50x12 - Ancient Earth: Frozen

[bunniefuu]

♪ ♪

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.



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.

NARRATOR: This is why we
consider them more complex.

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

that falls onto Earth's
surface, weathers these rocks,

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,

we start to see fewer and
fewer kinds of life flourishing.

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.

NARRATOR: Cooling the Earth.

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.

One of the ideas is something
we call a hard snowball.

♪ ♪

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.

NARRATOR: But it's
hard to prove where,

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.

Uh, hot springs, which surely
would have been around then

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

for life to escape
this icy prison.

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]



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


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.

Plants transformed bare
rock into life-giving soil.

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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