How a single-celled organism almost wiped out life on Earth - Anusuya Willis

翻译人员: Yuyang Zhao 校对人员: Cissy Yun

There's an organism that changed the world.

这是一个改变了世界的生物

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

它导致了地球历史上第一次大规模的灭绝

and also paved the way for complex life.

但也为之后的复杂生命铺平了道路

How?

怎么做到的呢？

By sending the first free oxygen molecules into our atmosphere,

它们把第一个自由氧分子送到我们的大气层中

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

并且，它们是以单细胞的生命形态 来完成这一切的

They're cyanobacteria,

它们就是蓝藻

and the story of these simple organisms

这些简单的生物

that don't even have nuclei or any other organelles

没有细胞核和其它任何细胞器 而它们的故事

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

正是地球上生命的故事的一个关键章节

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

地球的大气层并非一直充满着 我们现在呼吸的富氧的混合物

3.5 billion years ago, the atmosphere was mostly nitrogen,

3.5亿年以前，大气层中的大多是氮气，

carbon dioxide,

二氧化碳

and methane.

和甲烷

Almost all oxygen was locked up in molecules like water,

几乎所有的氧气都被锁在像水一样的分子里

not floating around in the air.

并不是在空气中飘着

The oceans were populated by anaerobic microbes.

海洋被厌氧微生物填充。

Those are simple, unicellular life forms that thrive without oxygen

它们是在没有氧气的环境中茁壮成长的， 以单细胞形式存在的简单生物。

and get energy by scavenging what molecules they find.

它们从它们找到的被净化的分子中汲取能力。

But somewhere between 2.5 and 3.5 billion years ago,

但在2.5到3.5亿年前

one of these microbial species,

这类微生物群中的一个

probably floating on the surface of the ocean,

可能正飘在海洋表面，

evolved a new ability: photosynthesis.

进化出了一种新的能力：光合作用

Structures in their cell membrane could harness the energy from sunlight

它们细胞膜中的结构可以利用阳光中的能量

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

将二氧化碳和水转化成氧气和糖分，

which they could use for energy.

并作为它们的能源。

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

这些生物就是我们现在说的蓝藻的祖先

Their bluish color comes from the blue-green pigments

它们的蓝色来源于蓝绿色素

that capture the sunlight they need.

可用于捕捉它们所需要的阳光

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

光合作用使这些古老的细菌 比别的物种有更大的优势

They could now produce their own energy

它们可以从基本上无尽的原材料中

from an almost endless supply of raw ingredients,

生产它们自己的能源

so their populations exploded

所以它们的数量剧增

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

并开始向大气中排出一种新型废弃物：氧气

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

一开始，流出的额外的氧气在和铁的化学反应

or decomposing cells,

或细胞分解中被吸收了

but after a few hundred million years,

但在数百万年之后，

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

蓝藻生产氧气的速度比它们吸收的要快

and the gas started building up in the atmosphere.

气体开始在大气层中聚集

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

这对地球上的其它生物来说是个大问题

Oxygen-rich air was actually toxic to them.

事实上，富氧的空气对它们来说是有毒的

The result?

结果呢？

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

2.5亿年前是一个地球上所有生命的大灭绝。

which barely spared the cyanobacteria.

但几乎没有影响到蓝藻。

Geologists call this the Great Oxygenation Event,

地质学家吧这个叫做大氧化事件，

or even the Oxygen Catastrophe.

甚至氧气大灾难。

That wasn't the only problem.

这不是仅有的问题

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

甲烷一直是使地球温暖的强效温室气体。

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

但现在，多余的氧气和甲烷反应 产生二氧化碳和水，

which don't trap as much heat.

而不会留住热量

The thinner atmospheric blanket

越来越薄的大气层

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

导致了地球上第一次，也可能是最长的一次冰期。

the Huronian Glaciation.

休伦冰期

The planet was basically one giant snowball

星球基本上是一个大雪球

for several hundred million years.

并持续了数亿年。

Eventually, life adjusted.

最终，生命适应了。

Aerobic organisms, which can use oxygen for energy,

需氧生物，可以将氧气作为能源，

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

开始吸收大气中一些多余的气体。

The oxygen concentration rose and fell

氧气的浓度又上又下

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

直到最终达到了我们今天的浓度也就是大约21%。

And being able to use the chemical energy in oxygen

可以利用氧气中的化学能量

gave organisms the boost they needed to diversify

给了这些生物它们需要的刺激来实现多元化。

and evolve more complex forms.

并进化出更复杂的形态。

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

蓝藻在这一段故事中也有一个角色。

Hundreds of millions of years ago,

几亿年前，

some other prehistoric microbe swallowed a cyanobacterium whole

其它一些史前微生物吞噬了蓝藻

in a process called endosymbiosis.

这整个过程称为内共生。

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

为了这样做，那个微生物必须拥有 自己的内部光合作用工厂。

This was the ancestor of plant cells.

这就是植物细胞的祖先

And cyanobacteria became chloroplasts,

蓝藻变成了叶绿体，

the organelles that carry out photosynthesis today.

就是今天那个可以光合作用的细胞器。

Cyanobacteria are still around in almost every environment on Earth:

蓝藻还在地球上的几乎每一个环境：

oceans,

海洋

fresh water,

淡水

soil,

土地

antarctic rocks,

南极石，

sloth fur.

树懒毛。

They still pump oxygen into the atmosphere,

它们依然在向大气中泵出氧气，

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

它们也依然将氮气析出， 以给它们创造的植物施肥。

We wouldn't recognize life on Earth without them.

如果不是它们，我们不会认识到生命。

But also thanks to them,

但也要谢谢它们，

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

地球上差点就没有生命了。