All language subtitles for NOVA S50E12 Ancient Earth Frozen 1080p WEB h264-BAE_track3_[eng]

♪ ♪

NARRATOR:
700 million years ago,

ice threatened to make
our planet uninhabitable...

...snuffing out
new complex life forms.

This is some of the deepest,
most intense cold

that Earth has ever experienced.

Some people think the planet
was completely

encased in ice,
a so-called Snowball Earth.

♪ ♪

MICHAEL WONG:
You would have seen it

as a glistening white marble
hanging

in the blackness of space.

NARRATOR:
A seemingly impossible fate

for a planet
so close to the sun.

When Snowball Earth
was first proposed,

people thought it was an
outrageous idea

that this actually
could have happened on Earth.

NARRATOR:
But now

new techniques that are peering

into the past say it was not
only likely...

CHARLOTTE SPRUZEN:
Now we have geological evidence

that nearly the whole world was
plunged into a deep glaciation.

NARRATOR:
...but also lucky for us.

This snowball event most
certainly shaped life on Earth.

NARRATOR:
From this climate catastrophe,

new forms of life arose,

eventually leading
to the living world today.

But it was a close call.

How did life survive
in this planetary deep freeze...

(eruption roars)

...and thrive in its aftermath?

"Ancient Earth: Frozen,"

right now on "NOVA."

♪ ♪

("Never Close Enough"
by SIPHO. playing)

♪ Oh, we won't ever hear
the silence ♪

♪ Or ever see the colors ♪

(exploding)

♪ That never lived in
our minds ♪

♪ ♪

♪ Just a moment ♪

♪ Never too far out ♪

♪ Never close enough ♪

♪ ♪

NARRATOR:
Nearly a billion years ago,

most of Earth's land forms a
single giant continent.

Rodinia.

Thousands of
miles coast to coast,

this enormous
landmass dominates the planet.

♪ ♪

But despite its size,
from its jagged peaks

across its desolate lowlands,

there are no plants

and no animals.

♪ ♪

Yet, in the shallow coastal
waters, there is life.

♪ ♪

Microscopic bacteria gather
to form living mats,

clinging to the seafloor.

For around three billion years,

this has essentially been
the extent of life on Earth.

♪ ♪

It's a planet
dramatically different

from the world we live in today.

♪ ♪

♪ ♪

(insects chirping)

Large, complex organisms that we
have today,

like plants and animals,

wouldn't have existed.

800 million years ago,

the world would have looked
pretty much alien

for us today.

SPRUZEN:
If you were transported

to the planet back then,

you might not even think
anything was living there.

When in fact, Earth was teeming
with life--

just microbial life.

♪ ♪

NARRATOR:
As distant as
this ancient world may seem...

...we are connected to it.

Because the origins of animal
life today can be traced

back to those shallow coastal
waters 800 million years ago.

Amongst the bacteria...

...are giant cells
that tower over the living mats.

♪ ♪

They are called eukaryotes

and are the ancestors of all
plants and animals living today,

including us.

Slowly evolving
for over a billion years,

larger and more complex
than bacteria,

they require more energy

and nutrients from their
surroundings.

They are sustained by bacteria
in the water and minerals

washed in from
the slowly eroding land.

♪ ♪

It's enough for them to survive,

but probably too limited for
them to get any more complex.

All life on Earth is essentially
made up

of two types of cell,

which we call prokaryotes and
eukaryotes.

NARRATOR:
Prokaryotes are very simple,
microscopic life

and include many bacteria,

with a loop of genetic material
floating in the cell.

A eukaryotic cell differs
from a prokaryotic cell in that

it contains all of its DNA

inside of a central package
that we call a nucleus.

SUSANNAH PORTER:
And these cells have a lot
of other

structures inside that we call
organelles,

and that have special, different
functions

inside the cell.

It's a radical change
from before.

When they first appeared, they
would have looked very simple.

NARRATOR:
But, over time, developed
into more familiar forms.

This is tiny fossil

from North China,
and it look very much like

modern seaweed,
but it just small.

Orders of magnitude
smaller than modern seaweed.

NARRATOR:
But for life to gain a foothold

and further diversify, the
Earth itself needed to change.

♪ ♪

In fact, the story of life
is the story

of our planet's changing
geology.

Life and rocks have co-evolved

over billions of years.

They're totally interconnected,

interdependent on each other.

Geology and biology

are entwined together in
the formation of the Earth.

CHRISTOPHER JACKSON:
When you think about
geological timescales,

it's actually quite easy

to think about the really
long-term,

complex evolutions that can
happen.

RAYMOND PIERREHUMBERT:
To understand these sweeps
of time,

you have
to learn to think like a rock.

Things happen very slowly if
you're a rock.

NARRATOR:
And on Rodinia,

an epic geological process
will change life forever.

♪ ♪

It's on vast geologic time
scales

that profound and lasting change
becomes apparent.

COHEN:
So we're in Iceland,

and that kind of long-term
geological change

is happening here right now.

It's really amazing knowing

that I am standing on a place

where two tectonic
plates are pulling apart.

NARRATOR:
Tectonic plates are giant slabs
of rock

that lie beneath land and ocean
and are constantly on the move.

COHEN:
That process happens about one
to two inches a year.

That's about the same speed
that your fingernails grow.

NARRATOR:
This might not seem fast,

but over time,
movement like this has formed

entire mountain ranges
and torn continents apart.

♪ ♪

The rocks beneath my feet are
being pulled apart

along the Mid-Atlantic Ridge,

which runs this way and
this way for thousands of miles

and has been opening
for almost 200 million years.

NARRATOR:
These tectonic forces from deep
within the Earth

have been creating
and then destroying

Earth's most prominent features

for billions of years.

♪ ♪

And around
800 million years ago,

they began to divide Rodinia.

♪ ♪

Deep underground...

...over millions of years...

...swirling currents of heat
cause tectonic plates

to shift.

Great rifts gape
open across the landscape.

(ground rumbling)

This vast continent is
slowly being ripped apart.

♪ ♪

Over millions of years,
it creates deep valleys...

...and new seas,
reshaping the continent.

Minerals and nutrients wash

from the land into the seas.

♪ ♪

Making the world around these
eukaryotes

a much more enriched place
to live.

This influx of resources

radically increases
their number and variety.

And they are not alone.

The whole ecosystem becomes
filled

with new and more complex life.

The world is beginning to see

more interconnectedness
between organisms.

Driven by
Earth's dynamic geology.

♪ ♪

♪ ♪

We can find
some incredible evidence

for this in the fossil record.

COHEN:
The fossils themselves

are really small
and they're hard to find.

It's like trying to put together
a puzzle

when you only
have half the pieces in the box.

So this rock here,
this has got the gray part

and the dark part,

and both of these contain
microscopic fossils.

They are structures that were
made by early eukaryotes,

and maybe 40 of them would fit

in the period
at an end of a sentence.

NARRATOR:
What these tiny fossil cells
tell us

is they were interacting

with one another
in an entirely new way.

We can see spikes,

spines, and other really amazing
structures

made from minerals
that eroded off of Rodinia.

It's energetically costly
to make structures like this,

and so there had to
be a really good reason

for organisms to spend all of
this time

and energy making them.

NARRATOR:
One idea to explain
this evolutionary development

suggests that early eukaryotes
were facing a deadly new threat.

So we think these
organisms made these structures

as defensive weapons to protect
themselves from being eaten.

♪ ♪

They are some
of the first evidence

of predation
in the fossil record.

NARRATOR:
So, what was eating them?

I think the most
exciting clues are that we see

holes that are
similar to bite marks.

But in this case,
they're microscopic.

They're tinier than a,

a thousandth of a millimeter
in size.

NARRATOR:
More evidence is found in
fossils

from the Grand Canyon.

So the fossil remains
in this rock

are little casings
of organisms

that look remarkably like

a modern group of
organisms that live today

called testate amoebae,
that are voracious predators.

NARRATOR:
They make holes in cells
to suck out their prey,

suggesting that something
similar

may have been
eating ancient eukaryotes.

If true, this would have changed
everything.

SANDERS:
Predation,

just like environmental
pressures,

can push life to experiment
and try new things.

GIRGUIS:
There's a good chance
that the seas around

Rodinia may have been teeming

with more diverse life
than ever before.

NARRATOR:
And this life is slowly becoming
more recognizable.

But it's about to come under
immense pressure.

(ground cracking, rumbling)

Although the supercontinent
break-up

could have helped fuel
eukaryotic life,

it also set into motion
a series of events

that could have led
to catastrophe for them.

♪ ♪

NARRATOR:
And that catastrophe...

(erupts)

...begins with fire.

On Rodinia, geological
change accelerates rapidly.

Superheated magma bursts through
the surface as rifting

thins Earth's crust
in multiple places.

Molten rock swamps
over 800,000 square miles

of the continent's surface,

one of the largest
outpourings in Earth's history.

(hissing)

As this lava cools,

it becomes a dark rock
called basalt.

♪ ♪

Creating a vast black stain

across the heart of the
continent.

Which will
have dire consequences.

♪ ♪

WONG:
Basalt is incredibly important

at impacting
the atmosphere of the planet.

NARRATOR:
Basalt breaks down

more readily than other rocks,
like granite,

in a process
called silicate weathering.

So when rain mixes with carbon
dioxide

in that atmosphere, it forms
this weak acid

weathers these silicates,
and creates these compounds

that wash into the oceans
and eventually form carbonates.

So, trapping
carbon dioxide in the form

of rocks on the seafloor.

SPRUZEN:
This can control our climate
on a geological

timescale, because it can affect
the amount

of carbon dioxide that can be
drawn down

from the atmosphere.

So, when millions of tons
of this rock

poured out over the surface
of Earth

in the heart of Rodinia,

it could have had a profound
effect on Earth's climate.

NARRATOR:
The amount of basalt created
only tells part of the story.

Where it formed
also plays a role.

BENJAMIN MILLS:
We think that it formed around
the tropics,

and this means they would have
been subject

to huge amounts of rainfall.

(thunder claps, rain falling)

JACKSON:
So, the combination
of large amounts of basalt

being exposed at
the Earth's surface

and heavy, intense rainfall

meant that a large amount of
carbon dioxide

was drawn out of the atmosphere.

NARRATOR:
This cools the Earth.

WORDSWORTH:
And this is because
carbon dioxide

is a greenhouse gas.

It traps heat trying
to escape from Earth,

kind of like a
warm blanket on a winter's day.

So, you make
that blanket thinner,

the Earth has
to get colder as a result.

NARRATOR:
Falling levels of carbon dioxide

cause temperatures to plummet
over millions of years.

Eventually, ice forms.

At first,
it's confined to the poles.

But it spreads.

And for life on the planet,

that's a potentially
deadly challenge.

♪ ♪

Today, in certain places
on Earth,

you can vividly see
the impacts that ice

and falling temperatures
have on ecosystems.

♪ ♪

SANDERS:
In the valleys surrounding

high mountains in a mountain
range,

you might expect to see

a lot of different types
of life.

It's a very lush
and hospitable environment.

But as you go up the
mountainside

and temperatures get a little
cooler,

NARRATOR:
Cold and ice
can destroy cells

and slow down chemical
reactions,

impacting life.

It's likely that 717 million
years ago,

as the Snowball Earth glaciation
began,

microscopic webs of life

were also threatened.

NARRATOR:
On ancient Earth,

the cold advances on early life.

♪ ♪

Sheets of sea ice
thicken year by year,

as they creep
towards the shores of Rodinia

from the North and South poles.

And on land,

huge ice caps expand down
from the mountains.

The places life can
hold on are rapidly shrinking...

...as the ice tightens
its grip across the planet.

The big question
for scientists is,

why did this cooling trend
turn into a runaway deep freeze?

It's not like there's never been
supercontinent break-up

or vast outpourings
of basalt before.

So what was
so different this time?

It's really difficult
to find out

exactly what happened millions
of years ago,

because all we have left is this
incomplete rock record,

and translating that

into its climate impacts
is a very complicated process.

It could have been that because

Rodinia was breaking
up around the Equator,

that caused a lot
more weathering

than other supercontinent
break-ups in the past.

NARRATOR:
One theory
suggests new complex life

may have
changed ocean chemistry,

which drew down carbon dioxide,
trapping it.

(eruption explodes)

Another involves volcanic gases.

PIERREHUMBERT:
These large volcanic eruptions
put a lot of sulfur dioxide,

which turns into sulfuric acid,

into the atmosphere,

which reflects sunlight back to
space.

WORDSWORTH:
Once this cooling began,

it would have likely been
incredibly difficult to stop.

NARRATOR:
The ice is bearing down.

But a web of life can
still thrive at lower latitudes.

As the white expanse spreads,

it reflects more of the sun's
energy back into space.

Driving down temperatures

and expanding the reach of the
ice,

until it becomes
a runaway process.

This is known as
the ice-albedo feedback effect.

WONG:
Once you cover

a little patch of Earth
with ice,

the albedo of the planet has
risen,

which means it reflects more
light to outer space

and cools down,

causing more water to freeze
into ice,

which reflects yet more light
into space,

cooling the planet,
on and on and on.

NARRATOR:
Driving a continuous
loop of self-reinforcing change

that remakes
the surface of the planet.

Sea ice surges over the oceans,

reaching a crushing
half-mile thick in places.

And near the Equator,
ice closes in...

♪ ♪

...creating a frozen planet.

We suspect Earth became a
snowball,

because we know
how far the ice reached,

thanks to clues
that were left behind.

SPRUZEN:
So, today, if you look

underneath glaciers,
you can see rocks

that are trapped
at the bottom of the ice.

And then as
it advances out to sea,

it will carry
these rocks along with it.

And then as it melts,
these big rocks at the bottom

will drop into the deep ocean.

NARRATOR:
Scientists call these
dropstones,

because once they drop
to the bottom of the ocean,

they become embedded in the
seafloor and look different

from the surrounding layers.

Over millions of years,
that ocean floor turns to rock

and can be exposed as dry land.

This rock, funnily enough,
is from the desert in Namibia,

uh, but you can see

there's this big other rock
embedded in this brown mass.

The only thing that could have
dropped this stone

into the ocean
would have been ice.

These dropstones
from the Snowball Earth period

have been documented
literally around the globe.

And that's how we know
that Snowball Earth

was an entire global event.

There's still
a lot of debate, however,

on what exactly the Snowball
Earth might have looked like.

WONG:
We rely on our knowledge
of the geological record,

as well as computer models,
to tell us

what Snowball Earth would have
been like.

♪ ♪

A complete entombment
of the planet in ice,

with not even any communication

between the sea
and the atmosphere.

NARRATOR:
Another theory is called
Slushball, or Waterbelt Earth.

WORDSWORTH:
So, Slushball Earth would have

kind of looked something
halfway between our Earth today

and a full global snowball.

There would have been
ice extending down,

but probably some open water
still around the Equator.

COHEN:
Other models suggest that

while the seas would have been

mainly covered by ice,

on land, there would have been
things

like dry valleys and exposed
mountains,

like Antarctica today.

NARRATOR:
Whatever the scenario,
this frozen world

produced
incredibly harsh conditions.

BRENHIN KELLER:
We're talking about temperatures

near or below freezing

even at the Equator.

SPRUZEN:
Climate models predict
that the ice covering the oceans

would have been several hundred
feet to half a mile thick.

♪ ♪

NARRATOR:
This would have had
a huge impact on life.

♪ ♪

As the ice chokes our planet,

it is a disaster for
the majority of living things.

Under ice, the ocean is mostly
cut off from the atmosphere.

Darkness closes in.

And the flow of nutrients from
the land slows to a trickle.

Under this pressure,

ecosystems begin to fall apart.

Imprisoned by the ice,

the predecessors
of all animal life falter.

And many die.

♪ ♪

♪ ♪

It's easy to imagine
how this could have ended

all complex life on Earth.

JAZ MILLAR:
We know that life

must have survived through
Snowball Earth

because we still have a lot of
life

in the Earth today.

We had life before and after,

so, somehow, it must have
survived.

as all the ice
from back then has melted.

MILLAR:
To understand how life
survived in ice in the past,

then we look to how life
survives in ice today.

And there is one habitat

that Snowball Earth scientists
are particularly interested in.

So, we're gonna see if
we can try and find that today.

♪ ♪

NARRATOR:
There is more to this frigid
landscape than meets the eye.

♪ ♪

What I found here is a hole in
the glacier surface.

We refer to this
as a cryoconite hole.

Sediment lands on
the glacier surface,

swept in by water
and by wind.

Then the radiation
from the sun

warms it,
and it melts the underlying ice.

So it melts
almost vertically down

to make this pocket
that we see here,

with meltwater at the bottom
and open at the top.

NARRATOR:
Look hard enough
in the right places,

and you'll find these
on glacial surfaces everywhere.

MILLAR:
The cryoconite holes are perfect

for microorganisms because
they have access to the sun,

and they have
access to meltwater.

NARRATOR:
That makes these holes refuges

for a variety of life.
(device beeps)

I'm gonna try and
sample this cryoconite hole.

♪ ♪

I'm seeing some bits
are a little bit green,

so they might be bits of

photosynthetic microorganisms
or they might be bits of plants.

NARRATOR:
Other samples
have revealed even more life.

A huge range
of organisms, from fungi

to micro-animals.

This captures the diversity
of the organisms

living in cryoconite holes,

not just in species,
but in size

and shape and function.

♪ ♪

It's surprising and
it's quite magnificent

that these quite
empty-looking spots

can have this much diversity.

NARRATOR:
Even when life like this
is subjected

to harsh
Snowball Earth conditions,

it survives.

MILLAR:
For us, as humans,
Snowball Earth will always seem

like a harsh, almost impossible
planet to live on.

However, when we look
at the depths of Antarctica

and the middle of ice sheets
in the Arctic,

there is life
wherever you look for it.

NARRATOR:
And it's not just cryoconites.

There's various different
places eukaryotic life

may have survived
on Snowball Earth.

GIRGUIS:
It could have been

in little glacial ponds
or lakes.

as they are today.

SANDERS:
Some may have survived

deep underground
in subterranean caves,

where there's water

and temperatures are higher.

And then, of course, the oceans.

♪ ♪

NARRATOR:
But this is no easy life.

For life to flourish again,

we would need some kind of
drastic change

NARRATOR:
Ironically, this change

would be driven by the same
powerful tectonic process

thought to have triggered
Snowball Earth

in the first place--

a process
that had never stopped.

♪ ♪

Deep below Earth's surface,

the fire still burns.

♪ ♪

(eruption roars)

The ice can't stop volcanoes

from punching through
the planet's frozen shell.

(volcano erupts)

WORDSWORTH:
Even in the depths of the
Snowball Earth period,

volcanism would never stop
completely,

because there would still be

Earth's internal heat
driving it.

(eruption roars)

NARRATOR:
But it is not heat
from the lava

that impacts Earth's climate.

Lava is not
what melts the snowball.

It's actually
an invisible force,

which is the carbon dioxide

being produced by volcanoes and
building up in the atmosphere.

♪ ♪

This would have served

to trap heat against the Earth,

causing a rise in global
temperatures.

NARRATOR:
Before the
Snowball Earth period,

carbon dioxide was removed
from the atmosphere by reacting

with sea water
or with rocks on land

via silicate weathering.

But if you cover the entire
planet in ice,

as was the case during Snowball
Earth,

this is effectively shut off.

NARRATOR:
The ice layer

would stop
the weathering process

that had been pulling
carbon dioxide

from the atmosphere.

And so the carbon dioxide

builds up into the atmosphere,

until it gets warm enough to
melt the ice.

(volcano erupts)

NARRATOR:
As each eruption tears
through the ice-bound plains...

♪ ♪

...it releases a mix
of volcanic gases,

including carbon dioxide,

each time adding
to the atmosphere,

leading to tiny increases
in global temperature.

As concentrations rise and
build over millions of years,

the gas traps ever more heat.

The frozen planet

begins to thaw.

♪ ♪

How long it took
is still debated.

PIERREHUMBERT:
Almost everybody agrees that,

in geological terms,
the melting,

the deglaciation of
Snowball Earth, was really fast.

Where the disputes are
is whether fast

means a matter
of hundreds of years

or maybe up to a million years.

It is incredible to imagine

that a global glaciation

could have been undone in just

a couple thousands of years.

This is a geologically
instantaneous amount of time.

♪ ♪

NARRATOR:
The rapid, thousand-year

melting speed
is a contentious theory,

but there are geological
clues left that support it.

♪ ♪

JACKSON:
We don't know
precisely what happened

millions of years ago
because we weren't there.

But one theory is that,

by looking at these types
of rocks,

called cap carbonates,

we can understand
how Snowball Earth melted.

NARRATOR:
These are found
in vast quantities.

JACKSON:
This was actually found

in a rock sequence
that was 72 feet thick,

so that's a whole cliffside.

One idea is that they
represent the products

of the intense
chemical weathering

that happened at the end
of Snowball Earth,

as the snowball was melting.

NARRATOR:
But this rock can also suggest
how fast it happened.

If the melt had been slow,

impurities would have had
time to accumulate in here,

and that's not what we see.

There's no impurities within it.

This tells us that the melting
was actually very rapid.

NARRATOR:
Further evidence
that this could have happened

in around 1,000 years.

(water trickling)

635 million years ago,

as the planet warms...

♪ ♪

...the melting
of gigantic glaciers

drives intense evaporation
of water into the atmosphere.

♪ ♪

Water vapor
is a strong greenhouse gas.

So this would have kind of acted

like an accelerator pedal
on the whole process.

Once the thawing begins,

and the ice caps
start to retreat,

the planet becomes darker,

enabling it to absorb
more energy from the sun.

PU:
As the dark oceans
and continents absorb more heat

from the sun,
this would have helped

warm the planet and decrease
the amount of ice left on it.

This can be considered

the exact reverse process

of how the snowball was formed.

NARRATOR:
But this is not a tranquil time.

When the Snowball Earth ended,

it ended in a catastrophic way.

♪ ♪

NARRATOR:
What starts as a trickle

becomes a torrent.

And the melt
becomes unstoppable.

A runaway event

that releases a deluge
of meltwater

on an almost unimaginable scale.

You can imagine
these huge waterfalls

coming off the edge of glaciers.

PIERREHUMBERT:
You would have

almost certainly had these
really massive tropical storms

unlike anything
that we see today.

We call them hypercanes.

PU:
There must have been

acid rain coming down
from the skies above...

...just completely obliterating
the surface of the Earth.

♪ ♪

NARRATOR:
At the peak of the melt,

sea levels rise
over six feet every decade,

swallowing great chunks
of coastline.

After a total of over 50 million
years in the deep freeze,

the world beneath the ice

finally emerges,

its warm shores
a stark contrast.

Perhaps over 120 degrees
Fahrenheit at the Equator.

COHEN:
If you were transported back,

the oceans would have been very
tropical.

XIAO:
That change from
freeze to fry is a big challenge

to all kinds of life.

GIRGUIS:
Eukaryotes and all life on Earth
of course

would have evolved

to the warming temperatures.

But evolution is slow.

Doesn't happen overnight.

NARRATOR:
Many would have died.

And those that survived

would remain reliant on
nutrient streams from the land.

♪ ♪

This is a changed world,

the land utterly transformed

by the incredible power
of glaciers,

a force
that can still be seen today.

♪ ♪

Well, I'm lucky enough to be
standing

at the foot of a glacier,

and this big face here next to
me,

it's really dark, really black.

But it's not rock--
it's actually ice.

And although it looks
pretty stable and static now,

it's actually moving towards
me at three feet per day.

I mean, it's a really kind of
eerie place to be standing.

If we look up
into the mountains,

we can see that this glacier
has carved its way down

into this deep valley
right to where I'm standing.

This has broken up all of this

material around us
and these boulders,

and then this really fine
clay-like material, as well.

But imagine this happening
during Snowball Earth,

across the whole planet.

NARRATOR:
It's thought
that the glaciers back then

were just
as scouring as today's.

JACKSON:
These glaciers
were able to remove

about two miles
of rock from below them.

That's about twice the depth
of the Grand Canyon.

So, the power of these
glaciers was enough to remove

vast swathes of the rock record,

leaving behind gaps.

♪ ♪

NARRATOR:
Many scientists believe
that this could explain

a geologic mystery
known as the Great Unconformity.

So, this rock is
about 520 million years old,

but it sits just above

this three-billion-year-old
granite.

So, the amount of time missing

is about 2.5 billion years.

And we think that the rocks
may have been partly eroded away

by glaciers during
the Snowball Earth period.

NARRATOR:
Another theory is, this gap

may be due to the movement
of tectonic plates.

KELLER:
But if it's true that
glacial erosion is responsible,

we're talking about
more than a vertical mile

of erosion on average globally.

That's a billion billion tons.

♪ ♪

NARRATOR:
This incredible amount
of ground-up rock

is deposited in
the world's rivers and oceans...

...helping single-celled algae
and bacteria to flourish...

...growing in massive blooms...

...likely visible from space.

And these may have impacted
the entire ocean systems.

The oxygen levels in the water

greatly increase,

generated by high levels
of photosynthesis.

♪ ♪

In the aftermath
of the great thaw,

conditions seem possible
for eukaryotic life

to take another leap forward.

COHEN:
The oceans would have been
a really great place for life,

with sunlight,
oxygen, warmth, and nutrients.

NARRATOR:
And the melting of the ice

would have created
more shallow marine habitats

to expand into.

SANDERS:
It's a time when life really
has a lot of opportunities

to experiment and diversify.

Once more stable
and productive interaction

starts to happen
between the Earth

and the life that existed on it,

it didn't actually
take that long

for much more complex
life forms to evolve.

NARRATOR:
This is a major turning point,

unlike anything
that came before.

Finally, individual organisms
are becoming complex

and big enough that we could
see them with our own eyes.

XIAO:
Some scientists think that

the increase in the size
of eukaryotic life

has to do
with the availability of oxygen.

The increased
availability of oxygen

may have allowed
organisms to access

a greater amount of energy

than, perhaps,
they were previously doing.

It's also possible that
phosphorus played a key role.

Photosynthesis
requires phosphorus,

and that forms
the basis of the food chain.

This could have fueled
a very productive ecosystem.

And it might not
be oxygen or phosphorus

or any of the other theories
we have,

and it may just be
they needed the time

and the space to evolve.

♪ ♪

NARRATOR:
In shallow coastal waters

across the planet,

life blossoms.

This myriad of strange forms
may all look like plants...

...but some are animals--

grazers, predators,

and prey.

Descendants of those
first tiny pioneers,

but more complex.

Now collections
of these cells work together,

finally broken free
of the microscopic world,

all living within
and supported by a web of life

just as intricate and productive
as any on Earth today.

These first truly complex
living creatures

are Ediacaran organisms.

SPRUZEN:
Life has been
on this incredible journey.

We've gone from these
tiny microscopic little fossils

that we can only see
with microscopes

all the way to these large,
complex organisms,

which show signs
of having animal features.

XIAO:
This fossil,

Yilingia, was found in South
China.

About 550 million years old.

What are you looking at
is a trail made by this animal,

perhaps in the last day
of its life,

and then it died.

And then the animal
and the trail it made

were preserved together.

NARRATOR:
Striking proof
these animals moved

and made choices
over where to go.

But these creatures
are so odd-looking,

they are hard
to study and classify.

XIAO:
So this is something
called Dickinsonia.

It is probably an animal.

But scientists still debating

what kind of animal
exactly it is.

♪ ♪

NARRATOR:
Trying to understand
how they interacted

with each other is tricky.

MITCHELL:
I'm gonna press the red button.

NARRATOR:
But thanks to
advanced laser technology,

scientists can now
learn more about them

than ever before.

(device tapping rapidly)

MITCHELL:
So, by laser-scanning

these fossils, we've got
a three-dimensional surface

covered in the fossils.

And what this means is,

you have, essentially,

a snapshot of Ediacaran life

captured on the rock surfaces.

♪ ♪

To date,
we have over 20,000 fossils

that we've laser-scanned.

And using different sorts
of statistics

and mathematical approaches,
we can then work out

what they were doing and how

they're interacting
with each other.

♪ ♪

NARRATOR:
Looking at the whole ecosystem

is helping scientists
gain precious knowledge

about these animals

and how they interacted
with each other.

♪ ♪

It doesn't seem that we have
any of the creatures

from the Ediacaran period
still alive today,

except for sponges.

COHEN:
The geologically short time

that the Ediacaran fauna
were around on our planet

indicates that,
after Snowball Earth,

the evolution of complex life

occurred at a rate
never seen before.

♪ ♪

NARRATOR:
Complexity increased
and ecosystems diversified,

leading to the ancestors
of large animals and plants

in our world today.

Great climate
and geological change

took eukaryotic life
to the edge.

But it seems that eukaryotic
life

always found a way to persist.

♪ ♪

Eukaryotic life has survived
events

that scientists previously
thought

were unsurvivable.

Re-emerging,
diversifying, and exploring

every possible niche
that the world has to offer.

GIRGUIS:
Our Earth

finally became the stage
that ushered in a new age.

One of complex life
that would eventually reach

the scale, beauty, and
complexity that we see today.

♪ ♪

ANNOUNCER:
On "NOVA: Ancient Earth"...

Our green planet
wasn't preordained.

SUSANNAH LYDON:
It's the result
of an immense struggle

over millions
of years.

KIRK JOHNSON:
It creates

one of the most bizarre
prehistoric landscapes of all.

DORI CONTRERAS:
An alien world.

ROBERT HAZEN:
Life and Earth-- they're not
separate entities,

they're totally interconnected.

ANNOUNCER:
"Life Rising" on "NOVA."

Next time.

SINGER: ♪ Wish I could go back
in time ♪

♪ ♪

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