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There's an organism 
that changed the world.

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It caused both the first mass extinction
in Earth's history

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and also paved the way for complex life.

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How?

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By sending the first free oxygen
molecules into our atmosphere,

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and they did all this
as single-celled life forms.

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They're cyanobacteria,

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and the story of these simple organisms

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that don't even have nuclei 
or any other organelles

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is a pivotal chapter 
in the story of life on Earth.

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Earth's atmosphere wasn't always 
the oxygen-rich mixture we breathe today.

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3.5 billion years ago, the atmosphere
was mostly nitrogen,

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carbon dioxide,

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and methane.

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Almost all oxygen was locked up 
in molecules like water,

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not floating around in the air.

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The oceans were populated by
anaerobic microbes.

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Those are simple, unicellular life forms
that thrive without oxygen

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and get energy by scavenging
what molecules they find.

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But somewhere between 
2.5 and 3.5 billion years ago,

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one of these microbial species,

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probably floating 
on the surface of the ocean,

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evolved a new ability: photosynthesis.

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Structures in their cell membrane
could harness the energy from sunlight

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to turn carbon dioxide and water
into oxygen gas and sugars,

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which they could use for energy.

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Those organisms were the ancestors
of what we now call cyanobacteria.

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Their bluish color comes from 
the blue-green pigments

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that capture the sunlight they need.

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Photosynthesis gave those ancient bacteria
a huge advantage over other species.

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They could now produce their own energy

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from an almost endless supply 
of raw ingredients,

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so their populations exploded

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and they started polluting the atmosphere
with a new waste product: oxygen.

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At first, the trickle of extra oxygen was
soaked up by chemical reactions with iron

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or decomposing cells,

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but after a few hundred million years,

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the cyanobacteria were producing oxygen
faster than it could be absorbed,

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and the gas started building up
in the atmosphere.

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That was a big problem for the rest
of Earth's inhabitants.

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Oxygen-rich air 
was actually toxic to them.

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The result?

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About 2.5 billion years ago was a mass
extinction of virtually all life on Earth,

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which barely spared the cyanobacteria.

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Geologists call this
the Great Oxygenation Event,

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or even the Oxygen Catastrophe.

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That wasn't the only problem.

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Methane had been acting as a potent
greenhouse gas that kept the Earth warm,

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but now, the extra oxygen reacted with
methane to form carbon dioxide and water,

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which don't trap as much heat.

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The thinner atmospheric blanket

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caused Earth's first, 
and possibly longest, ice age,

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the Huronian Glaciation.

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The planet was basically 
one giant snowball

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for several hundred million years.

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Eventually, life adjusted.

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Aerobic organisms, 
which can use oxygen for energy,

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started sopping up some of the excess
gas in the atmosphere.

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The oxygen concentration rose and fell

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until eventually it reached 
the approximate 21% we have today.

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And being able to use 
the chemical energy in oxygen

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gave organisms the boost they needed
to diversify

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and evolve more complex forms.

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Cyanobacteria had a part 
to play in that story, too.

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Hundreds of millions of years ago,

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some other prehistoric microbe
swallowed a cyanobacterium whole

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in a process called endosymbiosis.

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In doing so, that microbe acquired
its own internal photosynthesis factory.

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This was the ancestor of plant cells.

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And cyanobacteria became chloroplasts,

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the organelles that carry out 
photosynthesis today.

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Cyanobacteria are still around
in almost every environment on Earth:

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oceans,

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fresh water,

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soil,

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antarctic rocks,

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sloth fur.

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They still pump oxygen
into the atmosphere,

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and they also pull nitrogen out to
fertilize the plants they helped create.

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We wouldn't recognize life on Earth
without them.

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But also thanks to them,

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we almost didn't have 
life on Earth at all.