS)
The Neanderthals are supreme hunters,
which is why they have lasted over 260,000 years,
but they have a weakness - they do not adapt well to change.
As a result of the harsher climate and the arrival of other humans,
Neanderthals are fast disappearing.
In another 2,000 years, they'll be gone altogether.
The herd eventually arrive back on the northern plains.
Fortunately, both of the youngsters have survived the journey.
The herd has nonetheless suffered a major loss.
The matriarch is missing, killed when the Neanderthals attacked.
It is now her sister that leads the herd into the future.
Mammoths will last another 24,000 years.
They are superbly adapted for this cold world,
but when it gets warm again,
the grassy plains they depend on will disappear.
The humans have a different future ahead of them.
When change comes, they will be able to deal with it
because they can adapt and have the ability to shape the world around them.
One day...
they'll look back on all this.
We have since built museums to celebrate the past,
and spent decades studying prehistoric lives.
And, if all this has taught us anything, it's this -
no species lasts forever.