In the last episode of Walking with Monsters,
our earliest fish ancestors escaped their armoured enemies,
the Arthropods.
They evolved legs and lungs and took their first steps onto land.
But not far enough from the perils of the water.
Now our amphibian ancestors have made another huge leap forward.
They've evolved into the first reptiles.
But they will face other giants.
Monsters who consider this their turf.
The war for the world has moved on to dry land.
The Arthropods are back.
Three hundred million years ago
much of the Earth was covered in vast swamp forests.
Here, nothing is as it seems.
The fifty metre giants towering above this water world may look like trees,
but they're actually distant relatives of ferns.
Even the air is alien.
Over a third is oxygen.
Much higher than at any time in history.
This oxygen rich atmosphere has fuelled the growth of new,
super sized Arthropod predators.
This Mesothelae spider is the size of a human head.
She'd be hunting cats if she were alive today.
She's an ambush predator.
Operating from her underground headquarters.
Safe in her borrow,
she rests her feet on carefully constructed trip lines.
These allow her to pick up vibrations from the outside world.
Reptile,
Petrolacosaurus is hunting on the forest floor.
Unlike our amphibian ancestors,
he has tough scaly skin which traps moisture inside his body.
Vital for all land dwellers.
Because he doesn't dry out in the sun,
he can venture away from water.
But that means encountering new predators.
The reptile outruns the spider.
He's evolved a complex heart,
much more efficient than his enemy's.
In fact the template for our own.
It's a powerful pump that pushes blood and oxygen around his body to his muscles,
so he can run at high speed without stopping for a rest.
This time though he's trapped inside a log.
At first the spider's armour stops her squeezing in.
But she's not going to give up so easily.
Back in her lair,
Mesothelae will inject her victim with digestive juices
to dissolve him from inside.
There are other dangers in the swamp apart from predators.
The water levels can change rapidly.
They've risen.
The spider drags her dinner home, but something's wrong.
Her headquarters are flooding.
She'll have to evacuate.
She can't dig, so she'll have to find another hole she can adapt.
And once outside, she's vulnerable to larger predators.
Meganeurid is a monster dragon fly,
queen of the carboniferous skies.
With a wing span of almost a metre,
she's the size of an eagle with an appetite to match.
High above her carboniferous kingdom,
normally she's invincible.
But trouble's brewing.
The flood below is due to an approaching storm.
The super oxygenated atmosphere,
which has helped fuel the growth of the insects,
the Arthropods, is highly volatile.
A lightning strike could trigger an explosion.
The clock is ticking for the swamp.
The rising water levels suit one group of hunters.
Amphibians.
They've continued thriving over the last sixty million years.
Their thin skin still restrict them to the water's edge,
but they're now powerful predators with a devastating pair of jaws,
ready to ambush anything that wanders within reach.
The homeless spider tries her luck further inland.
But other Mesothelae fill the forest floor
and they don't like to share.
Neither does Arthropleura.
He's a distant relative of modern millipedes,
but as long as a car.
He can rear up, tall enough to look you right in the eye.
Although he's vegetarian,
his strong jaws could still deliver a nasty bite.
With rising waters,
Arthropleura's foraging leads him into dangerous territory.
In this clash of the titans,
the Arthropod's armour is his best defence.
Unless it gets split.
Impaled, his soft innards provide a feast.
The lightning storm is building, moving ever closer.
Just in time, the spider finds another hole.
A Petrolacosaurus is evicted,
but reptiles don't need burrows to avoid danger,
they have speed and stamina on their side.
Meanwhile, the spider starts to turn the hole into a hunter's lair.
Night falls.
And Mesothelae settles into her new silk-lined home
She tests the new trip lines.
They're already picking up vibrations from outside.
The storm is very close now.
Lightning strikes ignite the oxygen rich air.
The giant dragonflies are forced down from the canopy to seek shelter below.
An unexpected windfall for the water predators.
Amid the chaos,
Mesothelae seems safe in her bunker underground.
A huge fire, ignited by lightning,
has devastated Mesothelae's neighbourhood.
There are no signs of life above ground.
However, a Petrolacosaurus managed to outrun the flames.
But now he heads right into the spider's lair.
Mesothelae's hole was at the centre of the lightning strike.
The reptile gets a spider barbecue.
The era of the giant bugs is coming to an end.
The climate is drying and the atmosphere is losing oxygen.
The monster insects and spiders can't survive the change.
But with their efficient hearts and waterproof skin,
our versatile ancestors flourish in the new dry climate.
Evolution takes over.
Lacking restrictive armour
means the reptiles can start to make it big.
Their muscles and bones broaden and extend.
Their bodies mushroom into huge new shapes and forms.
Our distant ancestors have come a long way.
They've now conquered the land.
The world's first giant reptiles have arrived.
But they're not related to dinosaurs.
Their descendents will be of a very different kind.
Reptiles have now spread all across the globe.
Edaphosaurus bask in the early morning sun.
They're three metres long, as big as hippos,
and, like them, they're vegetarians.
Life in their new Permian home is a constant battle with the elements.
The climate swings between seasonal extremes.
From burning summers, to freezing winters.
A new species of plant has evolved,
able to withstand this cold, dry atmosphere.
Primitive conifers.
In this harsh world,
creatures need ways to regulate their body temperature.
Edaphosaurus are sluggish autumn morning chill,
but they'll soon warm up.
Thanks to the huge sail on their backs.
Their backbone has tall spines connected by a thin membrane of blood filled skin.
This large surface area can lose heat fast, if the Edaphosaurus need to cool down.
While to heat up,
they simply turn their sails towards the sun,
like a solar panel.
Amazingly, these strange sailbacks are related to us.
They're part of a very special group of creatures called mammal like reptiles.
It's down to them that we have control over our body temperature today.
But Edphosaurus aren't the only sailbacks,
and now their biggest enemy is one of their own kind.
This is Dimetrodon, a vicious carnivore.
The biggest reptile on Earth.
She'd normally attack adult Edaphosaurus,
but today she wants to avoid injury.
She's pregnant and almost ready to lay her eggs.
She finds a smaller, softer target.
This was only a mock charge,
a tactic to disperse the group and expose the vulnerable young.
The panicking Edaphosaurus flush their sails with blood,
flashing an eye spot to distract the predator's attention
from their vulnerable head.
But Dimetrodon can see through their disguise.
She locks on to her victim.
As well as her sheer size and power,
Dimetrodon has a killer edge.
As a mammal like reptile, she's evolved specialised teeth.
Incisors shear off chunks of flesh.
Serrated canines cut and slice.
We humans have inherited these same meat eating teeth.
We have a smaller version in our mouths today.
The young Edaphosaurus didn't stand a chance.
This is a crucial kill for the Dimetrodon.
Before she lays
she needs to eat enough to last her for the next seven months.
To boost her babies chances of survival,
she's evolved a basic form of parental care.
Guarding her eggs.
But the smell of blood has attracted more Dimetrodon.
Highly aggressive males.
The female knows better than to defend her kill.
By the time the group has finished
there'll be nothing left but bones.
While lions today consume seventy per cent of a carcass,
Dimetrodon eat ninety.
But they'll only eat intestines after shaking out the waste inside.
Dung is one thing a Dimetrodon can't stomach.
The female has built a nesting mound of earth and wood.
Inside, she digs a deep burrow.
And starts to lay her eggs.
She covers her brood carefully, ready to begin her guard duty.
Egg thieves lurk in the shadows.
Like this carnivorous amphibian.
But he's too small to make a full frontal attack.
He'll have to bide his time until the female turns her back.
The mother must also constantly monitor the nest's temperature.
Temperature control is vital for reptiles even before they hatch.
The mother piles on more earth if the babies are too cold.
And will remove a layer if they're too warm, otherwise the babies will die.
It's going to be a long seven months.
As autumn turns to bitterly cold winter,
the landscape is transformed.
Still the female keeps her vigil,
getting weaker all the time from lack of food.
Until finally winter melts into spring.
With the warmer weather,
the Edaphosaurus laze beneath the conifers,
shading their sails from the sun to avoid overheating.
But there's much activity at the nesting mound.
And it's not the mother who's digging.
Another female is desperate to lay her eggs.
If she takes over the mound,
she'll dig out and kill the incubating young already inside.
The resident mother may be weak,
but she won't give up her nest without a fight.
It's a battle neither can afford to lose.
Next morning, just one female remains at the nesting mound.
It's the resident mother. She's managed to wrestle off her rival.
But not without a sacrifice.
She won't be able to defend her eggs or herself for much longer.
And the thieving amphibian is poised to seize his chance.
As summer arrives, the Edaphosaurus are thriving in the heat.
The young have grown and test out their newfound strength.
But the mother Dimetrodon has reached the end of hers.
So weak, she's helpless as another intruder attacks her nest.
Luckily this large male has got his teeth
into something more substantial than her young. The amphibian.
The precious eggs are safe and ready to hatch.
The mother can hear them squeaking.
She's achieved her goal.
But at this moment her maternal bond is severed.
Unlike her mammal descendents, she won't nurture her young.
Her instinct now is to save herself, by finding food.
Without their mother's protection,
the babies will have to be ready to go it alone.
They take their first steps into a perilous world
and must make a mad dash for the safety of the trees.
A danger in a familiar form awaits them.
Adult Dimetrodon are cannibals.
Not all babies are helpless.
Some already instinctively know how to defend themselves.
They roll in dung. Something the adults can't stand.
Repelling their pursuers long enough to get a head start.
One adult is driven on by hunger and desperation.
The mother. She'll weed out weaker babies straight away,
which helps the strong ones and her species as a whole survive.
And now the reptiles evolve to tighten their grip on land.
Their legs straighten and lengthen,
holding them more upright and giving them speed.
Their backbones and muscles get stronger and they even start to look like mammals.
To house a larger brain their skull bones expand into an enormous head.
And out of their draw thrust huge, sharp teeth.
A new age of specialist reptiles had dawned.
Next time on Walking with Monsters,
we enter the world of the first terrifying sabre tooth,
but find it always size that counts.
And we meet one of the most successful reptiles of all time
who face a perilous journey.
Back to the dangers of the water.