♪ ♪
NARRATOR: Earth, our home.
A rocky world
enveloped by a thin blue layer
clinging to the surface.
Our atmosphere.
ROBERT HAZEN: The atmosphere is the only thing
that stands between us and that cold, dark void of space.
(rumbling)
NARRATOR: The story of the air that surrounds us
is the story of Earth itself.
♪ ♪
SONIA TIKOO: It is incredible to think that
this 4.5 billion-year-old rock could hold clues
as to how our early atmosphere formed.
(lightning crackling, rain falling)
NARRATOR: Clues that reveal colossal transformations.
MIKE WONG: It is absolutely mind-blowing that a tiny bacteria
could affect a planetary-scale phenomenon
like our atmosphere.
♪ ♪
NARRATOR: How did our atmosphere make Earth a unique place,
the only home to life as we know it?
We have never found another planet that has
an atmosphere like ours.
PETER GIRGUIS: And understanding how Earth got its atmosphere
is the first step in understanding how to protect it.
NARRATOR: "Ancient Earth: Birth of the Sky,"
right now, on "NOVA."
♪ ♪
(rumbling)
NARRATOR: In the depths of space
lies a young, unrecognizable planet.
♪ ♪
Bombarded by asteroids,
this battered world is our Earth.
♪ ♪
A desolate and volatile globe
overlooked by a new-born moon that dominates the sky.
With no recognizable atmosphere, the void of space
nearly touches the surface of slowly cooling lava.
♪ ♪
There's little protection from the young sun's harsh radiation.
This world looks nothing like the Earth we know today.
Around four and a half billion years ago,
Earth was inhospitable for life.
(rumbling)
HAZEN: There were meteors and asteroids bombarding the surface.
It was too hot.
It was a terrible place to be.
♪ ♪
LINGAPPA: The question is,
how did our planet transform from this barren rock,
into somewhere capable of supporting life?
♪ ♪
WONG: This is the story of how our planet became a living world,
and the dance, the co-evolution
between life and our atmosphere.
♪ ♪
("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: Today, our planet is an oasis for life,
enveloped by a layer of
nourishing and protective gases--
the atmosphere.
It's extremely thin, it's almost
like the skin on apple,
if you pretended the apple was planet Earth.
TIKOO: The diameter of the earth
is about 8,000 miles across,
and the atmosphere adds
just another 60 or so miles on top of that.
(laughing): We wouldn't... we wouldn't be
on planet Earth if it weren't for the atmosphere.
And when I say we,
not just you and me sitting here, but any life.
GIRGUIS: Everything from fungi to bacteria,
to tiny insects, to giant reptiles,
are entirely dependent
on this atmosphere of ours.
♪ ♪
It does many things for us,
and we can think of the simple answer that we,
as human beings, use the oxygen to live.
NARRATOR: Besides oxygen, other important gases
make up the atmosphere.
LYONS: Perhaps the single most critical aspect
of our atmosphere is the fact that carbon dioxide
is one of the greenhouse gases that keeps our planet habitable.
PIERREHUMBERT: Nitrogen is an essential
component of the atmosphere,
because all life as we know it--
amino acids, DNA,
all of the things that are essential to us--
rely on nitrogen.
The atmosphere protects us
from ultraviolet radiation
because it has a small amount
of a molecule called ozone.
LINGAPPA: It controls our climate,
which makes this planet, like,
a livable temperature for us at all.
It sort of insulates us from the cold of space.
NARRATOR: But the journey to today's atmosphere
has been full of drama.
WONG: It's a long, winding road
between the initial atmosphere that Earth had
through all of the various different kinds of atmospheres
in the middle of its history,
and finally arriving at the atmosphere that we have now.
♪ ♪
NARRATOR: Millions of years after the birth of the planet,
there's still no atmosphere as we know it.
But that is going to change.
Four and a half billion years ago,
the Earth would have been a very violent place.
(rumbling)
LYONS: Such that it's called the Hadean, which comes from
the god Hades, the Greek god of the underworld,
because it was imagined to be
a hellish kind of place.
(thundering, rumbling)
(bursting)
(magma rushing)
NARRATOR: Across the globe,
molten magma surges up from within.
(splashing)
Rivers of fire unleash a cocktail of gases.
(booming)
♪ ♪
Carbon dioxide, nitrogen, and methane
explode out of volcanoes in thick plumes.
♪ ♪
These are the building blocks
of our early atmosphere.
♪ ♪
And we can see echoes of this young planet
still on Earth today.
♪ ♪
The volcanoes that we have here in Iceland look
very similar to those that we had
four and a half billion years ago.
The early Earth was dominated by volcanic activity,
and this lasted for millions of years.
♪ ♪
Those volcanoes acted as pathways
for the gases that are trapped inside the magma
to escape into our atmosphere.
NARRATOR: But how do scientists know
what the atmosphere was like in the past?
Surprisingly, clues are hidden
in some of the rocks we find today.
Throughout Earth's history and evolution,
there's been a dynamic interplay between the Earth's surface
and the Earth's atmosphere.
And because of this interplay,
there are chemical fingerprints that are trapped in rocks.
The rock I'm holding in my hand
is 3.4 billion years old.
And there are tiny bubbles
trapped in this rock
that are samples
of Earth's ancient atmosphere.
So we can measure the gases
and piece together evidence
of what the atmosphere was like in the past.
♪ ♪
NARRATOR: However,
to understand the very earliest atmosphere,
scientists have to go back to a time
before Earth was formed.
The planet is made up of rock that started as debris
floating in the early solar system.
But this debris is also falling to Earth today as meteorites.
WILLIAMS: In my pocket here,
I've got the most incredible rock.
This is a chondrite meteorite.
These chondrite meteorites
formed when rock dust came together
to form small chunks of rock like this.
They eventually smashed together,
forming our planet Earth.
And so these are really the building blocks of our planet.
And so it's so humbling,
because it's not only older than all of the life
that we know on Earth,
but even the planet itself,
right back to 4.6 billion years ago.
NARRATOR: These rocks are not just the building blocks of our planet,
but of our early atmosphere as well.
WILLIAMS: Thanks to these space rocks,
the ingredients for our early atmosphere were there.
It's just that they were locked inside the planet.
NARRATOR: Chondrite meteorites are like time machines,
allowing scientists to travel back billions of years
to when the atmosphere first developed.
WILLIAMS: Experts can study these chondrites in great detail.
They can analyze them in the laboratory
to understand their chemical signatures,
and that's going to tell them about the gases
that they would have contributed to
our early Earth's atmosphere,
such as water vapor, carbon dioxide,
sulfur dioxide, hydrogen, methane, and nitrogen.
HAZEN: It's remarkable that you can learn
about the history of the atmosphere
going back billions and billions of years
just by studying rocks.
WONG: When I learn about this, I feel simultaneously
like a time traveler and a detective,
being able to piece together the story of our ancient Earth.
LYONS: Each year, techniques become more and more powerful
at deciphering bits of information of what the Earth
may have been like
four and a half billion years ago,
that even a short time ago would have been completely a mystery.
(rumbling)
(steam hissing)
NARRATOR: While the scorching Earth slowly cools,
the sky is undergoing a dramatic transformation.
The vast sea of stars becomes obscured.
♪ ♪
And as the young sun rises,
color bursts across Earth's sky.
These dense gases are the beginnings
of the atmosphere as we know it.
What this fledgling atmosphere looked like is debated.
Because it's hard to know the precise amounts
of the various gases it contained.
Gases like methane.
PIERREHUMBERT: If there's enough methane,
it can reach a concentration where it starts breaking up
under the action of ultraviolet light
and then recombining into longer molecules,
making these smoggy substances.
That can actually make these hazes
which cause an orangey glow.
♪ ♪
NARRATOR: With enough methane,
thick, intermittent orange hazes would have covered the planet.
TIKOO: There was probably carbon dioxide.
There might have been carbon monoxide, water vapor.
There might have been
some nitrogen or cyanide.
All in all, it was a pretty
noxious combination of stuff.
HAZEN: If you were dropped onto early Earth
and experienced that early atmosphere,
you'd die very quickly because there was no oxygen.
Carbon dioxide, some of the other molecules
that might have been present in the atmosphere,
you just simply couldn't live on those.
♪ ♪
NARRATOR: And the planet is inhospitable for another reason--
the landscape has never seen a drop of liquid water.
But Earth does have water.
(thunder crackling)
It is suspended high above the searing surface,
pumped into the atmosphere by volcanoes
as water vapor with the other early gases.
♪ ♪
(woman vocalizing, thunder crackling)
If this ocean in the sky could reach the ground,
it would have the power to utterly transform the planet.
(water rushing)
(roaring)
♪ ♪
Today, over 70% of our world
is covered in water.
A planet of abundant blue
with endless rivers,
freezing icebergs,
and vast oceans.
♪ ♪
GIRGUIS: Water that we get out of our tap
has been through a long journey.
♪ ♪
DOTTIN: And the water on our planet
gets incorporated into various aspects of our life,
including weather, into the food that we eat.
And because of this, the water is recycled
over and over and over again throughout Earth's history.
WONG: So the water that we drink
was consumed by dinosaurs
and had the first microbes
swimming around in it. (laughs)
(distant rumbling)
NARRATOR: On the early Earth,
water is stuck in the atmosphere.
(wind howling, thunder crackling)
But that is about to change.
♪ ♪
High up in the sky,
droplets of water,
just a fraction of a millimeter across, are condensing.
So light and small, they just float,
drifting in the air.
Colliding and merging with others...
(squishing)
...they slowly grow until a tipping point is reached.
♪ ♪
Earth's gravity draws them downwards...
but the journey is short-lived.
The planet is still hot.
(sizzling)
So any drops nearing the ground...
(hissing)
...simply evaporate.
♪ ♪ (wind whipping)
But over time,
that heat is radiating out into space.
Earth is cooling down.
(wind whistling)
(fizzing)
♪ ♪
(thunder booming)
(rain falling)
(lightning crackling)
A deluge of rain
begins to pour down on the surface.
(booming)
(rain pelting down)
Huge storms sweep through the atmosphere.
(thunder rumbling)
Oceans of water are dumped onto the land.
(rain pouring)
(thunder rumbling)
♪ ♪
(lightning striking)
The atmosphere has unleashed a key molecule for life.
Water changes the surface
of a planet.
It evaporates,
it allows clouds to form
and rain to fall,
it erodes things,
and it also became a medium
that would eventually harbor life.
(wildlife chittering)
Without liquid water,
we certainly would not
get life as we know it.
♪ ♪
LANE: Things that life needs it to do
here on Earth,
to have structures like cells,
for example,
and all kinds of machinery
that operate in water,
that works really well.
♪ ♪
NARRATOR: The early atmosphere had started a transformation
that would set our planet apart
from every other in the solar system.
♪ ♪
The young planet is only a few hundred million years old.
The atmosphere has made Earth a water world.
(waves crashing)
♪ ♪
And in that water, something remarkable is happening...
♪ ♪
Arguably, the most pivotal moment in Earth's history.
It would lead to the reshaping of the entire atmosphere.
♪ ♪
So much of this moment is still a mystery.
But we do know that one day on Earth,
complex molecules come together
to form something never seen on the planet before...
Life.
WONG: The origin of life
is one of the greatest questions in science,
and it's fair to say that we don't know when,
where, or how life started.
♪ ♪
(waves crashing)
NARRATOR: A shallow rock pool is one of the leading theories.
ANDERSON: People think that shallow pools
would have been
a potentially important site
for the origin of life,
because they can get wet and dry
over and over again.
(waves lapping)
LYONS: Through this repeated cycling of wetting and drying,
reflooding and evaporating,
maybe through a tide,
maybe through seasonal variation,
more and more complex molecules can form.
ANDERSON: And that process could've been the precursors
for things like DNA, which is what makes up
the information in our cells today.
NARRATOR: But there are many other theories
about how life could have started.
♪ ♪
LINGAPPA: Some scientists think
life began in a deep sea hydrothermal vent.
LANE: Hydrothermal vents are sources of gases--
like hydrogen sulfide, for example--
and provide the kind of reactive conditions
between hydrogen and CO2 to make the building blocks of life.
♪ ♪
LINGAPPA: Others think that
life originated somewhere
completely else,
not on the Earth at all,
and landed here on a meteorite.
(crashing, booming)
All of these different theories
have sort of different details,
but the punchline is
that life needed water,
and it needed a way to harness energy.
♪ ♪
NARRATOR: Scientists have some idea
what early life might have looked like.
The very first lifeform
that existed was probably
a little bit of genetic material
enclosed in a bubble
of fatty material.
♪ ♪
NARRATOR: And while simple,
some of these early organisms
would have a powerful effect on the atmosphere.
We think that one of the earliest organisms
were organisms that produce methane
as part of the way they eat, and breathe, and live.
♪ ♪
NARRATOR: They take in carbon dioxide and hydrogen,
producing water and methane as waste products.
LYONS: So it is reasonable to estimate
that a vast portion of the early atmosphere
contains large amounts of methane.
♪ ♪
NARRATOR: This increased methane
could have made the orange hazes thicker.
The evolution of our atmosphere is, in many respects,
the story of the evolution of life on our planet.
(wind howling)
Life can change a planet fundamentally.
But it's always this cause and effect kind of dance
between the environment changing life
and life changing the environment.
♪ ♪
NARRATOR: And it's the story of life that reveals how we got from
this orange hazy atmosphere
to the blue, oxygen-rich one we know today.
♪ ♪
This young world is not one we could survive in.
Earth's early oceans are blue-green,
full of dissolved iron,
and they've absorbed large amounts of carbon dioxide,
making them acidic.
(waves crashing)
ANDERSON: Early life would have been
adapted specifically to living in high temperatures
or fairly acidic conditions.
ROTHSCHILD: The ultraviolet radiation was incredibly harsh
because there was no ozone layer.
Furthermore, there was no oxygen in the atmosphere;
it was a very different composition.
NARRATOR: And some organisms today offer clues
to how life survived in this early environment.
♪ ♪
High in the Andes is one of
the largest geyser fields in the world.
♪ ♪
LINGAPPA: This environment is extreme for a lot of reasons.
First of all, the water that's coming
out of this pool is boilingly hot.
And the altitude means
that this environment is subject
to a lot of ultraviolet
radiation from the sun.
NARRATOR: These are conditions most animals would find toxic.
But amongst the boiling water and steam,
life is flourishing.
LINGAPPA: So the colors that we see
in these mats around this pool
are billions of thriving bacteria
that have sort of carved out a niche
in this extreme environment.
It's incredible to find so much life,
not just surviving, but thriving in these conditions.
NARRATOR: Organisms like these are called extremophiles.
They can survive in environments most complex life
would struggle to live in.
LANE: We call them extremophiles
because we didn't really think that life could exist
in such extreme conditions.
ROTHSCHILD: In fact, we know of organisms
that can live up to 122 centigrade.
So this is well over the boiling temperature of water,
and, conversely, we know of ones
that can live well below freezing.
Bacteria could survive in outer space
quite comfortably as well.
These are behaviors that we consider to be extreme
because we would die in them.
NARRATOR: Today, life on Earth is prolific,
thriving in the most unlikely of places.
From under ice sheets,
to acid-filled pools,
and the dark, high-pressure depths of the oceans.
And researching these modern extremophiles
can hint at what was happening
in the early atmosphere.
LINGAPPA: There's been extremophiles
as long as there's been life on Earth.
Studying today's extremophiles
expands our understanding of what life can do
and what life can look like,
including trying to understand
the earliest life and our most ancient ancestors.
GIRGUIS: Life originated on an earth without oxygen.
So looking at the microbes that live
in areas without oxygen
helps us understand how they evolved
to do well in those environments.
(water fizzing)
NARRATOR: But evolving to live in one very specialized habitat
can have its drawbacks.
At this period of time in Earth's history,
life would have been limited in its energy sources.
LINGAPPA: If you're dependent
on something that's being released from a vent in order
to generate energy to be alive, you're going to be
extremely limited in how fast you can grow.
NARRATOR: Because the energy sources early life used
to survive in the oceans were comparatively rare,
organisms struggled to proliferate.
PIERREHUMBERT: They had to take various chemicals
that were produced by geology
and then combine them to release energy.
WONG: But there's about to be
a grand innovation by life that will open up
a nearly limitless source of energy.
NARRATOR: An innovation that would also change
the course of the evolution of the atmosphere.
♪ ♪
Today, life can be found
in puddles, lakes, and oceans across the planet.
(waves crashing)
LINGAPPA: Our world today
is absolutely teeming with life.
Just a single drop of water from the ocean
would contain tens of thousands of cells,
and among them, you would find organisms called cyanobacteria.
NARRATOR: There are over 2,000 known types of cyanobacteria,
and they come in all shapes and sizes.
From long filaments
to geometric shapes.
LYONS: Cyanobacteria evolved,
as best as we understand,
around three billion years ago.
They are astounding, in that
they have the ability to adapt to environmental change,
and to change the environment
because they can be so abundant.
NARRATOR: Cyanobacteria sometimes changed
their local environments by creating structures,
which they used as habitats to live in.
This is one of the clear-cut examples that life was living
even three billion years ago.
This is a formation called a stromatolite.
It's really inspiring that you can pick up a rock like this
and read the history of what was happening
billions of years ago.
That's an amazing book to read.
♪ ♪
LINGAPPA: Cyanobacteria are some of the most
successful organisms ever to live on our planet.
They completely transformed the world.
NARRATOR: This transformation was due
to a revolutionary process
that changed the fundamental composition of the atmosphere.
Around three billion years ago,
Cyanobacteria are developing a unique way
to produce energy.
They capture sunlight
and use it to combine hydrogen from nearby water molecules...
with the carbon dioxide dissolved in the oceans.
With this, they make carbohydrates...
And a very reactive waste product--
oxygen.
LINGAPPA: So this vial contains a culture
of cyanobacteria that I grew.
And these are cyanobacteria of today,
but they are descended from the organisms
that first evolved the ability
to do oxygen-producing photosynthesis.
LANE: There are these
moments in the history of life
that seem to have
only happened once.
Oxygen-producing photosynthesis is one of them.
Was it a freak accident?
We just don't know.
Suddenly, the oceans became the fuel.
It allowed life to scale up at least tenfold.
NARRATOR: Lifeforms can now make energy
using one of the most abundant raw materials
on the planet's surface: water.
This kind of photosynthesis now enables organisms
to get energy from virtually anywhere,
allowing them to spread throughout the oceans.
One cyanobacteria cell becomes two;
two become four.
Multiplying again and again...
until there are trillions.
And we can still find evidence of them everywhere we look,
including in the harshest environments.
LINGAPPA: If you look really closely
in the mats in this pool,
you can see in some of these greener areas
little bubbles that are literally the oxygen
being produced by them
doing photosynthesis right now.
Cyanobacteria demonstrate the power of life on Earth
like nothing else does.
Over two billion years ago,
the ancestors of these extraordinary microbes
unlocked this spectacular ability.
LANE: Cyanobacteria were the first,
and the only, organisms
that ever evolved oxygen-producing photosynthesis.
As soon as they cracked the problem
of how to use water as a fuel,
the oceans became the fuel, and they could
live almost anywhere.
These are seaweeds from the ocean--
ooh, right off the West Coast of Scotland--
and oxygen-producing photosynthesis
was such a brilliant innovation,
that some organisms at that time
actually swallowed the cyanobacteria,
and that's how we got algae like this.
And then the algae ultimately gave rise
to the plants, as well.
(thunder rumbling)
NARRATOR: While life was spreading through the oceans,
above the waves,
the atmosphere was also undergoing
a dramatic transformation.
♪ ♪
Over millions of years,
tectonic movements in the earth
are forging the first large-scale landmass.
And this directly affects
the dynamics of the atmosphere.
♪ ♪
(wind howling)
Air warmed by the new land
drives novel weather patterns across the globe.
(wind blowing)
(thunder rumbling)
Winds sweep dust high up into the air,
where tiny particles seed vast pillars of clouds.
♪ ♪
Something resembling our modern-day weather system
is, at last, taking shape.
HAZEN: The appearance of large continental masses
had a huge impact on the atmosphere.
We would have seen
high-pressure systems,
low-pressure systems,
prevailing winds, rains,
very much like the modern world.
♪ ♪
NARRATOR: Meanwhile, in the oceans,
trillions of cyanobacteria
are photosynthesizing and producing oxygen.
The first time it has ever been made
in significant amounts on Earth.
Oxygen bubbles race upwards.
♪ ♪
But most vanish before they can reach the surface.
(waves crashing)
Earth's early atmosphere seems destined
to stay as thick orange hazes.
So what is making the oxygen disappear?
The answer lies in a unique property oxygen has.
Oxygen is an extremely reactive element.
DOTTIN: What's unique about oxygen
is that it has a chemical makeup
that allows it to bond with
so many different elements;
almost all of the elements that exist.
NARRATOR: And this incredible reactive power
is the reason many scientists think
oxygen struggled to escape the water.
♪ ♪
Around three billion years ago,
Earth's oceans were packed
full of dissolved metals.
Metals like iron.
This key that we see here
is rusty,
and in order to form rust
we need three ingredients.
The first is iron, which is
what this key is made of,
the second is water,
or water vapor in the air,
and the third crucial ingredient
is oxygen.
NARRATOR: And in Earth's early oceans,
oxygen began to react more and more with iron and other metals,
as cyanobacteria spread across the oceans.
♪ ♪
Below the surface...
it's raining.
As oxygen and iron interact,
rust particles form in the water,
and sink down to the seabed...
trapping the oxygen and iron
to form solid rock at the bottom of the ocean.
Vast swaths of the water turn red.
LYONS: There were many,
many ingredients in the ocean
at that time that would
readily react with oxygen.
HAZEN: So there was no way for atmospheric oxygen
to build up to any significant level.
(seagulls calling)
NARRATOR: A record of the reactions
that happened long ago can be seen
in formations today.
This is a sedimentary rock,
meaning it's made up of many different layers
that were all deposited
on the sea floor
over millions of years.
NARRATOR: And this sample is evidence
of how iron can react to form solid rock.
This particular sedimentary rock
is a banded iron formation,
and we can see red color
in the rock here,
and that red is evidence
of iron oxidation or rust.
NARRATOR: As these reactions continued,
the metals in the ocean water
started to diminish, locked away in the seafloor.
LINGAPPA: Eventually,
the oceans would have
been depleted of the things
that would readily react
with the oxygen that was being released.
NARRATOR: But cyanobacteria continued to produce oxygen.
♪ ♪
For up to half a billion years,
this oxygen has been trapped
in the oceans.
But now, increasing amounts are free to escape.
(water bubbling)
♪ ♪
Over just a few million years,
oxygen floods into the atmosphere.
(bubbling intensifies)
♪ ♪
The very composition of the atmosphere itself is changing.
♪ ♪
This is known as the Great Oxidation Event.
DOTTIN: The Great Oxidation Event
was one of the most significant periods in Earth's history.
♪ ♪
It was a period of time
over which oxygen built up
in the atmosphere.
LINGAPPA: Not only did it
completely change the environment,
it also completely changed the possibilities
available for life.
ROTHSCHILD: It is almost
impossible to overstate how important
the production of oxygen on the Earth was
to the history of planet Earth.
All of a sudden, new chemical possibilities arose
all over the surface of Earth,
and amazing changes were to follow.
NARRATOR: Oxygen is used by organisms today
to release energy
from the food they eat, so their bodies can use it.
And since oxygen is so reactive,
this energy release is immense.
ROTHSCHILD: If you use oxygen,
you are 19 times more efficient
than an organism that doesn't use oxygen.
♪ ♪
NARRATOR: Because of its reactive power,
one of the first consequences of oxygen in the atmosphere
is on the rocks themselves.
Now, depending on your perspective,
oxygen can be a creative or a destructive force.
♪ ♪
NARRATOR: As it spreads through the atmosphere,
oxygen begins to react vigorously
with the surface of the Earth.
Rocks with metals like iron react
with the oxygen and crumble.
All the elements are in place for vast dust storms to form.
The young world is being ripped up by its own atmosphere.
Earth's surface is changing color
as new minerals are created.
TIKOO: We had the formation of all these minerals
that did not exist prior
to the Great Oxidation Event.
Copper minerals like these,
blue and green minerals,
yellow and orange, uranium minerals,
minerals of molybdenum, minerals of nickel,
minerals of cobalt, color of the rainbow,
all in new minerals
form because of the Great Oxidation Event.
NARRATOR: It isn't just minerals.
Oxygen also makes key nutrients vital for life
far more available
by reacting with a number of elements to form acids.
Things like sulfuric acid
and nitric acid,
and acid has this property
that when it rains out onto the surface of the earth,
it can dissolve rocks, cause weathering
and runoff of nutrients and minerals
into lakes and oceans.
HAZEN: Those nutrients flowed
into the ocean, there were new opportunities for life.
It really was a game changer.
LYONS: And as oxygen began to increase in the atmosphere,
the relatively minor elements
that are essential for all the processes of life
became more abundant.
In many cases, those are metals.
♪ ♪
LINGAPPA: Life on Earth evolved
because of the conditions that the Earth provided.
But, in turn, life has completely remodeled
the planet, and the world that we know today
is, in so many ways, been created by life.
(flamingos squawking)
♪ ♪
NARRATOR: This co-evolution
of earth and the atmosphere is apparent everywhere we look.
(waves crashing)
ANDERSON: These particular cliffs were formed
when dinosaurs walked the earth.
But the beautiful red that you can see
in these bands here is formed
from the iron in these cliffs
combined with water and oxygen to create iron oxides.
So we see an example of oxygen changing the very landscape.
NARRATOR: Oxygen transformed our oceans, reshaped our land,
and became a vital ingredient for life today.
But this volatile gas
was to have one more dramatic impact on the world.
♪ ♪
Across the globe, oxygen levels are rising,
slowly enriching the air, reacting with the methane
to form carbon dioxide and water.
(waves crashing)
Over millions of years,
the orange hazes are fading away.
Sunlight is scattered by the nitrogen and oxygen,
producing Earth's first oxygen-rich
blue skies.
♪ ♪
In the upper atmosphere,
ultraviolet light from the sun
causes the oxygen to form a new protective molecule...
ozone.
HAZEN: Ozone is made
of three oxygen atoms bonded together,
and they absorb ultraviolet radiation really well.
So, if you put a little bit of ozone
in the atmosphere, it's like sunblock.
NARRATOR: The level of oxygen in the atmosphere
will fluctuate over the next two billion years,
but eventually, reach the 21% we have today.
♪ ♪
But the story of our atmosphere is not over.
It will continue to evolve,
both naturally, and under the influence of human activity.
ROTHSCHILD: If we don't understand
the history of the atmosphere,
how can we possibly be the stewards
of the atmosphere moving forward?
PARAI: Because our atmosphere is totally unique,
there's nothing like it that we found
on any other planetary body.
WONG: Our atmospheric composition may actually be
the thing that is screaming out to the cosmos,
a signal to the universe that we're here,
that life exists on our world.
DOTTIN: But then,
in many ways, we're going back in time,
where we're putting all of
the chemicals that are toxic to life into the atmosphere.
WONG: Over the past couple hundreds of years,
humanity has made a difference
in the chemical composition of our atmosphere,
most predominantly through the release of
carbon dioxide, and this causes myriad effects on our world.
HAZEN: The atmosphere isn't fragile.
But we are.
We can make changes
to the atmosphere that will make Earth
a very unpleasant place to live.
And so we need to look to ourselves
when we think about changes to the atmosphere.
LINGAPPA: We have immense power
over our planet.
Our atmosphere is precious.
And, if we change it too much,
the whole world will pay the price.
(magma rushing)
GIRGUIS: But by understanding the huge and complex steps
it took to develop our atmosphere, hopefully,
we can develop approaches
to take care of it for generations to come.
♪ ♪
ANNOUNCER: On "NOVA: Ancient Earth"...
People thought it was an outrageous idea.
The planet completely encased in ice.
CECILIA SANDERS: Some of the deepest, most intense cold
Earth has ever experienced.
JAZ MILLAR: Life survived events
that scientists previously thought were unsurvivable.
SUSANNAH PORTER: But, for life to flourish again,
we would need some kind of drastic change.
(explosion roars)
ANNOUNCER: "Frozen," on "NOVA."
Next time.
SINGER: ♪ Wish I could go back in time ♪
♪ ♪