How we think complex cells evolved - Adam Jacobson

譯者: XiuDe Huang 審譯者: Helen Chang

What if you could absorb another organism

如果你能吸收其他的有機體

and take on its abilities?

並得到它的能力？

Imagine you swallowed a small bird and suddenly gained the ability to fly.

想像你吞下一隻小鳥

即刻獲得飛翔的能力

Or if you engulfed a cobra

或者吞下一隻眼鏡蛇

and were then able to spit poisonous venom from your teeth.

就能夠從牙齒唾出毒液

Throughout the history of life,

綜觀生命的歷史

specifically during the evolution of complex eukaryotic cells,

尤其是複雜的真核細胞演化

things like this happened all the time.

像這樣的事情經常發生

One organism absorbed another,

一個有機體吞沒另一個

and they united to become a new organism with the combined abilities of both.

它們合而成為一個 綜合二者能力的全新有機體

We think that around 2 billion years ago,

我們推測大約二十億年前

the only living organisms on Earth were prokaryotes,

地球上活著的有機體 只有「原核生物」

single-celled organisms lacking membrane-bound organelles.

這些單細胞有機體裡面 沒有以核膜為界的細胞器

Let's look closely at just three of them.

讓我們仔細看看其中三種

One was a big, simple blob-like cell

一種是巨大而簡單的斑點狀細胞

with the ability to absorb things by wrapping its cell membrane around them.

能夠用它的細胞膜 包圍且吸收其他有機體

Another was a bacterial cell

另一種為細菌

that converted solar energy into sugar molecules through photosynthesis.

能透過光合作用將太陽能轉化為醣類

A third used oxygen gas to break down materials like sugar

第三種利用氧氣分解醣類

and release its energy into a form useful for life activities.

並把它的能量 以生命活動可用的形式釋放出去

The blob cells would occasionally absorb the little photosynthetic bacteria.

斑點狀細胞偶而也會 吞噬光合作用的小細菌

These bacteria then lived inside the blob and divided like they always had,

細菌住在在細胞裡 如同往常，不斷地分裂增多

but their existence became linked.

二種細胞連結並存

If you stumbled upon this living arrangement,

如果碰巧遇到

you might just think that the whole thing was one organism,

你可能會認為整體是一個有機體

that the green photosynthetic bacteria were just a part of the blob

綠色行光合作用的細菌 只是斑點狀細胞的一部分

that performed one of its life functions,

它執行一項細胞的生命機能

just like your heart is a part of you

就好像心臟是你的一部分

that performs the function of pumping your blood.

如同幫浦般將血液運送到全身

This process of cells living together is called endosymbiosis,

這種幾個細胞共生的過程 被稱為「內共生」

one organism living inside another.

也就是一個有機體 住在另一個有機體內

But the endosymbiosis didn't stop there.

但內共生不侷限於此

What would happen if the other bacteria moved in, too?

倘若其他種類的細菌 也搬進來會如何呢？

Now the cells of this species started becoming highly complex.

這樣，細胞就變得極為複雜

They were big and full of intricate structures

它們很大，且內含許多

that we call chloroplasts and mitochondria.

被稱為「葉綠體」 和「粒線體」的複雜結構

These structures work together to harness sunlight,

這些結構一起工作

利用陽光製造醣分

make sugar,

and break down that sugar using the oxygen

又利用氧氣分解醣分

that right around this time started to appear in the Earth's atmosphere.

正好約在此時 氧氣開始現身於大氣層中

Organisms absorbing other organisms

有機體吸收其他有機體

was one way species adapted to the changing environmental conditions

是讓生物能夠適應

它周遭環境變化的一種方式

of their surroundings.

This little story highlights what biologists call the endosymbiotic theory,

剛剛的小故事凸顯 生物學家的內共生理論

the current best explanation of how complex cells evolved.

這是目前為止，如何進化出 複雜細胞的最佳解釋

There's a lot of evidence that supports this theory,

有很多證據支持此理論

but let's look at three main pieces.

讓我們先看看三個主要的部分

First, the chloroplasts and mitochondria in our cells multiply the very same way

首先，細胞裡的葉綠體和粒線體

分裂繁衍的方式

as those ancient bacteria,

與古代的細菌相同

which are still around, by the way.

順道一提，我們的週遭仍有古細菌

In fact, if you destroy these structures in a cell, no new ones will appear.

事實上，若毀掉細胞內的 這些葉綠體、粒線體結構

它就無法再生

The cell can't make them.

細胞不能製造那些結構

They can only make more of themselves.

它們只會複製更多的自己

Second piece of evidence.

第二個證據是

Chloroplasts and mitochondria both contain their own DNA and ribosomes.

葉綠體和粒線體都有自己的 DNA（去氧核糖核酸）和核糖體

Their DNA has a circular structure

它們環狀架構的 DNA

that is strikingly similar to the DNA of the ancient bacteria,

和古細菌的 DNA 架構非常相似

and it also contains many similar genes.

並含有很多相似的基因

The ribosomes, or protein assembly machines of chloroplasts and mitochondria,

葉綠體和粒線體的核醣體 是合成蛋白質的機器

also have the same structure as ribosomes of ancient bacteria,

和古細菌的核醣體結構相同

but are different from the ribosomes

但和真核細胞其他的核醣體不同

hanging around the rest of eukaryotic cell.

Lastly, think about the membranes involved in the engulfing process.

最後，關於吞噬過程中 所牽涉到的細胞膜

Chloroplasts and mitochondria both have two membranes surrounding them,

葉綠體和粒線體都包覆著兩層細胞膜

an inner and outer membrane.

內細胞膜和外細胞膜

Their inner membrane contains some particular lipids and proteins

內細胞膜含有特殊的膜脂和膜蛋白

that are not present in the outer membrane.

而外細胞膜沒有

Why is that significant?

這為什麼重要呢？

Because their outer membrane used to belong to the blob cell.

因為它們的外細胞膜 曾屬於斑點狀細胞

When they were engulfed in the endosymbiosis process,

在共生過程中被後者吞噬

they got wrapped up in that membrane and kept their own as their inner one.

就被後者的細胞膜所包覆

自己原有的膜成為內細胞膜

Surely enough, those same lipids

證據再明確也不過

相同的膜脂和膜蛋白

and proteins are found on the membranes of the ancient bacteria.

也在古細菌的細胞膜上找得到

Biologists now use this theory

生物學家用這理論

to explain the origin of the vast variety of eukaryotic organisms.

去解釋許多真核有機體的起源

Take the green algae that grow on the walls of swimming pools.

就拿長在游泳池牆上的綠藻為例

A larger eukaryotic cell with spinning tail structures, or flagella,

某隻大大、長著鞭毛尾巴的真核細胞

at some point absorbed algae like these to form what we now call euglena.

在某個時間點吞噬了綠藻

而成為現在所謂的「眼蟲」

Euglena can perform photosynthesis,

眼蟲能進行光合作用

break down sugar using oxygen,

能利用氧氣分解醣

and swim around pond water.

也能在池塘裡游泳

And as the theory would predict,

如同理論所預測的

the chloroplasts in these euglena have three membranes

眼蟲的葉綠體有三層細胞膜

since they had two before being engulfed.

因為它們在被吞噬前原已有兩層

The absorbing process of endosymbiotic theory

內共生理論的吸收過程

allowed organisms to combine powerful abilities

讓有機體組合多種有用的能力

to become better adapted to life on Earth.

使自己更能適應地球上的生活

The results were species capable of much more

結果是物種

比各自仍是獨立個體時的能力更強大

than when they were separate organisms,

and this was an evolutionary leap

這是演化的大躍進

that lead to the microorganisms, plants,

使得我們能在現今地球上 看到各種的微生物、植物以及動物

and animals we observe on the planet today.