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We have lots in common with our closest primate relatives. But comparatively, humans seem a bit... underdressed.
我们与我们最亲近的灵长类亲戚有很多相同点。但相比之下，人类似乎毛发...有些少。
Instead of thick fur covering our bodies, many of us mainly have hair on top of our heads -- and a few other places.
相较身上覆盖着厚厚的毛，大多数人的毛发主要集中在头顶--和其他几个地方。
So, how did we get so naked? And why do we have hair where we do?
那么我们何以变得如此“赤裸”呢？我们的毛发为什么长在它们所在的地方呢？
Human hair and animal fur are made of the same stuff:
人类的毛发和动物的皮毛都是由同一种东西形成的：
filaments of the protein keratin that grow out of organs known as follicles, which go through cycles of growth and shedding.
角蛋白的单纤维从毛囊中长出来，经过一个生长和脱落的周期。
Across mammalian species, hairs have been modified for numerous purposes, ranging from the soft fluff covering rabbits to the rigid quills protecting porcupines.
对于哺乳动物来说，毛发在经历改变后有了多种作用，从被柔软的绒毛覆盖的兔子到用坚硬的刺来保护自己的豪猪。
But for many mammals, hair grows in two layers consisting of a shorter undercoat of ground hairs covered by longer guard hairs.
但很多哺乳动物的头发有两层，一层相对短的里层毛发被另一层相对长的外层粗毛所覆盖。
Together, they help insulate the animal's body and protect its skin.
这两层毛发有助于为动物的身体保温和保护牠的皮肤。
Human hairs, on the other hand, are kind of a combination of these hair types.
在另一方面，人类毛发是这类毛发的一种组合。
Unfortunately, hair is rarely found in fossils, making it hard for researchers to pinpoint when and how our ancient ancestors lost their coats.
不幸的是，毛发很少在化石中被发现，这使得研究者很难去查明我们的祖先在什么时候以及如何失去他们的皮毛。
But scientists have developed some working hypotheses.
但是科学家们已经推导出一些有用的假设了。
It seems that, millions of years ago in Africa, early hominins first transitioned out of trees and adopted a more active lifestyle.
看起来，在数百万年前的非洲，早期古人类首次从树上下来，还选用更活跃的生活方式。
Keeping cool became increasingly important. Eventually, they developed more sweat glands, which helped them lose heat by evaporating moisture through the skin.
保持凉爽变得极具重要。最终，他们衍变出了更多的汗腺，这有助于他们通过皮肤蒸发水分而散热。
In fact, humans have 10 times more sweat glands than chimpanzees, for instance.
事实上，例如，人类拥有比黑猩猩多10倍的汗腺。
But efficiently losing heat by sweating is harder to do when you're covered in fur.
但是被皮毛所覆盖的你难以通过出汗的方式有效地散热。
Scientists believe that early humans lost much of their coat around this time to help their sweat evaporate faster.
科学家们相信早期人类在这段时间失去他们大量的毛发，以助于让他们的汗蒸发地更快。
However, if losing our hair was so advantageous, why do we have any left at all?
但是，如果失去毛发这么有益，那我们为什么还会有任何余下来的毛发呢？
It seems that there are unique uses for hair in different parts of our bodies.
似乎我们身体不同部位的毛发都有独特的用途。
When it comes to the tops of our heads, temperature regulation likely played a part again.
当我们谈论到我们的头顶时，温度调节可能再次发挥了作用。
Since early humans began venturing into the open, their heads would've been exposed to the scorching sun.
由于早期人类开始进行野外冒险，他们的头会暴露在烈日下。
Thicker, longer-growing hair protects our sensitive scalps and keeps our brains from overheating.
更厚，更长的头发可以起到保护我们敏感头皮的作用并且防止我们的大脑变得过热。
Dark tightly curled hair is most effective at keeping solar radiation off of skin.
深色紧紧卷曲的头发最能有效地防止太阳辐射照射皮肤。
Other kinds of head hair evolved as humans moved to different places.
其他种类的头发也随着人类搬去不同的地方而进化。
Meanwhile, researchers think eyebrows are especially useful for communication because they sit atop active facial muscles that convey our feelings.
同时，研究者们认为眉毛在沟通交流的时候尤其地有效，因为它们坐落在活跃的面部的肌肉上传达我们感受。
Eyelashes have been shown to minimize airflow over our eyeballs, preventing them from drying out and catching debris.
睫毛已经被证实能使流经眼球的气流最小化，防止眼球变干并粘上碎物。
And maybe facial hair proved helpful in distinguishing identity from a distance, but we really don't know.
还有，或许面部的毛发能够有助于从远处区分身份，但是我们不知道真假。
Evidence is stubbly at best. Why we have hair in other regions is... more pungent.
证据充其量也只是零星的。为什么我们在其他区域有毛发这件事...更呛。
Our armpits, nipples, and pubic areas are dotted with apocrine glands.
我们的腋窝、乳头和阴部布满了大汗腺。
They produce oily, smelly secretions which the thick, curly hair that often grows in these spots helps disperse.
它们会产生油腻、有臭味的分泌物，浓密的卷发通常长在这些位置来散味。
The secretions that waft off these hairy patches may be useful for identification.
这些从小区域的毛发中飘出的分泌物可能有助于辨别身份。
For example, several studies have shown that people are able to identify their own armpit odors as well as those of people they're close with.
例如，一些研究表明人们有能力辨别出他们自己的腋窝气味以及与他们亲近的人的腋窝气味。
The final type of notable human hair is the vellus hair that covers our bodies.
最后一种值得注意的人类毛发是覆盖我们身体的绒毛。
We don't know if these hairs serve any purpose themselves,
我们不知道这些毛发本身是否有任何用途，
but the follicles vellus hair grows from are essential banks of stem cells that repair damaged skin after injury.
但长出毛发的毛囊是必不可少的干细胞库，可以修复受伤后受损的皮肤。
They're also important sites of nerve endings that convey signals of gentle touch to the brain.
它们也是向大脑传递轻柔触摸信号的神经末梢的重要部位。
In fact, although it's much finer, humans have roughly the same density of body hair as apes of comparable sizes.
事实上，尽管它相对更细，但人类的体毛密度与大小相当的猿类大致相同。
So despite all this talk of human nakedness, we're not actually as hairless as we look.
所以尽管说了这么多关于人类如何赤裸的讨论，我们实际并没有看起来那么的无毛。
