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this program possible.

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Support your local PBS station.

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Earth, our home.

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A rocky world

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enveloped by a thin blue layer

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clinging to the surface.

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Our atmosphere.

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The atmosphere is the only thing

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that stands between us and
that cold, dark void of space.

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The story of the air
that surrounds us

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is the story of Earth itself.

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It is incredible to think that

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this 4.5 billion-year-old rock
could hold clues

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as to how
our early atmosphere formed.

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Clues that reveal colossal transformations.

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It is absolutely mind-blowing
that a tiny bacteria

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could affect a
planetary-scale phenomenon

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like our atmosphere.

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How did our atmosphere make
Earth a unique place,

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the only home to life
as we know it?

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We have never found
another planet that has

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an atmosphere like ours.

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And understanding how
Earth got its atmosphere

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is the first step in
understanding how to protect it.

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"Ancient Earth:
Birth of the Sky,"

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right now, on "NOVA."

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In the depths of space

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lies a young,
unrecognizable planet.

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Bombarded by asteroids,

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this battered world
is our Earth.

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A desolate and volatile globe

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overlooked by a new-born moon
that dominates the sky.

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With no recognizable atmosphere,
the void of space

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nearly touches the surface
of slowly cooling lava.

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There's little protection from
the young sun's harsh radiation.

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This world looks nothing
like the Earth we know today.

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Around four and a half billion
years ago,

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Earth was inhospitable
for life.

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There were meteors and asteroids
bombarding the surface.

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It was too hot.

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It was a terrible place to be.

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The question is,

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how did our planet transform
from this barren rock,

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into somewhere capable of
supporting life?

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This is the story of how our
planet became a living world,

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and the dance, the co-evolution

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between life and our atmosphere.

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

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♪ Or ever see the colors

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♪ That never lived in
our minds ♪

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♪ Just a moment

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♪ Never too far out

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♪ Never close enough

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Today, our planet is
an oasis for life,

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enveloped by a layer of

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nourishing and protective
gases--

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the atmosphere.

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It's extremely thin, it's almost

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like the skin on apple,

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if you pretended the apple
was planet Earth.

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The diameter of the earth

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is about 8,000 miles across,

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and the atmosphere adds

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just another 60 or so miles
on top of that.

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We wouldn't... we wouldn't be

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on planet Earth if it
weren't for the atmosphere.

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And when I say we,

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not just you and me
sitting here, but any life.

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Everything from fungi
to bacteria,

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to tiny insects,
to giant reptiles,

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are entirely dependent

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on this atmosphere of ours.

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It does many things for us,

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and we can think of
the simple answer that we,

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as human beings,
use the oxygen to live.

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Besides oxygen,
other important gases

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make up the atmosphere.

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Perhaps the single most
critical aspect

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of our atmosphere is
the fact that carbon dioxide

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is one of the greenhouse gases
that keeps our planet habitable.

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Nitrogen is an essential

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component of the atmosphere,

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because all life as we know it--

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amino acids, DNA,

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all of the things
that are essential to us--

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rely on nitrogen.

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The atmosphere protects us

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from ultraviolet radiation

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because it has a small amount

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of a molecule called ozone.

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It controls our climate,

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which makes this planet, like,

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a livable temperature for us
at all.

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It sort of insulates us from
the cold of space.

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But the journey to today's
atmosphere

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has been full of drama.

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It's a long, winding road

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between the initial atmosphere
that Earth had

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through all of the various
different kinds of atmospheres

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in the middle of its history,

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and finally arriving at
the atmosphere that we have now.

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Millions of years after
the birth of the planet,

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there's still no atmosphere
as we know it.

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But that is going to change.

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Four and a half billion
years ago,

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the Earth would have been a very
violent place.

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Such that it's called the
Hadean, which comes from

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the god Hades, the Greek god
of the underworld,

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because it was imagined to be

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a hellish kind of place.

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Across the globe,

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molten magma surges up
from within.

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Rivers of fire unleash
a cocktail of gases.

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Carbon dioxide, nitrogen,
and methane

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explode out of volcanoes
in thick plumes.

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These are the building blocks

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of our early atmosphere.

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And we can see echoes of
this young planet

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still on Earth today.

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The volcanoes that we have
here in Iceland look

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very similar to those
that we had

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four and a half
billion years ago.

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The early Earth was dominated
by volcanic activity,

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and this lasted
for millions of years.

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Those volcanoes
acted as pathways

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for the gases that are
trapped inside the magma

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to escape into our atmosphere.

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But how do scientists know

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what the atmosphere was like
in the past?

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Surprisingly,
clues are hidden

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in some of the rocks
we find today.

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Throughout Earth's history
and evolution,

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there's been a dynamic interplay
between the Earth's surface

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and the Earth's atmosphere.

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And because of this interplay,

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there are chemical fingerprints
that are trapped in rocks.

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The rock I'm holding in my hand

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is 3.4 billion years old.

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And there are tiny bubbles

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trapped in this rock

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that are samples

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of Earth's ancient atmosphere.

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So we can measure the gases

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and piece together evidence

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of what the atmosphere was like
in the past.

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However,

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to understand
the very earliest atmosphere,

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scientists
have to go back to a time

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before Earth was formed.

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The planet is made up of rock
that started as debris

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floating in
the early solar system.

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But this debris is also falling
to Earth today as meteorites.

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In my pocket here,

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I've got
the most incredible rock.

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This is a chondrite meteorite.

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These chondrite meteorites

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formed when
rock dust came together

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to form small chunks of
rock like this.

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They eventually
smashed together,

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forming our planet Earth.

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And so these are really the
building blocks of our planet.

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And so it's so humbling,

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because it's not only older
than all of the life

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that we know on Earth,

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but even the planet itself,

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right back to
4.6 billion years ago.

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These rocks are not just the
building blocks of our planet,

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but of our
early atmosphere as well.

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Thanks to these space rocks,

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the ingredients for
our early atmosphere were there.

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It's just that they
were locked inside the planet.

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Chondrite meteorites are
like time machines,

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allowing scientists to
travel back billions of years

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to when the atmosphere
first developed.

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Experts can study these
chondrites in great detail.

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They can analyze them
in the laboratory

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to understand
their chemical signatures,

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and that's going to
tell them about the gases

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that they would have
contributed to

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our early Earth's atmosphere,

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such as water vapor,
carbon dioxide,

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sulfur dioxide, hydrogen,
methane, and nitrogen.

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It's remarkable that you can learn

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about the history
of the atmosphere

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going back billions and
billions of years

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just by studying rocks.

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When I learn about this,
I feel simultaneously

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like a time traveler and
a detective,

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being able to piece together
the story of our ancient Earth.

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Each year, techniques become
more and more powerful

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at deciphering bits
of information of what the Earth

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may have been like

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four and a half
billion years ago,

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that even a short time ago would
have been completely a mystery.

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While the scorching Earth
slowly cools,

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the sky is undergoing
a dramatic transformation.

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The vast sea of stars
becomes obscured.

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And as the young sun rises,

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color bursts
across Earth's sky.

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These dense gases
are the beginnings

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of the atmosphere as we know it.

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What this fledgling atmosphere
looked like is debated.

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Because it's hard to know
the precise amounts

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of the various
gases it contained.

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Gases like methane.

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If there's enough methane,

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it can reach a concentration
where it starts breaking up

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under the action of
ultraviolet light

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and then recombining
into longer molecules,

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making these
smoggy substances.

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That can actually make
these hazes

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which cause an orangey glow.

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With enough methane,

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thick, intermittent orange hazes
would have covered the planet.

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There was probably carbon dioxide.

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There might have been
carbon monoxide, water vapor.

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There might have been

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some nitrogen or cyanide.

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All in all, it was a pretty

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noxious combination of stuff.

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If you were dropped onto
early Earth

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and experienced
that early atmosphere,

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you'd die very quickly
because there was no oxygen.

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Carbon dioxide,
some of the other molecules

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that might have been
present in the atmosphere,

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you just simply
couldn't live on those.

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And the planet is inhospitable
for another reason--

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the landscape has never seen
a drop of liquid water.

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But Earth does have water.

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It is suspended high above
the searing surface,

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pumped into the atmosphere
by volcanoes

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as water vapor with the other
early gases.

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If this ocean in the sky
could reach the ground,

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it would have the power
to utterly transform the planet.

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Today, over 70% of our world

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is covered in water.

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A planet of abundant blue

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with endless rivers,

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freezing icebergs,

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and vast oceans.

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Water that we get out of our tap

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has been through a long journey.

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And the water on our planet

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gets incorporated into various
aspects of our life,

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including weather,
into the food that we eat.

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And because of this,
the water is recycled

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over and over and over again
throughout Earth's history.

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So the water that we drink

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was consumed by dinosaurs

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and had the first microbes

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swimming around in it.

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On the early Earth,

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water is stuck in
the atmosphere.

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But that is about to change.

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High up in the sky,

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00:16:52,467 --> 00:16:55,200
droplets of water,

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00:16:55,200 --> 00:17:00,467
just a fraction of a millimeter
across, are condensing.

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So light and small,
they just float,

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drifting in the air.

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Colliding and merging
with others...

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...they slowly grow until
a tipping point is reached.

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Earth's gravity draws them downwards...

263
00:17:37,467 --> 00:17:40,233
but the journey is short-lived.

264
00:17:43,167 --> 00:17:45,900
The planet is still hot.

265
00:17:47,967 --> 00:17:52,367
So any drops
nearing the ground...

266
00:17:56,233 --> 00:17:58,433
...simply evaporate.

267
00:18:07,000 --> 00:18:08,833
But over time,

268
00:18:08,833 --> 00:18:11,933
that heat is radiating out
into space.

269
00:18:15,367 --> 00:18:18,600
Earth is cooling down.

270
00:18:49,100 --> 00:18:50,267
A deluge of rain

271
00:18:50,267 --> 00:18:52,767
begins to pour down
on the surface.

272
00:19:02,200 --> 00:19:05,700
Huge storms sweep through
the atmosphere.

273
00:19:10,200 --> 00:19:13,200
Oceans of water
are dumped onto the land.

274
00:19:30,167 --> 00:19:35,467
The atmosphere has unleashed
a key molecule for life.

275
00:19:39,467 --> 00:19:40,600
Water changes the surface

276
00:19:40,600 --> 00:19:41,833
of a planet.

277
00:19:41,833 --> 00:19:43,067
It evaporates,

278
00:19:43,067 --> 00:19:44,067
it allows clouds to form

279
00:19:44,067 --> 00:19:45,433
and rain to fall,

280
00:19:45,433 --> 00:19:47,267
it erodes things,

281
00:19:47,267 --> 00:19:49,467
and it also became a medium

282
00:19:49,467 --> 00:19:52,200
that would eventually
harbor life.

283
00:19:56,867 --> 00:19:57,933
Without liquid water,

284
00:19:57,933 --> 00:19:58,967
we certainly would not

285
00:19:58,967 --> 00:20:00,533
get life as we know it.

286
00:20:02,967 --> 00:20:04,633
Things that life needs it to do

287
00:20:04,633 --> 00:20:05,767
here on Earth,

288
00:20:05,767 --> 00:20:06,933
to have structures like cells,

289
00:20:06,933 --> 00:20:08,000
for example,

290
00:20:08,000 --> 00:20:09,167
and all kinds of machinery

291
00:20:09,167 --> 00:20:10,400
that operate in water,

292
00:20:10,400 --> 00:20:11,567
that works really well.

293
00:20:15,367 --> 00:20:18,600
The early atmosphere had started
a transformation

294
00:20:18,600 --> 00:20:21,800
that would set our planet apart

295
00:20:21,800 --> 00:20:25,600
from every other
in the solar system.

296
00:20:42,067 --> 00:20:47,033
The young planet is only a few
hundred million years old.

297
00:20:47,033 --> 00:20:50,967
The atmosphere has
made Earth a water world.

298
00:20:56,800 --> 00:21:01,467
And in that water, something
remarkable is happening...

299
00:21:04,967 --> 00:21:10,067
Arguably, the most pivotal
moment in Earth's history.

300
00:21:11,433 --> 00:21:17,133
It would lead to the reshaping
of the entire atmosphere.

301
00:21:19,900 --> 00:21:23,433
So much of this moment
is still a mystery.

302
00:21:25,467 --> 00:21:30,067
But we do know
that one day on Earth,

303
00:21:30,067 --> 00:21:33,833
complex molecules
come together

304
00:21:33,833 --> 00:21:38,367
to form something never seen
on the planet before...

305
00:21:40,100 --> 00:21:42,467
Life.

306
00:21:44,333 --> 00:21:46,033
The origin of life

307
00:21:46,033 --> 00:21:48,200
is one of the greatest questions
in science,

308
00:21:48,200 --> 00:21:50,633
and it's fair to say
that we don't know when,

309
00:21:50,633 --> 00:21:52,933
where, or how life started.

310
00:21:59,133 --> 00:22:03,967
A shallow rock pool is
one of the leading theories.

311
00:22:03,967 --> 00:22:05,500
People think that shallow pools

312
00:22:05,500 --> 00:22:06,800
would have been

313
00:22:06,800 --> 00:22:07,967
a potentially important site

314
00:22:07,967 --> 00:22:09,267
for the origin of life,

315
00:22:09,267 --> 00:22:10,400
because they can get wet and dry

316
00:22:10,400 --> 00:22:11,533
over and over again.

317
00:22:14,400 --> 00:22:17,800
Through this repeated cycling
of wetting and drying,

318
00:22:17,800 --> 00:22:19,333
reflooding and evaporating,

319
00:22:19,333 --> 00:22:20,900
maybe through a tide,

320
00:22:20,900 --> 00:22:22,467
maybe through seasonal variation,

321
00:22:22,467 --> 00:22:25,267
more and more
complex molecules can form.

322
00:22:27,333 --> 00:22:29,533
And that process could've
been the precursors

323
00:22:29,533 --> 00:22:32,000
for things like DNA,
which is what makes up

324
00:22:32,000 --> 00:22:33,733
the information
in our cells today.

325
00:22:35,667 --> 00:22:37,867
But there are many other theories

326
00:22:37,867 --> 00:22:41,200
about how life
could have started.

327
00:22:45,200 --> 00:22:46,867
Some scientists think

328
00:22:46,867 --> 00:22:50,333
life began in a deep sea
hydrothermal vent.

329
00:22:52,367 --> 00:22:55,333
Hydrothermal vents are
sources of gases--

330
00:22:55,333 --> 00:22:57,533
like hydrogen sulfide,
for example--

331
00:22:57,533 --> 00:22:59,867
and provide the kind of
reactive conditions

332
00:22:59,867 --> 00:23:02,667
between hydrogen and CO2 to make
the building blocks of life.

333
00:23:06,933 --> 00:23:09,600
Others think that

334
00:23:09,600 --> 00:23:10,700
life originated somewhere

335
00:23:10,700 --> 00:23:11,700
completely else,

336
00:23:11,700 --> 00:23:13,333
not on the Earth at all,

337
00:23:13,333 --> 00:23:17,333
and landed here on a meteorite.

338
00:23:21,600 --> 00:23:23,100
All of these different theories

339
00:23:23,100 --> 00:23:24,433
have sort of different details,

340
00:23:24,433 --> 00:23:26,033
but the punchline is

341
00:23:26,033 --> 00:23:27,933
that life needed water,

342
00:23:27,933 --> 00:23:31,267
and it needed a
way to harness energy.

343
00:23:33,667 --> 00:23:35,900
Scientists have some idea

344
00:23:35,900 --> 00:23:38,100
what early life
might have looked like.

345
00:23:41,100 --> 00:23:42,700
The very first lifeform

346
00:23:42,700 --> 00:23:43,967
that existed was probably

347
00:23:43,967 --> 00:23:45,767
a little bit of genetic material

348
00:23:45,767 --> 00:23:48,033
enclosed in a bubble

349
00:23:48,033 --> 00:23:50,033
of fatty material.

350
00:23:52,467 --> 00:23:54,300
And while simple,

351
00:23:54,300 --> 00:23:56,367
some of these early organisms

352
00:23:56,367 --> 00:24:00,167
would have a powerful effect
on the atmosphere.

353
00:24:02,100 --> 00:24:04,867
We think that one of
the earliest organisms

354
00:24:04,867 --> 00:24:07,267
were organisms
that produce methane

355
00:24:07,267 --> 00:24:09,700
as part of the way
they eat, and breathe, and live.

356
00:24:12,667 --> 00:24:16,200
They take in carbon dioxide
and hydrogen,

357
00:24:16,200 --> 00:24:19,467
producing water and methane
as waste products.

358
00:24:19,467 --> 00:24:22,300
So it is reasonable to estimate

359
00:24:22,300 --> 00:24:24,233
that a vast portion of
the early atmosphere

360
00:24:24,233 --> 00:24:26,000
contains large amounts of methane.

361
00:24:28,833 --> 00:24:30,467
This increased methane

362
00:24:30,467 --> 00:24:33,433
could have made
the orange hazes thicker.

363
00:24:36,567 --> 00:24:38,267
The evolution of our
atmosphere is, in many respects,

364
00:24:38,267 --> 00:24:40,533
the story of the evolution
of life on our planet.

365
00:24:42,800 --> 00:24:45,767
Life can change
a planet fundamentally.

366
00:24:47,633 --> 00:24:51,267
But it's always this
cause and effect kind of dance

367
00:24:51,267 --> 00:24:53,433
between the
environment changing life

368
00:24:53,433 --> 00:24:55,233
and life
changing the environment.

369
00:24:59,100 --> 00:25:02,533
And it's the story of life
that reveals how we got from

370
00:25:02,533 --> 00:25:06,033
this orange hazy atmosphere

371
00:25:06,033 --> 00:25:09,933
to the blue, oxygen-rich one
we know today.

372
00:25:20,100 --> 00:25:23,633
This young world is not one
we could survive in.

373
00:25:25,800 --> 00:25:28,833
Earth's early oceans
are blue-green,

374
00:25:28,833 --> 00:25:32,733
full of dissolved iron,

375
00:25:32,733 --> 00:25:36,067
and they've absorbed large
amounts of carbon dioxide,

376
00:25:36,067 --> 00:25:39,367
making them acidic.

377
00:25:42,900 --> 00:25:44,700
Early life would have been

378
00:25:44,700 --> 00:25:46,633
adapted specifically to
living in high temperatures

379
00:25:46,633 --> 00:25:49,167
or fairly acidic conditions.

380
00:25:51,467 --> 00:25:53,767
The ultraviolet radiation was
incredibly harsh

381
00:25:53,767 --> 00:25:56,133
because
there was no ozone layer.

382
00:25:57,900 --> 00:25:59,800
Furthermore, there was
no oxygen in the atmosphere;

383
00:25:59,800 --> 00:26:01,367
it was a very different composition.

384
00:26:03,500 --> 00:26:06,700
And some organisms today
offer clues

385
00:26:06,700 --> 00:26:09,233
to how life survived
in this early environment.

386
00:26:13,233 --> 00:26:15,800
High in the Andes is one of

387
00:26:15,800 --> 00:26:18,067
the largest geyser fields
in the world.

388
00:26:26,167 --> 00:26:29,000
This environment is extreme
for a lot of reasons.

389
00:26:29,000 --> 00:26:31,467
First of all,
the water that's coming

390
00:26:31,467 --> 00:26:33,633
out of this pool
is boilingly hot.

391
00:26:33,633 --> 00:26:35,467
And the altitude means

392
00:26:35,467 --> 00:26:37,633
that this environment is subject

393
00:26:37,633 --> 00:26:38,833
to a lot of ultraviolet

394
00:26:38,833 --> 00:26:40,233
radiation from the sun.

395
00:26:42,367 --> 00:26:47,033
These are conditions
most animals would find toxic.

396
00:26:48,867 --> 00:26:52,633
But amongst
the boiling water and steam,

397
00:26:52,633 --> 00:26:56,300
life is flourishing.

398
00:26:56,300 --> 00:26:58,000
So the colors that we see

399
00:26:58,000 --> 00:26:59,800
in these mats around this pool

400
00:26:59,800 --> 00:27:02,033
are billions
of thriving bacteria

401
00:27:02,033 --> 00:27:04,567
that have sort of
carved out a niche

402
00:27:04,567 --> 00:27:06,500
in this extreme environment.

403
00:27:07,667 --> 00:27:11,500
It's incredible
to find so much life,

404
00:27:11,500 --> 00:27:16,633
not just surviving, but
thriving in these conditions.

405
00:27:16,633 --> 00:27:20,500
Organisms like these
are called extremophiles.

406
00:27:22,267 --> 00:27:25,000
They can survive in environments
most complex life

407
00:27:25,000 --> 00:27:29,033
would struggle to live in.

408
00:27:29,033 --> 00:27:30,467
We call them extremophiles

409
00:27:30,467 --> 00:27:33,867
because we didn't really think
that life could exist

410
00:27:33,867 --> 00:27:35,267
in such extreme conditions.

411
00:27:37,933 --> 00:27:40,900
In fact, we know of organisms

412
00:27:40,900 --> 00:27:45,167
that can live
up to 122 centigrade.

413
00:27:45,167 --> 00:27:48,200
So this is well over the
boiling temperature of water,

414
00:27:48,200 --> 00:27:50,233
and, conversely, we know of ones

415
00:27:50,233 --> 00:27:52,600
that can live
well below freezing.

416
00:27:52,600 --> 00:27:54,500
Bacteria could survive
in outer space

417
00:27:54,500 --> 00:27:55,900
quite comfortably as well.

418
00:27:55,900 --> 00:27:58,867
These are behaviors
that we consider to be extreme

419
00:27:58,867 --> 00:28:00,400
because we would die in them.

420
00:28:03,100 --> 00:28:06,900
Today, life on Earth
is prolific,

421
00:28:06,900 --> 00:28:10,267
thriving in
the most unlikely of places.

422
00:28:11,500 --> 00:28:14,067
From under ice sheets,

423
00:28:14,067 --> 00:28:17,933
to acid-filled pools,

424
00:28:17,933 --> 00:28:23,100
and the dark, high-pressure
depths of the oceans.

425
00:28:23,100 --> 00:28:26,967
And researching
these modern extremophiles

426
00:28:26,967 --> 00:28:30,300
can hint at what was happening

427
00:28:30,300 --> 00:28:33,600
in the early atmosphere.

428
00:28:33,600 --> 00:28:36,067
There's been extremophiles

429
00:28:36,067 --> 00:28:37,933
as long as there's been
life on Earth.

430
00:28:38,967 --> 00:28:41,867
Studying today's extremophiles

431
00:28:41,867 --> 00:28:44,433
expands our understanding
of what life can do

432
00:28:44,433 --> 00:28:45,600
and what life can look like,

433
00:28:45,600 --> 00:28:47,533
including trying to understand

434
00:28:47,533 --> 00:28:52,000
the earliest life and
our most ancient ancestors.

435
00:28:52,000 --> 00:28:56,033
Life originated
on an earth without oxygen.

436
00:28:56,033 --> 00:28:58,500
So looking at
the microbes that live

437
00:28:58,500 --> 00:29:00,767
in areas without oxygen

438
00:29:00,767 --> 00:29:03,367
helps us understand
how they evolved

439
00:29:03,367 --> 00:29:06,067
to do well
in those environments.

440
00:29:08,033 --> 00:29:12,633
But evolving to live
in one very specialized habitat

441
00:29:12,633 --> 00:29:15,633
can have its drawbacks.

442
00:29:15,633 --> 00:29:17,833
At this period of time
in Earth's history,

443
00:29:17,833 --> 00:29:22,100
life would have been
limited in its energy sources.

444
00:29:23,400 --> 00:29:24,867
If you're dependent

445
00:29:24,867 --> 00:29:28,067
on something that's being
released from a vent in order

446
00:29:28,067 --> 00:29:32,000
to generate energy to be alive,
you're going to be

447
00:29:32,000 --> 00:29:36,733
extremely limited
in how fast you can grow.

448
00:29:36,733 --> 00:29:40,367
Because the energy sources
early life used

449
00:29:40,367 --> 00:29:44,000
to survive in the oceans
were comparatively rare,

450
00:29:44,000 --> 00:29:48,100
organisms
struggled to proliferate.

451
00:29:48,100 --> 00:29:50,333
They had to take
various chemicals

452
00:29:50,333 --> 00:29:52,167
that were produced by geology

453
00:29:52,167 --> 00:29:54,533
and then combine them
to release energy.

454
00:29:57,400 --> 00:29:59,633
But there's about to be

455
00:29:59,633 --> 00:30:04,367
a grand innovation by life
that will open up

456
00:30:04,367 --> 00:30:07,100
a nearly limitless
source of energy.

457
00:30:09,233 --> 00:30:11,533
An innovation
that would also change

458
00:30:11,533 --> 00:30:16,267
the course of
the evolution of the atmosphere.

459
00:30:23,833 --> 00:30:25,500
Today, life can be found

460
00:30:25,500 --> 00:30:28,700
in puddles, lakes,
and oceans across the planet.

461
00:30:33,400 --> 00:30:34,533
Our world today

462
00:30:34,533 --> 00:30:36,867
is absolutely teeming with life.

463
00:30:36,867 --> 00:30:40,933
Just a single drop
of water from the ocean

464
00:30:40,933 --> 00:30:44,933
would contain
tens of thousands of cells,

465
00:30:44,933 --> 00:30:48,667
and among them, you would find
organisms called cyanobacteria.

466
00:30:50,600 --> 00:30:54,300
There are over 2,000 known types
of cyanobacteria,

467
00:30:54,300 --> 00:30:57,000
and they come in
all shapes and sizes.

468
00:30:59,867 --> 00:31:01,000
From long filaments

469
00:31:01,000 --> 00:31:05,233
to geometric shapes.

470
00:31:06,233 --> 00:31:07,767
Cyanobacteria evolved,

471
00:31:07,767 --> 00:31:09,400
as best as we understand,

472
00:31:09,400 --> 00:31:11,567
around three billion years ago.

473
00:31:11,567 --> 00:31:13,600
They are astounding, in that

474
00:31:13,600 --> 00:31:16,167
they have the ability to adapt
to environmental change,

475
00:31:16,167 --> 00:31:18,400
and to change the environment

476
00:31:18,400 --> 00:31:21,467
because they can be so abundant.

477
00:31:21,467 --> 00:31:24,333
Cyanobacteria sometimes changed

478
00:31:24,333 --> 00:31:26,867
their local environments
by creating structures,

479
00:31:26,867 --> 00:31:31,800
which they used
as habitats to live in.

480
00:31:31,800 --> 00:31:36,667
This is one of the clear-cut
examples that life was living

481
00:31:36,667 --> 00:31:37,933
even three billion years ago.

482
00:31:37,933 --> 00:31:40,033
This is a formation
called a stromatolite.

483
00:31:40,033 --> 00:31:44,900
It's really inspiring that
you can pick up a rock like this

484
00:31:44,900 --> 00:31:47,067
and read the history
of what was happening

485
00:31:47,067 --> 00:31:48,300
billions of years ago.

486
00:31:48,300 --> 00:31:50,567
That's an amazing book to read.

487
00:31:53,967 --> 00:31:56,100
Cyanobacteria
are some of the most

488
00:31:56,100 --> 00:32:00,100
successful organisms
ever to live on our planet.

489
00:32:00,100 --> 00:32:04,167
They completely
transformed the world.

490
00:32:06,867 --> 00:32:09,033
This transformation was due

491
00:32:09,033 --> 00:32:10,933
to a revolutionary process

492
00:32:10,933 --> 00:32:16,567
that changed the fundamental
composition of the atmosphere.

493
00:32:19,367 --> 00:32:21,867
Around three billion years ago,

494
00:32:21,867 --> 00:32:25,367
Cyanobacteria are developing
a unique way

495
00:32:25,367 --> 00:32:28,600
to produce energy.

496
00:32:28,600 --> 00:32:31,833
They capture sunlight

497
00:32:31,833 --> 00:32:36,867
and use it to combine hydrogen
from nearby water molecules...

498
00:32:38,533 --> 00:32:43,300
with the carbon dioxide
dissolved in the oceans.

499
00:32:43,300 --> 00:32:46,267
With this,
they make carbohydrates...

500
00:32:50,333 --> 00:32:55,900
And a very reactive
waste product--

501
00:32:55,900 --> 00:32:59,533
oxygen.

502
00:32:59,533 --> 00:33:02,267
So this vial contains a culture

503
00:33:02,267 --> 00:33:04,967
of cyanobacteria that I grew.

504
00:33:04,967 --> 00:33:06,700
And these are
cyanobacteria of today,

505
00:33:06,700 --> 00:33:10,933
but they are descended
from the organisms

506
00:33:10,933 --> 00:33:13,633
that first evolved the ability

507
00:33:13,633 --> 00:33:16,867
to do oxygen-producing
photosynthesis.

508
00:33:19,567 --> 00:33:20,600
There are these

509
00:33:20,600 --> 00:33:22,433
moments in the history of life

510
00:33:22,433 --> 00:33:23,600
that seem to have

511
00:33:23,600 --> 00:33:24,900
only happened once.

512
00:33:24,900 --> 00:33:28,800
Oxygen-producing
photosynthesis is one of them.

513
00:33:28,800 --> 00:33:30,267
Was it a freak accident?

514
00:33:30,267 --> 00:33:32,800
We just don't know.

515
00:33:32,800 --> 00:33:36,300
Suddenly, the oceans
became the fuel.

516
00:33:36,300 --> 00:33:38,500
It allowed life
to scale up at least tenfold.

517
00:33:40,500 --> 00:33:42,967
Lifeforms can now make energy

518
00:33:42,967 --> 00:33:46,167
using one of the
most abundant raw materials

519
00:33:46,167 --> 00:33:51,400
on the planet's surface: water.

520
00:33:51,400 --> 00:33:54,900
This kind of photosynthesis
now enables organisms

521
00:33:54,900 --> 00:33:58,567
to get energy
from virtually anywhere,

522
00:33:58,567 --> 00:34:03,000
allowing them
to spread throughout the oceans.

523
00:34:05,800 --> 00:34:09,600
One cyanobacteria cell
becomes two;

524
00:34:09,600 --> 00:34:11,700
two become four.

525
00:34:13,533 --> 00:34:18,600
Multiplying again and again...

526
00:34:20,133 --> 00:34:25,233
until there are trillions.

527
00:34:32,100 --> 00:34:36,067
And we can still find evidence
of them everywhere we look,

528
00:34:36,067 --> 00:34:40,667
including
in the harshest environments.

529
00:34:44,167 --> 00:34:45,300
If you look really closely

530
00:34:45,300 --> 00:34:48,000
in the mats in this pool,

531
00:34:48,000 --> 00:34:50,033
you can see in some
of these greener areas

532
00:34:50,033 --> 00:34:54,333
little bubbles that are
literally the oxygen

533
00:34:54,333 --> 00:34:56,067
being produced by them

534
00:34:56,067 --> 00:34:58,667
doing photosynthesis right now.

535
00:35:01,333 --> 00:35:04,200
Cyanobacteria demonstrate
the power of life on Earth

536
00:35:04,200 --> 00:35:06,800
like nothing else does.

537
00:35:06,800 --> 00:35:08,233
Over two billion years ago,

538
00:35:08,233 --> 00:35:11,267
the ancestors of these
extraordinary microbes

539
00:35:11,267 --> 00:35:13,733
unlocked
this spectacular ability.

540
00:35:15,833 --> 00:35:18,200
Cyanobacteria were the first,

541
00:35:18,200 --> 00:35:19,733
and the only, organisms

542
00:35:19,733 --> 00:35:25,433
that ever evolved
oxygen-producing photosynthesis.

543
00:35:25,433 --> 00:35:27,067
As soon as they cracked
the problem

544
00:35:27,067 --> 00:35:28,800
of how to use water as a fuel,

545
00:35:28,800 --> 00:35:30,900
the oceans became the fuel,
and they could

546
00:35:30,900 --> 00:35:34,833
live almost anywhere.

547
00:35:37,100 --> 00:35:40,400
These are seaweeds
from the ocean--

548
00:35:40,400 --> 00:35:43,333
ooh, right off
the West Coast of Scotland--

549
00:35:43,333 --> 00:35:46,667
and oxygen-producing
photosynthesis

550
00:35:46,667 --> 00:35:49,500
was such a brilliant innovation,

551
00:35:49,500 --> 00:35:52,067
that some organisms at that time

552
00:35:52,067 --> 00:35:54,967
actually swallowed
the cyanobacteria,

553
00:35:54,967 --> 00:35:59,233
and that's how
we got algae like this.

554
00:35:59,233 --> 00:36:01,467
And then the algae
ultimately gave rise

555
00:36:01,467 --> 00:36:03,067
to the plants, as well.

556
00:36:05,900 --> 00:36:09,200
While life was spreading
through the oceans,

557
00:36:09,200 --> 00:36:12,033
above the waves,

558
00:36:12,033 --> 00:36:14,267
the atmosphere
was also undergoing

559
00:36:14,267 --> 00:36:16,000
a dramatic transformation.

560
00:36:31,433 --> 00:36:34,100
Over millions of years,

561
00:36:34,100 --> 00:36:36,033
tectonic movements in the earth

562
00:36:36,033 --> 00:36:40,033
are forging the first
large-scale landmass.

563
00:36:45,567 --> 00:36:47,900
And this directly affects

564
00:36:47,900 --> 00:36:50,567
the dynamics of the atmosphere.

565
00:36:57,733 --> 00:37:00,100
Air warmed by the new land

566
00:37:00,100 --> 00:37:03,667
drives novel weather patterns
across the globe.

567
00:37:07,967 --> 00:37:13,867
Winds sweep dust
high up into the air,

568
00:37:13,867 --> 00:37:17,833
where tiny particles
seed vast pillars of clouds.

569
00:37:20,667 --> 00:37:24,033
Something resembling
our modern-day weather system

570
00:37:24,033 --> 00:37:28,567
is, at last, taking shape.

571
00:37:28,567 --> 00:37:30,933
The appearance
of large continental masses

572
00:37:30,933 --> 00:37:33,333
had a huge impact
on the atmosphere.

573
00:37:33,333 --> 00:37:34,633
We would have seen

574
00:37:34,633 --> 00:37:35,633
high-pressure systems,

575
00:37:35,633 --> 00:37:36,733
low-pressure systems,

576
00:37:36,733 --> 00:37:38,467
prevailing winds, rains,

577
00:37:38,467 --> 00:37:41,200
very much like the modern world.

578
00:37:48,300 --> 00:37:50,167
Meanwhile, in the oceans,

579
00:37:50,167 --> 00:37:52,233
trillions of cyanobacteria

580
00:37:52,233 --> 00:37:56,600
are photosynthesizing
and producing oxygen.

581
00:37:56,600 --> 00:37:59,967
The first time
it has ever been made

582
00:37:59,967 --> 00:38:03,367
in significant amounts on Earth.

583
00:38:05,533 --> 00:38:08,933
Oxygen bubbles race upwards.

584
00:38:15,667 --> 00:38:21,533
But most vanish before
they can reach the surface.

585
00:38:23,767 --> 00:38:26,667
Earth's early atmosphere
seems destined

586
00:38:26,667 --> 00:38:31,000
to stay as thick orange hazes.

587
00:38:33,567 --> 00:38:39,133
So what is making
the oxygen disappear?

588
00:38:39,133 --> 00:38:44,200
The answer lies
in a unique property oxygen has.

589
00:38:44,200 --> 00:38:46,567
Oxygen is
an extremely reactive element.

590
00:38:48,633 --> 00:38:51,000
What's unique about oxygen

591
00:38:51,000 --> 00:38:53,267
is that it has a chemical makeup

592
00:38:53,267 --> 00:38:55,433
that allows it to bond with

593
00:38:55,433 --> 00:38:57,600
so many different elements;

594
00:38:57,600 --> 00:39:01,033
almost all
of the elements that exist.

595
00:39:01,033 --> 00:39:03,767
And this incredible
reactive power

596
00:39:03,767 --> 00:39:06,067
is the reason
many scientists think

597
00:39:06,067 --> 00:39:10,333
oxygen struggled
to escape the water.

598
00:39:21,300 --> 00:39:24,367
Around three billion years ago,

599
00:39:24,367 --> 00:39:26,933
Earth's oceans were packed

600
00:39:26,933 --> 00:39:30,467
full of dissolved metals.

601
00:39:30,467 --> 00:39:33,467
Metals like iron.

602
00:39:34,500 --> 00:39:36,267
This key that we see here

603
00:39:36,267 --> 00:39:37,300
is rusty,

604
00:39:37,300 --> 00:39:39,067
and in order to form rust

605
00:39:39,067 --> 00:39:40,400
we need three ingredients.

606
00:39:40,400 --> 00:39:42,100
The first is iron, which is

607
00:39:42,100 --> 00:39:43,867
what this key is made of,

608
00:39:43,867 --> 00:39:45,067
the second is water,

609
00:39:45,067 --> 00:39:46,633
or water vapor in the air,

610
00:39:46,633 --> 00:39:48,900
and the third crucial ingredient

611
00:39:48,900 --> 00:39:51,300
is oxygen.

612
00:39:51,300 --> 00:39:53,100
And in Earth's early oceans,

613
00:39:53,100 --> 00:39:58,667
oxygen began to react more and
more with iron and other metals,

614
00:39:58,667 --> 00:40:02,767
as cyanobacteria
spread across the oceans.

615
00:40:10,300 --> 00:40:12,867
Below the surface...

616
00:40:14,667 --> 00:40:17,600
it's raining.

617
00:40:17,600 --> 00:40:21,867
As oxygen and iron interact,

618
00:40:21,867 --> 00:40:25,133
rust particles form
in the water,

619
00:40:25,133 --> 00:40:27,000
and sink down to the seabed...

620
00:40:29,433 --> 00:40:32,000
trapping the oxygen and iron

621
00:40:32,000 --> 00:40:36,433
to form solid rock
at the bottom of the ocean.

622
00:40:37,567 --> 00:40:42,400
Vast swaths of the water
turn red.

623
00:40:42,400 --> 00:40:43,700
There were many,

624
00:40:43,700 --> 00:40:45,200
many ingredients in the ocean

625
00:40:45,200 --> 00:40:46,767
at that time that would

626
00:40:46,767 --> 00:40:48,400
readily react with oxygen.

627
00:40:50,033 --> 00:40:52,533
So there was no way
for atmospheric oxygen

628
00:40:52,533 --> 00:40:54,667
to build up
to any significant level.

629
00:40:59,533 --> 00:41:01,300
A record of the reactions

630
00:41:01,300 --> 00:41:04,200
that happened long ago
can be seen

631
00:41:04,200 --> 00:41:08,400
in formations today.

632
00:41:08,400 --> 00:41:09,933
This is a sedimentary rock,

633
00:41:09,933 --> 00:41:12,100
meaning it's made up
of many different layers

634
00:41:12,100 --> 00:41:13,367
that were all deposited

635
00:41:13,367 --> 00:41:14,467
on the sea floor

636
00:41:14,467 --> 00:41:15,800
over millions of years.

637
00:41:18,733 --> 00:41:20,600
And this sample is evidence

638
00:41:20,600 --> 00:41:25,433
of how iron can react
to form solid rock.

639
00:41:25,433 --> 00:41:27,433
This particular sedimentary rock

640
00:41:27,433 --> 00:41:29,167
is a banded iron formation,

641
00:41:29,167 --> 00:41:30,900
and we can see red color

642
00:41:30,900 --> 00:41:32,067
in the rock here,

643
00:41:32,067 --> 00:41:33,800
and that red is evidence

644
00:41:33,800 --> 00:41:36,033
of iron oxidation or rust.

645
00:41:37,367 --> 00:41:39,533
As these reactions continued,

646
00:41:39,533 --> 00:41:41,667
the metals in the ocean water

647
00:41:41,667 --> 00:41:46,400
started to diminish,
locked away in the seafloor.

648
00:41:46,400 --> 00:41:47,867
Eventually,

649
00:41:47,867 --> 00:41:49,700
the oceans would have

650
00:41:49,700 --> 00:41:51,300
been depleted of the things

651
00:41:51,300 --> 00:41:52,467
that would readily react

652
00:41:52,467 --> 00:41:56,967
with the oxygen
that was being released.

653
00:41:56,967 --> 00:42:01,033
But cyanobacteria
continued to produce oxygen.

654
00:42:11,867 --> 00:42:14,300
For up to half a billion years,

655
00:42:14,300 --> 00:42:16,000
this oxygen has been trapped

656
00:42:16,000 --> 00:42:18,967
in the oceans.

657
00:42:26,267 --> 00:42:29,233
But now, increasing amounts
are free to escape.

658
00:42:38,667 --> 00:42:41,833
Over just a few million years,

659
00:42:41,833 --> 00:42:45,633
oxygen floods
into the atmosphere.

660
00:43:00,867 --> 00:43:06,133
The very composition of the
atmosphere itself is changing.

661
00:43:21,500 --> 00:43:26,733
This is known
as the Great Oxidation Event.

662
00:43:27,933 --> 00:43:29,900
The Great Oxidation Event

663
00:43:29,900 --> 00:43:33,067
was one of the most significant
periods in Earth's history.

664
00:43:37,333 --> 00:43:39,000
It was a period of time

665
00:43:39,000 --> 00:43:40,567
over which oxygen built up

666
00:43:40,567 --> 00:43:43,367
in the atmosphere.

667
00:43:43,367 --> 00:43:44,867
Not only did it

668
00:43:44,867 --> 00:43:46,867
completely change
the environment,

669
00:43:46,867 --> 00:43:50,433
it also completely changed
the possibilities

670
00:43:50,433 --> 00:43:52,467
available for life.

671
00:43:52,467 --> 00:43:53,733
It is almost

672
00:43:53,733 --> 00:43:57,233
impossible to overstate
how important

673
00:43:57,233 --> 00:44:00,567
the production of oxygen
on the Earth was

674
00:44:00,567 --> 00:44:04,200
to the history of planet Earth.

675
00:44:04,200 --> 00:44:07,833
All of a sudden, new
chemical possibilities arose

676
00:44:07,833 --> 00:44:09,600
all over the surface of Earth,

677
00:44:09,600 --> 00:44:11,933
and amazing changes
were to follow.

678
00:44:13,067 --> 00:44:15,933
Oxygen is used
by organisms today

679
00:44:15,933 --> 00:44:17,533
to release energy

680
00:44:17,533 --> 00:44:20,300
from the food they eat,
so their bodies can use it.

681
00:44:20,300 --> 00:44:22,900
And since oxygen is so reactive,

682
00:44:22,900 --> 00:44:27,000
this energy release is immense.

683
00:44:27,000 --> 00:44:30,533
If you use oxygen,

684
00:44:30,533 --> 00:44:33,400
you are 19 times more efficient

685
00:44:33,400 --> 00:44:37,700
than an organism
that doesn't use oxygen.

686
00:44:45,967 --> 00:44:48,300
Because of its reactive power,

687
00:44:48,300 --> 00:44:52,700
one of the first consequences
of oxygen in the atmosphere

688
00:44:52,700 --> 00:44:57,000
is on the rocks themselves.

689
00:44:57,000 --> 00:44:58,633
Now, depending
on your perspective,

690
00:44:58,633 --> 00:45:02,167
oxygen can be a creative
or a destructive force.

691
00:45:08,400 --> 00:45:10,533
As it spreads
through the atmosphere,

692
00:45:10,533 --> 00:45:13,167
oxygen begins
to react vigorously

693
00:45:13,167 --> 00:45:17,100
with the surface of the Earth.

694
00:45:17,100 --> 00:45:20,433
Rocks with metals
like iron react

695
00:45:20,433 --> 00:45:24,400
with the oxygen and crumble.

696
00:45:24,400 --> 00:45:29,833
All the elements are in place
for vast dust storms to form.

697
00:45:32,167 --> 00:45:37,333
The young world is being
ripped up by its own atmosphere.

698
00:45:39,067 --> 00:45:43,533
Earth's surface
is changing color

699
00:45:43,533 --> 00:45:47,067
as new minerals are created.

700
00:45:50,600 --> 00:45:52,767
We had the formation
of all these minerals

701
00:45:52,767 --> 00:45:54,467
that did not exist prior

702
00:45:54,467 --> 00:45:56,100
to the Great Oxidation Event.

703
00:45:56,100 --> 00:45:57,233
Copper minerals like these,

704
00:45:57,233 --> 00:45:58,933
blue and green minerals,

705
00:45:58,933 --> 00:46:01,733
yellow and orange,
uranium minerals,

706
00:46:01,733 --> 00:46:03,833
minerals of molybdenum,
minerals of nickel,

707
00:46:03,833 --> 00:46:07,000
minerals of cobalt,
color of the rainbow,

708
00:46:07,000 --> 00:46:08,900
all in new minerals

709
00:46:08,900 --> 00:46:12,200
form because of
the Great Oxidation Event.

710
00:46:14,067 --> 00:46:18,000
It isn't just minerals.

711
00:46:18,000 --> 00:46:21,533
Oxygen also makes key nutrients
vital for life

712
00:46:21,533 --> 00:46:23,833
far more available

713
00:46:23,833 --> 00:46:29,500
by reacting with a number
of elements to form acids.

714
00:46:29,500 --> 00:46:31,167
Things like sulfuric acid

715
00:46:31,167 --> 00:46:32,500
and nitric acid,

716
00:46:32,500 --> 00:46:35,033
and acid has this property

717
00:46:35,033 --> 00:46:37,067
that when it rains out onto the
surface of the earth,

718
00:46:37,067 --> 00:46:39,600
it can dissolve rocks,
cause weathering

719
00:46:39,600 --> 00:46:42,467
and runoff
of nutrients and minerals

720
00:46:42,467 --> 00:46:44,467
into lakes and oceans.

721
00:46:46,567 --> 00:46:48,200
Those nutrients flowed

722
00:46:48,200 --> 00:46:50,600
into the ocean, there were
new opportunities for life.

723
00:46:50,600 --> 00:46:52,267
It really was a game changer.

724
00:46:57,533 --> 00:47:00,633
And as oxygen began
to increase in the atmosphere,

725
00:47:00,633 --> 00:47:03,033
the relatively minor elements

726
00:47:03,033 --> 00:47:06,067
that are essential
for all the processes of life

727
00:47:06,067 --> 00:47:07,833
became more abundant.

728
00:47:07,833 --> 00:47:10,267
In many cases, those are metals.

729
00:47:19,100 --> 00:47:22,133
Life on Earth evolved

730
00:47:22,133 --> 00:47:25,267
because of the conditions
that the Earth provided.

731
00:47:25,267 --> 00:47:29,033
But, in turn,
life has completely remodeled

732
00:47:29,033 --> 00:47:31,633
the planet,
and the world that we know today

733
00:47:31,633 --> 00:47:36,800
is, in so many ways,
been created by life.

734
00:47:47,100 --> 00:47:49,000
This co-evolution

735
00:47:49,000 --> 00:47:54,233
of earth and the atmosphere
is apparent everywhere we look.

736
00:47:55,767 --> 00:47:57,733
These particular cliffs
were formed

737
00:47:57,733 --> 00:47:59,733
when dinosaurs walked the earth.

738
00:47:59,733 --> 00:48:01,367
But the beautiful red
that you can see

739
00:48:01,367 --> 00:48:03,000
in these bands here is formed

740
00:48:03,000 --> 00:48:04,833
from the iron in these cliffs

741
00:48:04,833 --> 00:48:08,500
combined with water and
oxygen to create iron oxides.

742
00:48:08,500 --> 00:48:12,300
So we see an example of oxygen
changing the very landscape.

743
00:48:14,233 --> 00:48:19,733
Oxygen transformed our oceans,
reshaped our land,

744
00:48:19,733 --> 00:48:22,633
and became a vital ingredient
for life today.

745
00:48:24,533 --> 00:48:26,233
But this volatile gas

746
00:48:26,233 --> 00:48:31,033
was to have one more
dramatic impact on the world.

747
00:48:38,667 --> 00:48:44,133
Across the globe,
oxygen levels are rising,

748
00:48:44,133 --> 00:48:47,533
slowly enriching the air,
reacting with the methane

749
00:48:47,533 --> 00:48:49,867
to form carbon dioxide
and water.

750
00:48:54,133 --> 00:48:55,800
Over millions of years,

751
00:48:55,800 --> 00:48:59,100
the orange hazes
are fading away.

752
00:49:03,467 --> 00:49:08,667
Sunlight is scattered
by the nitrogen and oxygen,

753
00:49:08,667 --> 00:49:11,633
producing Earth's
first oxygen-rich

754
00:49:11,633 --> 00:49:14,733
blue skies.

755
00:49:21,900 --> 00:49:24,267
In the upper atmosphere,

756
00:49:24,267 --> 00:49:26,500
ultraviolet light from the sun

757
00:49:26,500 --> 00:49:30,167
causes the oxygen to form
a new protective molecule...

758
00:49:31,800 --> 00:49:33,800
ozone.

759
00:49:33,800 --> 00:49:34,933
Ozone is made

760
00:49:34,933 --> 00:49:38,033
of three oxygen atoms bonded together,

761
00:49:38,033 --> 00:49:40,767
and they absorb ultraviolet
radiation really well.

762
00:49:40,767 --> 00:49:42,400
So, if you put a little bit
of ozone

763
00:49:42,400 --> 00:49:44,633
in the atmosphere,
it's like sunblock.

764
00:49:48,200 --> 00:49:50,467
The level of oxygen
in the atmosphere

765
00:49:50,467 --> 00:49:55,400
will fluctuate over
the next two billion years,

766
00:49:55,400 --> 00:49:59,433
but eventually,
reach the 21% we have today.

767
00:50:07,533 --> 00:50:11,133
But the story
of our atmosphere is not over.

768
00:50:11,133 --> 00:50:13,067
It will continue to evolve,

769
00:50:13,067 --> 00:50:17,267
both naturally, and under
the influence of human activity.

770
00:50:19,133 --> 00:50:20,300
If we don't understand

771
00:50:20,300 --> 00:50:21,867
the history of the atmosphere,

772
00:50:21,867 --> 00:50:24,633
how can we possibly
be the stewards

773
00:50:24,633 --> 00:50:26,433
of the atmosphere
moving forward?

774
00:50:29,533 --> 00:50:31,533
Because our atmosphere
is totally unique,

775
00:50:31,533 --> 00:50:33,167
there's nothing like it
that we found

776
00:50:33,167 --> 00:50:36,600
on any other planetary body.

777
00:50:36,600 --> 00:50:40,233
Our atmospheric composition
may actually be

778
00:50:40,233 --> 00:50:43,033
the thing that is
screaming out to the cosmos,

779
00:50:43,033 --> 00:50:46,100
a signal to the universe
that we're here,

780
00:50:46,100 --> 00:50:48,533
that life exists on our world.

781
00:50:48,533 --> 00:50:49,700
But then,

782
00:50:49,700 --> 00:50:51,267
in many ways,
we're going back in time,

783
00:50:51,267 --> 00:50:53,067
where we're putting all of

784
00:50:53,067 --> 00:50:57,433
the chemicals that are toxic
to life into the atmosphere.

785
00:50:59,667 --> 00:51:02,167
Over the past
couple hundreds of years,

786
00:51:02,167 --> 00:51:03,900
humanity has made a difference

787
00:51:03,900 --> 00:51:05,933
in the chemical composition
of our atmosphere,

788
00:51:05,933 --> 00:51:09,033
most predominantly
through the release of

789
00:51:09,033 --> 00:51:13,633
carbon dioxide, and this causes
myriad effects on our world.

790
00:51:15,267 --> 00:51:17,833
The atmosphere isn't fragile.

791
00:51:17,833 --> 00:51:19,167
But we are.

792
00:51:19,167 --> 00:51:20,400
We can make changes

793
00:51:20,400 --> 00:51:22,767
to the atmosphere
that will make Earth

794
00:51:22,767 --> 00:51:25,333
a very unpleasant place to live.

795
00:51:25,333 --> 00:51:27,433
And so we need
to look to ourselves

796
00:51:27,433 --> 00:51:30,933
when we think about changes
to the atmosphere.

797
00:51:32,900 --> 00:51:35,033
We have immense power

798
00:51:35,033 --> 00:51:36,267
over our planet.

799
00:51:36,267 --> 00:51:40,067
Our atmosphere is precious.

800
00:51:40,067 --> 00:51:43,633
And, if we change it too much,

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00:51:43,633 --> 00:51:45,767
the whole world
will pay the price.

802
00:51:47,633 --> 00:51:50,667
But by understanding
the huge and complex steps

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00:51:50,667 --> 00:51:54,200
it took to develop
our atmosphere, hopefully,

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00:51:54,200 --> 00:51:55,400
we can develop approaches

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00:51:55,400 --> 00:51:57,400
to take care of it
for generations to come.