After millions of years of evolution,
our distant ancestors have filled the oceans
and crawled out onto land.
They have survived the giant insects of the swamps
and mushroomed into massive reptiles, armed to the teeth.
But now the war they've waged so long
is much more than a battle between predator and prey.
The whole planet is entering a different kind of crisis,
one that will destroy most of these monster reptiles
and wipe out 90 per cent of life on earth.
All the continents on earth have now drifted together
to create on giant land mass called Pangaea.
And at its centre lies the largest desert the world has ever seen.
It's a brutal world where animals have to be especially tough.
Perfect for reptiles.
This is a one-ton Siberian Scutosaurus,
a distant ancestor of turtles.
Although he has no shell, his back is covered in hard, bony plates.
And just as well: these sand dunes hide a fearsome predator.
Scutosaurus normally travel in herds.
But this old male's got left behind, and his keen nose senses danger.
Once he tires, he's vulnerable.
His attacker is a carnivorous Gorgonopsid.
She's fast, powerful, and equipped
with deadly weapons no hunter has had before.
Exhausted, trapped and weakened by his wound,
the old Scutosaurus has no fight left.
All he can do is wait for the killer blow.
The Gorgonopsid's huge jaws hold a pair of fangs 12 centimetres long.
She has evolved the world's first sabre teeth -
a lethal weapon some mammal predators will later use.
In this harsh landscape,
the reptile's biggest battle is now with the elements.
A massive surge in volcanic activity is beginning to superheat the atmosphere,
creating the highest temperature life on earth has ever known.
Each year the deserts spread.
Even here in Siberia, far north of the equator,
animals are forced to cluster around any lasting source of water,
no matter how small.
The huge female Gorgonopsid has come looking for a drink to wash down the Scutosaurus,
and even smaller Gorgonopsids will do well to keep out of her way.
Five metres long from nose to tail,
she's the top predator on earth.
There'll be nothing to rival her
until the dinosaurs evolve in 30 million years' time.
But unlike the dinosaurs,
these early reptiles are more closely linked to our ancestors,
mammals, than to lizards and crocodiles.
They don't have fur,
but they've already evolved scent glands in their skin.
This way, everyone can tell when the giant Gorgonopsid is around.
Despite the presence of these heavyweights,
smaller creatures also eke out a living around the waterhole.
Diictodon, a hardly little burrower, just half a metre long.
They live as pairs in spiral burrows which remain cool even in the desert heat.
Like the large predators outside,
Diictodon are distant reptile relatives of mammals,
and although it will be 30 million years before the first true mammals appear,
there are already family resemblances.
In particular, his hearing is linked to tiny bones in his lower jaw.
One day these will evolve into our middle ear bones,
something no reptile has.
Several pairs of Diictodon live around the waterhole,
but this is not a colony,
they compete furiously for the sparse vegetation that is their food.
This frequently leads to confrontation.
These clashes are one reason
why both males and females sport a pair of short tusks.
There is, however, one big advantage to living in a crowd:
if a neighbour spots danger, everyone soon knows about it.
A Gorgonopsid has come to investigate.
A single snap of those enormous jaws would crush Diictodon -
if they could get a grip.
But it's not always size that counts.
These tiny mouthfuls are more trouble than they're worth.
The Gorgonopsid knows that if she hangs around the waterhole for long enough,
eventually larger prey will come to her.
There is another, unseen, resident of this waterhole.
A monster much less suited than the reptiles to the changing climate.
A fugitive from when this desert world was lush and green.
It lies in ambush underwater, like a crocodile.
A giant amphibian Labyrinthodont.
But pound for pound, this female's picked the wrong target.
The amphibian's ambush attempt was probably driven by sheer desperation.
For generations this waterhole has supported a wide range of animals,
including several other Labyrinthodonts.
But this one is the last,
and as the water she depends on disappears, she's trapped.
The wet season arrives, but there's no rain.
With life under such pressure,
species are dying out at a rate
that won't be matched until humans evolve in 250 million years' time.
The waterhole is now no more than a large puddle.
Diictodon are more adaptable than most.
They can dig deeper to escape the heat,
carving out tunnels in the rock-hard mud.
But the giant amphibian is in big trouble,
and as she grows weaker, her enemy watches and waits.
It's a deadly game of cat and mouse.
But life here is about to change for everyone.
A migrating herd of Scutosaurus has sniffed out the waterhole.
It's possible the smelt the water several miles away,
and this could well be the first drink they've had in six months.
The desert plants are a poor source of nutrients,
but these large reptiles are designed to squeeze out every ounce,
although they cannot chew.
Their huge stomachs hold stones they've picked up from the desert.
These help grind the vegetation so their digestive juices can get to work.
The tough little Diictodons are used to dodging danger.
But the Gorgonopsid now keeps her distance.
Even she dare not attack such a large armoured herd.
It's a standoff.
It turns out the Scutosaurus are a bigger threat to life
around the waterhole than the predators.
Within a few days they have drunk it dry.
And moved on once again in their eternal search for food.
They represent the plight of all creatures on the continent of Pangaea.
The global drought is now obliterating millions of years of evolution.
Life everywhere is on the brink.
For animals that do not migrate,
the situation's reaching a crisis point.
Hunger drives the Gorgonopsid back to the Diictodon -
and this time she tries to dig them out.
But the Diictodon's hard work pays off.
Their tunnel in the sun-baked earth is deep enough to keep them safe.
Instead, the Gorgonopsid's nose leads her back to the dried-up waterhole.
She senses things are not as lifeless as they seem.
The giant amphibian.
Just before the waterhole dried up,
she must have burrowed into the damp mud,
forming a cocoon around herself,
in a last-ditch attempt to sit out the drought.
But in her torpid state,
she's helpless as the Gorgonopsid feasts.
Eventually storm clouds gather,
but instead of rainwater they bring sand.
The sandstorm sweeps in,
burying the waterhole completely.
The vast Pangaean deserts continue to spread,
snuffing out entire ecosystems.
Even the mighty Gorgonopsid has at last succumbed.
With no prey, even she could not survive.
Mummified in the sand,
this monster will soon be nothing but dust.
Over the next few million years,
the earth will suffocate under a stagnant blanket of heat,
sparking a mass extinction,
even greater than the one that will destroy the dinosaurs.
The future belongs to a tiny group of survivors,
small and adaptable enough to sit it out.
The Diictodon carry on digging throughout the drought.
This pair has unearthed plant tubers,
which hold enough water and nutrients to keep them going for several months.
As millions of years go by,
the climate shifts again,
and the reptiles are among the first to recover and repopulate the empty earth.
From creatures like little Diictodon, larger, stronger herbivores evolve.
These tough forerunners of mammals seem poised to seize control for good,
but they are in fact set to play out their final scene.
The dawn of the Triassic era,
and the earth has only just begun to show signs of recovery.
The deserts have stopped growing,
and huge forests of primitive conifer start to re-colonise the land.
In these new forests, plant eaters recover first.
One in particular. Like most of the large reptiles,
they may resemble dinosaurs,
but Lystrosaurus are more closely linked to mammals - and to us.
Astonishingly, their vast herds make up more than half of all life on earth.
Never again will a single species do so well.
Deep within the forests, though, a rival is evolving.
A new type of animal, destined to change the face of life on earth.
Meet Euparkeria, a tiny insect-eater.
He may not look much of a threat to the dominant reptiles,
but the key to his success is in his hip.
The way his thigh bone is attached
allows him to run on two legs, freeing his hands.
This kind of agility has never been seen before in reptiles,
and gives Euparkeria an edge.
Euparkeria will lay the foundations for a new group of reptiles - the dinosaurs.
Giants such as Tyrannosaurus and Diplodocus can all trace
their family tree back to this little insect hunter in the Triassic forests.
The monster herds of Lystrosaurus hoover up so much vegetation
that they constantly migrate to find fresh food.
They follow the seasonal rains,
and each year in Antarctica this brings them to the edge of a huge escarpment.
The only way down for these cumbersome animals is through a deep ravine.
The herd moves mostly at night, and come sunset,
they begin the dangerous descent.
It will take several hours for all of them to pass through the ravine,
and in some parts there's only space to move in single file,
which leaves them very vulnerable.
Lystrosaurus have sharp eyesight and a very good sense of smell.
But in these cramped conditions it's hard to spot trouble ahead.
They're being watched by a vicious nocturnal hunter - a therocephalian.
Lystrosaurus are strong opponents,
but this hunter doesn't rely on power to make a kill.
The attack is swift, and seems to have been unsuccessful.
But this predator's secret weapon is a poisonous bite.
Its venom is more lethal than a black mamba's,
and it quickly floods the Lystrosaurus's blood stream.
All the therocephalians have to do is wait for it to take effect.
The victim collapses, but the herd moves on without concern.
They have no social bonds.
The therocephalians close in for the kill.
Dawn at the foot of the escarpment,
and here the Lystrosaurus will face their biggest challenge yet.
They need to cross a river, but its banks are lined with open jaws.
Chasmatosaurs are the earliest ancestors of crocodiles and alligators.
Their strange overbite means once they get their teeth into a victim,
there is no chance of escape.
They don't normally gather in such numbers,
but they're here for a special event - the annual migration of the Lystrosaurus.
If you're a Euparkeria, the river's not a great place for hunting insects.
But he has speed on his side.
As the sun rises, the Chasmatosaurs are drawn towards the water.
Some of them haven't eaten
since the Lystrosaurus last passed this way a year ago.
Now they sense it's time to feed again.
The front of the vast Lystrosaurus herd finally reaches the river.
They're strong swimmers, but drowning is not the danger here.
They linger at the river's edge, reluctant to go in.
For reptiles, they have relatively large brains,
and they remember what happened last year.
But with more Lystrosaurus arriving all the time,
the backlog will soon force them in.
On the other side, lush grazing beckons,
and eventually the first one takes the plunge.
A strong swimmer, he quickly makes it to the other side.
But with the steep banks,
he's still got to find a place where he can get ashore,
and every extra moment in the water increases the risk.
More and more Lystrosaurus warily strike out for the other side.
But now the predators are homing in.
The first swimmer has made it, and clambers ashore.
But many of the herd now entering the water will not be so lucky.
Despite the carnage,
the herd is soon reforming on the other side.
The predators are quickly sated and exhausted.
Although many Lystrosaurus are dead,
it's still only a small proportion of the herd.
The dominance of these strange mammal-like reptiles is short-lived.
As the plant recovers, so does competition,
and the brief glory days of Lystrosaurus will soon be gone.
This is followed by a decline in all mammal-like reptiles,
consigning our ancestors to the shadows for millions of years.
Instead, it is creatures like Euparkeria that are about to usher in a new golden age.
These specialists on two legs will provide a cornerstone for
one of the greatest dynasties the world will ever see.
Their descendants will become the dominant life form on earth
for more than 170 million years,
and be known as the most notorious monsters of them all.
Welcome to the Age of Dinosaurs.