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

譯者: Helen Chang 審譯者: Hsiao russell

There's an organism that changed the world.

曾有個有機體改變了世界。

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

它引起地球史上 第一次物種大滅絕，

and also paved the way for complex life.

也為複雜的生命體鋪了路。

How?

如何辦到的？

By sending the first free oxygen molecules into our atmosphere,

把最初的游離氧分子送入大氣層，

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

而且它們僅僅以 單細胞形體就辦到了。

They're cyanobacteria,

它們是藍藻，

and the story of these simple organisms

這些簡單的有機體，

that don't even have nuclei or any other organelles

儘管沒有細胞核或任何它種胞器，

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

卻是地球的生命故事裡 舉足輕重的一章。

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

之前地球的大氣層並不富含氧， 和今天我們呼吸的不一樣。

3.5 billion years ago, the atmosphere was mostly nitrogen,

三十五億年前，大氣中主要是氮、

carbon dioxide,

二氧化碳，

and methane.

和甲烷。

Almost all oxygen was locked up in molecules like water,

幾乎所有的氧全被鎖在 像水分子這樣的分子中，

not floating around in the air.

沒在空氣中流動。

The oceans were populated by anaerobic microbes.

海洋充滿厭氧微生物。

Those are simple, unicellular life forms that thrive without oxygen

都是些簡單、單細胞， 不靠氧氣成長茁壯的生命形式，

and get energy by scavenging what molecules they find.

它們是清道夫， 從找到的分子中擷取能量。

But somewhere between 2.5 and 3.5 billion years ago,

但在二十五億到 三十五億年前間的某個時段，

one of these microbial species,

這些微生物的物種之一，

probably floating on the surface of the ocean,

可能浮到海洋的表面上，

evolved a new ability: photosynthesis.

發展出新的能力：「光合作用」。

Structures in their cell membrane could harness the energy from sunlight

它們的細胞膜結構會利用太陽能

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

把二氧化碳和水轉化成氧氣， 和它們可用的能量形式：糖。

which they could use for energy.

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

它們是今日「藍藻」的祖先。

Their bluish color comes from the blue-green pigments

它們的藍顏色來自 能捕捉所需陽光的藍綠色素。

that capture the sunlight they need.

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

光合作用給了那些古老的細菌 其他物種所欠缺的巨大優勢。

They could now produce their own energy

它們能用幾乎無限供應的原料 自己生產能源，

from an almost endless supply of raw ingredients,

so their populations exploded

所以菌口數爆炸成長。

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

以至於它們的新廢品「氧氣」 開始污染大氣層。

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

起初，涓涓細流的額外氧氣

被鐵的化學反應或細胞分解所吸收，

or decomposing cells,

but after a few hundred million years,

但幾億年後，

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

藍藻產生氧氣的速度 超過氧氣被吸收的速度，

and the gas started building up in the atmosphere.

氧氣開始在大氣層中堆積，

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

這對於其他的地球居民而言 是個大問題，

Oxygen-rich air was actually toxic to them.

實際上富含氧氣的空氣 對其他生物是有毒的。

The result?

結果呢？

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

約二十五億年前， 幾乎地球上所有的物種都滅絕了，

which barely spared the cyanobacteria.

藍藻只差一點倖免。

Geologists call this the Great Oxygenation Event,

地質學家稱之為「大氧化事件」，

or even the Oxygen Catastrophe.

或「氧化災變」。

That wasn't the only problem.

那不是唯一的問題。

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

原先大氣中的「甲烷」 有效地保持地球溫暖，

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

但是，多出的氧氣與甲烷 產生化學反應，形成二氧化碳和水，

which don't trap as much heat.

大氣再也無法保溫。

The thinner atmospheric blanket

變薄的大氣層毯子

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

造成地球的第一次， 並可能是最長的一次冰河時期：

the Huronian Glaciation.

「休倫冰期」。

The planet was basically one giant snowball

那時的大氣毯子 基本上是個巨大的雪球，

for several hundred million years.

持續了幾億年。

Eventually, life adjusted.

最終，生物體適應了。

Aerobic organisms, which can use oxygen for energy,

可利用氧為能源的「好氧生物」

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

開始吸收大氣中多餘的氧氣。

The oxygen concentration rose and fell

氧濃度上升、下跌，

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

最終達到我們今天的 21％ 左右。

And being able to use the chemical energy in oxygen

因能使用氧氣的化學能

gave organisms the boost they needed to diversify

促使生物多樣化，

and evolve more complex forms.

並且進化成更複雜的形式。

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

藍藻在這個故事中 也扮演了一個角色。

Hundreds of millions of years ago,

數億年前，

some other prehistoric microbe swallowed a cyanobacterium whole

某些史前微生物吞下藍藻，

in a process called endosymbiosis.

這過程被稱為「內共生」。

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

這使得該微生物有了 自己的光合作用工廠。

This was the ancestor of plant cells.

這就是植物細胞的始祖。

And cyanobacteria became chloroplasts,

藍藻成為「葉綠體」，

the organelles that carry out photosynthesis today.

也就是今日進行光合作用的胞器。

Cyanobacteria are still around in almost every environment on Earth:

藍藻仍然存在於 幾乎每個地球的環境：

oceans,

海洋、

fresh water,

淡水、

soil,

土壤、

antarctic rocks,

南極岩石、

sloth fur.

樹懶的毛皮。

They still pump oxygen into the atmosphere,

藍藻仍然把氧氣送到大氣層中；

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

它們還自大氣中拉氮出來， 為它們協助創造的植物施肥。

We wouldn't recognize life on Earth without them.

若沒有他們，我們將無法認出 地球上的生命。

But also thanks to them,

然而也因為它們，

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

地球差點兒沒有任何的生命。