How we think complex cells evolved - Adam Jacobson

翻译人员: Claire Zhang 校对人员: YUCHEN QIN

What if you could absorb another organism

如果你能吸收吞并其他生物

and take on its abilities?

然后因此得到它所具有的能力，这听起来怎么样？

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

想像你吞下一只小鸟，并突然得到飞的能力。

Or if you engulfed a cobra

或者你吞下一条眼镜蛇

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

就能够吐出毒液。

Throughout the history of life,

在生命的历史中，

specifically during the evolution of complex eukaryotic cells,

特别是复杂真核细胞的演化

things like this happened all the time.

像这样的事情是经常发生的。

One organism absorbed another,

一个有机体吞没另一个

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

它们合而为一，成为一个全新并有组合能力的新有机体

We think that around 2 billion years ago,

想想20亿年前

the only living organisms on Earth were prokaryotes,

地球上唯一存在的有机体是原核生物

single-celled organisms lacking membrane-bound organelles.

这是一种没有膜结合细胞器的单细胞生物。

Let's look closely at just three of them.

让我们仔细看看其中三种

One was a big, simple blob-like cell

一种是巨大,简单的点状细胞

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

它们能够把其他有机体包在其细胞膜内

Another was a bacterial cell

另一种是细菌。

that converted solar energy into sugar molecules through photosynthesis.

它能透过光合作用将太阳能转化为糖类

A third used oxygen gas to break down materials like sugar

第三种则是利用氧气将糖类等物质分解

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

并将一些有助生物活动的能量释放出去

The blob cells would occasionally absorb the little photosynthetic bacteria.

这些细胞有时也会吞噬小的光合细菌

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

这些细菌在细胞内不断分裂

but their existence became linked.

然而它们的存在被连结在一起

If you stumbled upon this living arrangement,

如果你碰巧遇到它，

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

你可能以为它只是一个有机体

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

以为这些绿色的光合细菌只是这巨大细胞的一部分

that performed one of its life functions,

以为它要执行生命机能

just like your heart is a part of you

就好像你的心脏也是你的一部分

that performs the function of pumping your blood.

它将血液运送到全身

This process of cells living together is called endosymbiosis,

这种细胞共存的过程，我们称之为内共生

one organism living inside another.

也就是一个有机体住在另一个有机体内

But the endosymbiosis didn't stop there.

但共生一事不仅于此

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

如果其他细胞也搬进来了会怎样呢?

Now the cells of this species started becoming highly complex.

那么，这样的细胞开始变得极为复杂

They were big and full of intricate structures

它们变大，且变得充满复杂的结构

that we call chloroplasts and mitochondria.

我们称这些为叶绿体和线粒体

These structures work together to harness sunlight,

这些结构能一起利用阳光

make sugar,

制造出糖分

and break down that sugar using the oxygen

并且利用氧气分解糖分

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

这时也是氧气开始出现在大气层的时候

Organisms absorbing other organisms

有机体吸收其他有机体

was one way species adapted to the changing environmental conditions

是一种生物进行适应

of their surroundings.

周遭环境变化的方式

This little story highlights what biologists call the endosymbiotic theory,

这个小故事总结了生物学家的内共生理论

the current best explanation of how complex cells evolved.

此为目前针对复杂细胞如何演化最好的解释

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

有很多证据支持此理论

but let's look at three main pieces.

但是让我们先看三个主要部分

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

首先, 在我们的细胞内，叶绿体和线粒体 以非常相同的方式繁殖

as those ancient bacteria,

就像古代的细菌。

which are still around, by the way.

顺道一提它们现在依然存在。

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

事实上，你若把一个细胞内的这些结构破坏了，细胞并不会产生新的结构。

The cell can't make them.

它们并不能制造这些结构

They can only make more of themselves.

这些细胞只能繁殖。

Second piece of evidence.

第二个证据是

Chloroplasts and mitochondria both contain their own DNA and ribosomes.

叶绿体和线粒体都有自己的DNA和核糖体

Their DNA has a circular structure

它们的DNA都有环状结构。

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

这些结构和古老细菌的DNA有着明显的相似

and it also contains many similar genes.

并且也包含很多相似的基因

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

核糖体，叶绿体和线粒体的蛋白质组成机器，

also have the same structure as ribosomes of ancient bacteria,

也和古代细菌的核糖体有类似的结构

but are different from the ribosomes

但是它们和在细胞的其他地方的

hanging around the rest of eukaryotic cell.

核糖体不同。

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

最后,想想在这吞噬过程中的细胞膜。

Chloroplasts and mitochondria both have two membranes surrounding them,

叶绿体和粒线体两者都有两个细胞膜包围

an inner and outer membrane.

一个是内部细胞膜,一个是外部细胞膜

Their inner membrane contains some particular lipids and proteins

内部细胞膜含有特别的脂质和蛋白质

that are not present in the outer membrane.

这些并不存在外部细胞膜中

Why is that significant?

为什么那么重要呢?

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

因为它们的外部细胞膜曾经属于巨大细胞

When they were engulfed in the endosymbiosis process,

当它们在共生过程中被吞没时

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

它们被包在系统膜内，并被保留在它们内部

Surely enough, those same lipids

能够确定的是,相同的脂质和蛋白质

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

也在古代的细菌细胞膜中被找到

Biologists now use this theory

生物学家用这个理论

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

去解释了很多真核有机体的起源

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

比如说长在游泳池墙上的绿藻

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

有旋转尾部结构或鞭毛的巨大有核系统

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

吸收了绿藻，形成我们现在所谓的绿虫藻

Euglena can perform photosynthesis,

绿虫藻可以进行光合作用

break down sugar using oxygen,

利用氧气分解糖

and swim around pond water.

并能在池塘上游泳

And as the theory would predict,

用这理论推测，

the chloroplasts in these euglena have three membranes

这个绿虫藻中的叶绿体有三个细胞膜

since they had two before being engulfed.

因为在它们被吞没前已经有两个

The absorbing process of endosymbiotic theory

这个共生的吸收过程

allowed organisms to combine powerful abilities

让有机体组合有用的能力

to become better adapted to life on Earth.

使它们能更加适应地球生活

The results were species capable of much more

结果是生物的种类能够

than when they were separate organisms,

比它们是独立个体时还要多

and this was an evolutionary leap

这是演化的大跃进

that lead to the microorganisms, plants,

所以现今，我们现在地球上能够看到

and animals we observe on the planet today.

微生物、植物以及动物。