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Inside our cells, each of us has a second set of genes completely separate from the 23 pairs of chromosomes we inherit from our parents.
在我们的细胞内，我们每个人都有第二套基因，与我们从父母那里继承的23对染色体完全分开。
And this isn't just the case for humans -- it's true of every animal, plant, and fungus, and nearly every multicellular organism on Earth.
而且这不仅仅是人类的情况--每种动物、植物和真菌，以及几乎所有地球上每个多细胞的生物体都是如此。
This second genome belongs to our mitochondria, an organelle inside our cells.
这第二个基因组属于我们的线粒体，是我们细胞内的一个细胞器。
They're not fully a part of us, but they're not separate either -- so why are they so different from anything else in our bodies?
他们并不完全是我们的一部分，但它们也不是独立的--那么，为什么它们与我们身体里的其他东西如此不同？
Approximately 1.5 billion years ago,
大约15亿年前，
scientists think a single-celled organism engulfed the mitochondria's ancestor, creating the predecessor of all multicellular organisms.
科学家认为一个单细胞生物体吞噬了线粒体的祖先，进而创造了所有多细胞生物的前身。
Mitochondria play an essential role:
线粒体起着至关重要的作用：
they convert energy from the food we eat and oxygen we breathe into a form of energy our cells can use, which is a molecule called ATP.
它们从我们吃的食物和呼吸的氧气中的能量转化为我们细胞可以使用的能量形式，这就是一种叫做ATP的分子。
Without this energy, our cells start to die. Humans have over 200 types of cells, and all except mature red blood cells have mitochondria.
没有这种能量，我们的细胞就会开始死亡。人类有超过200种类型的细胞，而除了成熟的红细胞外，都有线粒体。
That's because a red blood cell's job is to transport oxygen, which mitochondria would use up before it could reach its destination.
这是因为红细胞的工作是运输氧气，而线粒体在它到达目的地之前就会耗尽。
So all mitochondria use oxygen and metabolites to create energy and have their own DNA, but mitochondrial DNA varies more across species than other DNA.
因此，所有线粒体都使用氧气和代谢物来产生能量，并有自己的DNA，与其他DNA相比，线粒体DNA在不同物种间的差异更大。
In mammals, mitochondria usually have 37 genes.
在哺乳动物中，线粒体通常有37个基因。
In some plants, like cucumbers, mitochondria have up to 65 genes, and some fungal mitochondria have only 1.
在一些植物中，如黄瓜，线粒体有多达65个基因，而一些真菌的线粒体只有1个。
A few microbes that live in oxygen-poor environments seem to be on the way to losing their mitochondria entirely, and one group, oxymonad monocercomonoides, already has.
一些生活在贫氧环境中的微生物似乎正在走向完全失去他们的线粒体的路上，而类单鞭滴虫属的这些生物已经开始没有线粒体了。
This variety exists because mitochondria are still evolving, both in tandem with the organisms that contain them, and separately, on their own timeline.
这种多样性的存在是因为线粒体仍在进化中，与含有它们的生物体，分别在自己的时间线上同步进行。
To understand how that's possible, it helps to take a closer look at what the mitochondria inside us are doing, starting from the moment we're conceived.
要了解这一点是如何实现的，仔细看看我们体内的线粒体在做什么会有所帮助，从我们受孕的那一刻开始。
In almost all species, mitochondrial DNA is passed down from only one parent.
在几乎所有物种中，线粒体DNA只从父母一方传下来。
In humans and most animals, that parent is the mother. Sperm contain approximately 50 to 75 mitochondria in the tail, to help them swim.
在人类和大多数动物中，该受体是母亲。精子的尾部大约含有50至75个线粒体，来帮助他们游动。
These dissolve with the tail after conception.
这些东西在受孕后与尾巴一起解体。
Meanwhile, an egg contains thousands of mitochondria, each containing multiple copies of the mitochondrial DNA.
同时，一个卵含有成千上万的线粒体，每个都含有多组线粒体DNA。
This translates to over 150,000 copies of mitochondrial DNA that we inherit from our mothers,
这意味着我们体内超过15万份的线粒体DNA是从母亲那里继承的，
each of which is independent and could vary slightly from the others.
每一个都是独立的，且彼此之间可能略有不同。
As a fertilized egg grows and divides, those thousands of mitochondria are divvied up into the cells of the developing embryo.
随着受精卵的生长和分裂，这些数以千计的线粒体被分配到发育中的胚胎细胞中。
By the time we have differentiated tissues and organs, variations in the mitochondrial DNA are scattered at random throughout our bodies.
当我们有了分化的组织和器官，线粒体DNA的变异是随机地散布在我们的身体里。
To make matters even more complex, mitochondria have a separate replication process from our cells.
使之更复杂的是，线粒体有一个与我们细胞不同的独立复制。
So as our cells replicate by dividing, mitochondria end up in new cells, and all the while they're fusing and dividing themselves, on their own timeline.
因此，当我们的细胞通过分裂进行复制时，线粒体最终出现在新细胞中，而它们在自己的时间轴上融合和分裂。
As mitochondria combine and separate, they sequester faulty DNA or mitochondria that aren't working properly for removal.
随着线粒体的结合和分离，它们封存了有问题的DNA或对不正常工作的线粒体进行清除。
All this means that the random selection of your mother's mitochondrial DNA you inherit at birth can change throughout your life and throughout your body.
所有这一切意味着，你在出生时从母亲那随机继承的线粒体DNA会在你的一生中在你身上发生改变。
So mitochondria are dynamic and, to a degree, independent, but they're also shaped by their environments: us.
所以线粒体是动态的，而且，在某种程度上是独立的，但它们也被他们的环境所塑造：我们。
We think that long ago, some of their genes were transferred to their host's genomes.
我们认为，很久以前，它们的一些基因被转移到宿主的基因组中。
So today, although mitochondria have their own genome and replicate separately from the cells that contain them,
所以今天，虽然线粒体有自己的基因组，并与包含它们的细胞分别进行复制，
they can't do this without instruction from our DNA.
如果没有我们的DNA指示，它们就不能这样做。
And though mitochondrial DNA is inherited from one parent, the genes involved in building and regulating the mitochondria come from both.
而尽管线粒体DNA是由父母一方遗传的，参与建立和调节线粒体的基因来自父母双方。
Mitochondria continue to defy tidy classification.
线粒体继续藐视着井井有条的分类。
Their story is still unfolding inside of each of our cells, simultaneously separate and inseparable from our own.
它们的故事仍在我们每个人的细胞里展开，同时独立且与我们自己不可分割。
Learning more about them can both give us tools to protect human health in the future, and teach us more about our history.
更多地了解它们，既可以给我们提供在未来保护人类健康的办法，还能让我我们更多地了解我们的历史。
