[thunder rumbles]
[poignant music playing]
[Morgan Freeman] For billions of years,
much of our world was fiercely inhospitable…
[crackling thunder]
…ravaged by the elements and the forces of nature.
[howling wind]
But this is the story of what was happening beneath the waves.
Here, shielded from the storms, life had taken hold…
[rumbling thunder]
…and was about to change our planet forever.
[sinister music playing]
[sinister music intensifies]
[music ends]
[rousing music playing]
[low growling]
[snarling]
[trilling]
[screeching]
[rousing music continues]
[growling]
[wind howling]
[rumbling]
[rousing music intensifies]
[music fades]
[ominous music playing]
Far back in the depths of time,
Earth looked nothing like the world we know today.
Nearly 90% of its surface was water,
yet this was no blue planet.
Instead, vast clouds of methane had turned the atmosphere yellow.
[thunder rumbles]
[explosive blast]
On land, conditions were even worse.
[repeated blasts]
Volcanoes had been erupting for millions of years.
[rumbling, creaking]
Their explosive power,
one of the few sounds in this otherwise silent realm.
[ominous music continues]
With the world made up of nothing but toxic air and rivers of molten rock…
life as we know it was impossible.
[crackling, hissing]
[stillness]
But out in the prehistoric seas,
something is stirring.
[intriguing music playing]
Beneath the surface hangs a great swathe of plankton.
Microscopic cells like these have just evolved something new,
photosynthesis.
This means they can harness the sun's energy to grow,
a breakthrough so important
that plankton will one day give rise to every single plant on Earth.
[intriguing music continues]
But that's not all.
One byproduct of photosynthesis is a gas,
the missing ingredient for animal life to exist…
[rousing music playing]
…oxygen.
For nearly two billion years,
plankton produced so much oxygen that it transformed our planet.
[stirring music playing]
No longer hostile, Earth became a new world,
where life could finally run wild.
[music fades]
[wind blowing]
Today, millions of years later,
it's still possible to get a glimpse of what that next wave of life was like.
Because in some parts of our shallow seas…
life still resembles that more primitive time.
[whimsical music playing]
They may not look like it…
but these are animals…
very similar to the first complex life forms.
Most are soft-bodied and rooted to the seafloor.
All have senses.
And some can even see their surroundings.
[majestic music playing]
Many get their food by filter-feeding,
picking out plankton,
sometimes in the most curious of ways.
[slurps]
[slurping]
But not all animals here are stuck in one place.
Some can move,
exploring the seafloor
like their pioneering relatives millions of years ago.
It may seem like these shallows are a kind of Eden,
a safe haven for life.
But this is no paradise.
[ominous music playing]
Living off modern-day California…
this is a rainbow nudibranch.
And its sluggish nature hides a sinister side…
because, like its ancestors,
it's here to hunt.
It's adapted to a life of predation.
And its prey of choice,
anemones.
But like the very first predators,
the nudibranch has room for improvement…
[tense music playing]
…because here, timing…
is everything.
Luckily for the hunter,
its prey is anchored to the seafloor
and can't hide forever.
[sinister music playing]
The anemone's fate?
To be eaten from the inside out.
[chomping]
The arrival of predation was a seismic shift in the ancient seas.
The hunted had to adapt
or risk extinction.
And one group adapted in a way never seen before.
Despite being single animals,
they were able to separate into layers of individuals,
that then broke free.
Jellyfish.
530 million years ago,
they were the very first animals to escape the seafloor and swim.
[graceful music playing]
[tense music playing]
But predators were already everywhere.
[dramatic music playing]
Armed with venomous harpoons,
these anemones make quick work of baby jellyfish.
[tense music playing]
And they are so abundant
escape seems impossible.
[tense music continues]
Caught.
But not for long.
[tranquil music playing]
Free at last.
Thanks to their ability to swim,
jellyfish were the first to venture out of the shallows
and into the big blue.
[music swells]
[stirring music playing]
Here, with fish yet to evolve,
jellyfish were entirely free of predation.
In time, others would follow.
But more than half a billion years ago,
the open ocean belonged to them.
[music swells]
[music fades]
[water laps gently]
For those still stuck on the seabed,
escaping predation required a different approach.
This curious creature
is a trilobite.
And its breakthrough adaptation, armor.
Trilobites are one of the first in a brand-new dynasty,
the arthropods.
They've evolved an external skeleton,
a shield-like shell that protects their soft insides.
And in these waters, they need it.
[sinister music playing]
This is Anomalocaris.
A bizarre-looking animal,
its name translates as "the abnormal shrimp."
And in the tropical shallows 508 million years ago,
it's the world's first apex predator.
[tense music playing]
Despite the trilobite's armor, it makes sense to keep out of sight.
But away from the canyons, there's nowhere to hide…
because what makes this hunter so special…
…is its speed.
[dramatic music playing]
Unlike jellyfish, Anomalocaris can swim with both direction and pace.
But that isn't always enough…
because when cornered, the trilobite's armor comes into its own.
Rolling into a ball, it becomes completely impenetrable.
Even for the abnormal shrimp.
With the coast clear, the trilobite continues on its merry way once more.
It's searching for one of the ancient ocean's greatest sights…
[stirring music playing]
…the trilobite mating grounds,
where thousands come together to breed.
In the story of life, armor has been such a success
that today, nearly 80% of all animals,
including insects, spiders, and crabs,
are related to this ancient arthropod.
But armor couldn't protect them from everything.
[ominous music playing]
Forty million years later, and a new era of life has begun,
with more diversity than ever before.
Here, trilobites are still in their prime,
but their armor is even more robust…
because on this reef, danger comes from a new type of predator.
Ancestors of today's octopus and squid,
these tentacled giants are part of a dynasty known as the cephalopods.
[sinister music playing]
The largest, with a shell eight meters long,
are Cameroceras.
The trilobite is dwarfed by this towering hunter.
But it's not their size that makes them such a threat.
[trilobite squeaks]
Cameroceras have evolved a new way to catch their prey…
even when they can't see it.
[ominous music playing]
Because in these waters, they hunt using not just sight,
but touch.
All the trilobite can do is try to stay out of reach.
[ominous music fades]
[dramatic music playing]
With a scissor-like beak…
[crunching]
…able to slice through the toughest armor,
Cameroceras dominate these seas.
Although, there is another important new arrival.
Graced with a look of permanent surprise,
this is Arandaspis…
an early kind of fish.
[playful music playing]
It hasn't yet evolved a jaw,
so can only suck up the scraps.
But what it has evolved
will change the course of history.
It has a new internal skeleton that makes it both fast and agile.
A backbone.
And while it may not look like much,
fish like Arandaspis will one day give rise to all other vertebrates.
Amphibians, reptiles, mammals, and birds.
But for now, vertebrates are only at the beginning
of a very, very long journey.
[music ends]
[tense ambient music playing]
Twenty million years later, life faced one of its greatest challenges…
[rumbling]
[howling wind]
…as the climate suddenly began to cool…
[thunder rumbles]
…a development that was to have dire consequences.
[suspenseful music playing]
Little by little, the sea started to freeze.
[suspenseful music continues]
For the inhabitants of Earth's once tropical waters,
this was a catastrophe.
[music fades]
[mournful vocalizing]
The shallow seas became a frozen graveyard.
The world's first mass extinction.
[mournful vocalizing continues]
The fatal freeze was caused by a 60% drop in carbon dioxide.
Without this greenhouse gas to warm it,
the planet was plunged into an ice age.
One that would last for 200,000 years.
[sorrowful music playing]
At its peak, almost half the world was covered in ice,
causing the demise of 85% of all life,
lost from our family tree forever.
With the tropical shallows gone,
the few survivors were those who could head to deeper waters.
Animals like the nautilus,
a smaller relative of the giant Cameroceras.
It first evolved hundreds of millions of years ago,
but still exists today.
It owes its success to the deep ocean's unchanging nature.
Below 600 meters, the temperature rarely fluctuates,
making it a sanctuary of stability.
When the Ice Age struck,
all kinds of animals headed into the abyss.
But it's the cephalopods who have thrived.
[majestic music playing]
Today, many still haunt the deep sea…
including octopus and squid.
Their gelatinous bodies are unaffected by the deep's immense pressure.
And super senses let them find their way in the dark.
No longer constrained by shells,
they've evolved into almost every size and shape.
But this pitch-black world isn't theirs alone.
Arthropods, like this spider crab, live here too…
feeling their way across the ocean floor.
Alongside them, the vertebrates.
With little food on offer, down here, they're rare.
Only around 10% of all fish today live in the deep.
This is not a place they excel…
unlike the cephalopods, who have become the deep's near-perfect predators.
Something this shrimp may soon discover.
[quiet, suspenseful music playing]
All around it, the water twinkles with bioluminescent plankton.
Any disturbance, and for the briefest of moments, they light up.
But the shrimp has company.
A half-concealed squid
is watching the show.
Its large eyes allow it to see exceptionally well in the dark.
[tense music playing]
Picking out the dots of light,
it can follow every move the shrimp makes.
All it needs is the right moment.
[tense music continues]
The shrimp never saw it coming.
With such extraordinary senses,
perhaps it's no surprise that squid have mastered this cold, dark world.
But for those who lived back in the Ice Age,
things were beginning to heat up.
[rumbling]
[solemn music playing]
[ice creaking]
200,000 years after the big freeze,
there appeared in the ice a vein of blue.
[majestic music playing]
Water.
Carbon dioxide levels had bounced back,
causing global temperatures to rise.
The great melt had begun.
Slow at first, the thaw rapidly gathered pace.
Before long, glaciers hundreds of meters tall
cascaded into the warming seas.
[rousing vocalizing]
Within a few millennia,
almost half the planet's ice had melted.
Within a few more, it had entirely gone.
For those who had sought refuge in the deep,
now was a chance to head back up.
[mysterious music playing]
Off the coast of modern-day Australia,
this is a young giant cuttlefish.
Like his Ice Age ancestors,
he's come to the shallows in search of a mate.
But others have got here first…
[sinister music playing]
…including an alpha male…
jealously protecting his chosen partner.
And he does not intend to share.
But the smaller male isn't deterred.
Instead, he bides his time
and watches
as a new male enters the arena…
and signals his intention…
to challenge the alpha.
[tense music playing]
With neither backing down, a fight is inevitable…
[dramatic music playing]
…allowing the young male to make his move.
[whimsical music playing]
[dramatic music playing]
As he approaches, he tucks in his tentacles
and changes color…
to perfectly mimic
a female.
Using brain, not brawn,
he sneaks straight in.
[whimsical music playing]
[music ends]
The alpha now thinks he's guarding not one female…
but two.
He's unaware that this sly young interloper
is mating with his chosen partner.
[tranquil music playing]
Their exceptional intelligence and remarkable senses
have helped cephalopods colonize every ocean on Earth.
Yet, in the shallows,
they are rarely the top predators their towering ancestors once were…
because in the ancient seas,
there was to be a changing of the guard.
[suspenseful music playing]
Seventy million years after the great melt,
in a period known as the Devonian,
the vertebrates made their move.
[sinister music playing]
This is Dunkleosteus…
a descendant of the tiny Arandaspis.
But at nine meters long, this fish is no underdog.
Even its young, barely half its size, are fearsome predators.
And this juvenile has spotted something out in the blue.
Ammonoids,
close relatives of today's deep-sea nautilus.
[tense music playing]
Being a vertebrate,
Dunkleosteus has the pace to catch up with them.
[sinister music playing]
But the ammonoids boast a trick of their own…
…jet propulsion.
Just what it needs to outmaneuver the young fish.
[dramatic music playing]
As it starts to circle…
the ammonoid does the same…
shielding its body with its shell.
[growling]
Once upon a time, this defense would have worked.
But Dunkleosteus has a weapon its tiny ancestors did not.
Jaws.
It can bite down and crush its prey.
As long as it can find the right angle.
[suspenseful music playing]
Luckily for the ammonoid,
the juvenile's jaws are not yet strong enough to break its shell.
[teeth clacking on shell]
But the adult's are.
[sinister music playing]
[music fades]
The pairing of backbones with jaws was to prove a winning combination.
So perfect that, from this moment on,
the vertebrates would never look back.
[rousing music playing]
Today, there are more than 30,000 species of fish.
Forty times as many as the cephalopods.
While their variety is remarkable…
each still has that perfect pairing.
Backbones give them speed and power…
no matter their size.
While jaws
give them bite.
But there is more to jaws than that.
Some fish use them to protect their eggs.
[serene music playing]
Others to attract a mate.
Then there's the sarcastic fringehead.
[playful music playing]
He doesn't just use his to catch crabs.
He also uses them for one of the most intimidating displays in nature…
to rid his patch of rivals.
For these fish, size is everything.
[music intensifies]
[music fades]
Big jaws have let this fringehead rule his strip of seafloor,
but they've also allowed a more infamous group of fish to rule the waves.
[ominous music playing]
They first appeared more than 400 million years ago
and have been feared ever since.
[stabbing string music playing]
[music intensifies]
They are
the sharks.
To the winning combination of backbone and jaws,
they've added senses so sharp
they can detect prey from hundreds of meters away.
With many also boasting size and strength,
they are as close to perfection
as an ocean predator can get.
[tense music playing]
In the sea off Western Australia,
these fish are balled together for protection.
But the sharks hold back…
because they have a special plan of attack.
Appearing to work as a team,
some of the sharks rise from below to trap the fish against the surface.
Others come in from the side,
corralling the shoal away from the open ocean
and into the shallows.
Now numbering in their hundreds,
the sharks play for time…
as their prey get ever more tired.
[sinister music playing]
It's a hunting technique that has been honed over millions of years.
But the carnage starts in the blink of an eye.
[dramatic music playing]
[music continues]
[music fades]
Virtually unchanged since the great melt,
sharks are one of the most successful dynasties in history.
But they owe their rise to the fall of their greatest rivals.
[mysterious music playing]
Back in the ancient seas,
and Dunkleosteus had reigned for 20 million years.
[sinister music playing]
So vicious a predator that not even early sharks could compete.
But Dunkleosteus was to suffer from a devastating quirk of fate…
[mysterious music playing]
…caused by plankton.
Billions of years earlier,
plankton had been the catalyst for animal life.
But now, they would almost bring it to an end.
A sudden surge in ocean nutrients
caused the population of plankton to increase…
on an unprecedented scale.
[ominous music playing]
As it spread through the sunlit shallows, this thick soup began to rot…
causing the ocean's oxygen to plummet.
Without this critical ingredient for life, animals started to suffocate.
[mournful music playing]
For those unable to escape the sludge, there was only one possible outcome.
Extinction.
Dunkleosteus was one of many to be lost from our planet.
[mournful music continues]
Across the world, coastlines became littered with the dead.
Only the lucky few, including early sharks,
were able to survive.
In all, 80% of marine life was wiped out in this,
the world's second mass extinction.
[music fades]
And yet this is only half the story.
The nutrients that caused the plankton explosion
didn't start out in the sea.
[uplifting music playing]
They were carried there from the land…
…released by plants during one of the most amazing events of all time…
…the greening of planet Earth.
[majestic music playing]
What was once a volcanic wasteland
was now overflowing with life.
-[music fades] -[creatures chirping]
And this new world offered opportunity.
For where plants had paved the way,
animals would follow.
[chirping continues]
And before long, the race to dominate the land would begin.
-[dramatic music playing] -[low growling]
[growling, roaring]
[dramatic music intensifies]
[music ends]
[rousing music playing]