Name: 【TED-Ed】How a single-celled organism almost wiped out life on Earth - Anusuya Willis
Uploaded: 2016-08-18T06:46:13.000Z
Duration: 4 min 14 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

There's an organism  that changed the world.

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

By sending the first free oxygen molecules into our atmosphere,

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

that don't even have nuclei  or any other organelles

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

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

3.5 billion years ago, the atmosphere was mostly nitrogen,

Almost all oxygen was locked up  in molecules like water,

The oceans were populated by anaerobic microbes.

Those are simple, unicellular life forms that thrive without oxygen

and get energy by scavenging what molecules they find.

But somewhere between  2.5 and 3.5 billion years ago,

probably floating  on the surface of the ocean,

Structures in their cell membrane could harness the energy from sunlight

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

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

Their bluish color comes from  the blue-green pigments

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

from an almost endless supply  of raw ingredients,

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

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

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

and the gas started building up in the atmosphere.

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

Oxygen-rich air  was actually toxic to them.

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

Geologists call this the Great Oxygenation Event,

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

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

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

The blanket was basically  one giant snowball

Aerobic organisms,  which can use oxygen for energy,

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

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

And being able to use  the chemical energy in oxygen

gave organisms the boost they needed to diversify

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

some other prehistoric microbe swallowed a cyanobacterium whole

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

the organelles that carry out  photosynthesis today.

Cyanobacteria are still around in almost every environment on Earth:

They still pump oxygen into the atmosphere,

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

We wouldn't recognize life on Earth without them.

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