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  • Are you like me?

  • Do you think oxygen ispretty great?

  • Of course you do!

  • But what if I told you that there was a time when oxygen almost wiped out all life on Earth?

  • This is a story about how too much of a good thing can ruin everything for everybody.

  • Let's start 3 billion years ago, in the Archaean Eon, when the world was a place you'd

  • never recognize.

  • Oreven be able to survive in.

  • Back then, only 2 or 3% of the planet's surface was dry land.

  • The rest was covered in oceans, andthey were chock full of iron.

  • Because of all that iron, the oceans were probably not blue, but green -- and not because

  • of algae or other life, but because of rust.

  • You might think of rust as being brown, but green rust is a thing too!

  • It forms where there's a lack of oxygen, and back in the Archaean, oxygen was in short

  • supply everywhere.

  • So in the oceans, iron reacted with hydroxides and elements like sulfur and chlorine, covering

  • much of the world in green rust.

  • Above the green oceans, the atmosphere was mostly nitrogen -- as it is today -- along

  • with water vapor, carbon dioxide, and methane.

  • But, again, very little oxygen.

  • Which was fine at the time, because microscopic life on Earth back then

  • was probably anaerobic.

  • Then about 2.3 billion years ago, some new kids showed up: Photosynthesizers.

  • But not the kind that we're used to

  • These were bacteria, like the familiar blue-green cyanobacteria, or maybe even a kind of purple

  • microbe known as halobacteria.

  • Either way, they used the Sun's energy to convert CO2 and water into food -- releasing

  • oxygen as waste.

  • Which was ... okfor a while.

  • But not for long.

  • Because when these little microbes started farting out oxygen, it changed the chemical

  • balance of the entire biosphere and, eventually, altered the face of our planet.

  • First, within about 200 million years, the oxygen from these new bacteria began reacting

  • with the iron in the oceans.

  • This turned the oceans from a mellow green to a deep blood-red, as the seas began to

  • fill with iron oxide -- what we know as rust.

  • In fact, you can still see exactly when and where this happened, in strata known as Banded

  • Iron Formations, where huge swaths of rust settled out of oceans to form layers of red-brown

  • rock.

  • But beyond making water rusty, rising oxygen levels also shifted the balance of power among

  • living things.

  • Anaerobic microbes started to die off, as they were basically poisoned by the oxygen.

  • Meanwhile, the newer, more efficient, photosynthetic life began to spread.

  • And things changed even more when the oxygen in the ocean left the waterand entered

  • the air.

  • For the first time in Earth's history, large amounts of free oxygen started building up

  • in the atmosphere.

  • And this is what scientists call a game-changer.

  • Because then, the climate started to change too.

  • Drastically.

  • As the photosynthetic bacteria kept spreading, they used up more and more of the carbon dioxide

  • in the atmosphere.

  • And CO2 is a greenhouse gas, so as its levels went down, so did the temperature.

  • On top of that, oxygen also started reacting with the methane in the atmosphere, taking

  • it out of circulation.

  • With greenhouse gases at an all-time low, our planet started to see the first major

  • ice ages in its history.

  • Temperatures dropped so drastically that it triggered a global glaciation, and our once

  • balmy waterworld became shrouded in ice.

  • There may have been several pulses of these icy events, but by far the biggest of them

  • came to be known as the Huronian glaciation -- and it lasted for 300 million years.

  • Between these huge changes in the climate, and the radically different chemistry of the

  • planet, most life on Earth was pushed to the brink of extinction.

  • And this included the new guys -- the photosynthetic bacteria that were pumping out all the oxygen

  • in the first place!

  • Because, remember, they needed CO2 to survive.

  • So they were basically being suffocated by their own waste, whichwhen I say that

  • out loud, that's just gotta be a terrible way to go.

  • We don't know the full extent of the damage.

  • But the effects of this whole episode -- sometimes called the Oxygen Catastrophe -- is considered

  • one of the biggest extinction events in history.

  • But, you know how this movie ends, right?

  • By the time the Huronian glacial period ended, about 2.1 billion years ago, some

  • life remained.

  • And the life-forms that survived inherited a better, more hospitable planet.

  • Oxygen was now abundant in the air and water, and a new thing also appeared in the atmosphere:

  • an ozone layer.

  • This coating of molecular oxygen helped block dangerous ultraviolet radiation from the sun,

  • and suddenly new things became possible.

  • From there, well, you know what happens.

  • Life became more complex and awesome and wonderful, and here we are.

  • It seems strange that bacteria could end up covering the whole world in ice.

  • And who'da thought oxygen would be responsible for wiping outalmost everybody?

  • But it just goes to show you that, sometimes, things gotta get worse before they get better.

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  • What do you want to know about the story of life on Earth?

  • Let us know in the comments.

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That Time Oxygen Almost Killed Everything

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    joey joey に公開 2021 年 05 月 01 日
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