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为什么人类的身体几乎没有毛发?

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Our weird lack of hair may be the key to our success
为什么人类的身体几乎没有毛发?

Look closely at your friends, family and even strangers. Spot anything strange?

仔细观察你的亲友,甚至陌生人。有没有发现奇怪的地方?

Ignore any large noses or unusually-shaped brows. Instead, look at their hair – or rather, the lack of it.

不要观察他们的大鼻子或奇怪形状的眉毛。看看他们身上的毛发——或者哪里不长毛。

It might not seem strange, because we are used to having relatively little hair covering our bodies. But when we compare ourselves to the rest of the mammals, and our closest living ape cousins, it is downright bizarre that we are the only large-bodied mammal with so little of it.

这可能并不奇怪,因为我们已经习惯于身上毛发较少。但是当我们把自己与其他哺乳类动物和与我们亲缘关系最近的猿类相比时,我们会惊奇的发现人类是唯一毛发如此稀疏的哺乳类动物。

Unlike hairy chimpanzees and bonobos – and all other primates – most of our skin is on display. We have evolved this way, even though fur is beneficial: it insulates and protects the skin, and in some cases acts as a useful camouflage. So if it is so advantageous, why did we lose so much of it?

和毛发浓密的黑猩猩、倭黑猩猩以及所有其他的灵长类不同,人类的皮肤大部分都是裸露的。这是我们进化的结果,尽管毛发有它的好处:它可以隔热、保暖、保护皮肤,某些情况下还是一种有用的迷彩。既然它有这些优势,那么为什么人类在进化中失去了大部分的毛发呢?

我们在出生时就几乎没有毛发
It was Charles Darwin who first taught the public that humans are descended from an ape-like ancestor. He also wondered why we had so little hair.
 
查尔斯·达尔文(Charles Darwin)是第一个告诉公众人类起源于像猿类一样的祖先的人。他也奇怪为什么人类的毛发如此稀疏。

"No one supposes that the nakedness of the skin is any direct advantage to man; his body, therefore, cannot have been divested of hair through natural selection," Darwin wrote in The Descent of Man.

“没有人认为皮肤裸露对人来说有任何直接的好处;所以,人体不可能是通过自然选择失去了毛发,”达尔文在《人类的由来》(The Descent of Man)中写道。

He proposed that we lost much of our fur due to sexual selection: we preferred hairless mates, and that is why hairlessness became common.

他提出我们失去大部分毛发是性选择的结果:我们偏好毛发较少的异性,所以毛发较少的人变得更为常见。

But that cannot be the whole picture. Before a preference for hairlessness began, we first had to start losing hair.

但是,这不可能解释一切。在出现对少毛发人的偏好之前,首先人类要开始失去毛发。

Our earliest human-like ancestors, known as hominins, were ape-like. For them, fur would have been useful, keeping them warm on cold nights.

人类最早的祖先人族(hominis)长得像猿类。毛发对他们来说很有用,可以帮助他们在寒冷的夜晚保持温暖。

Something must have created an evolutionary pressure for these hominins to lose their fur.

一定是出现了某种力量,推动人族在进化的过程中失去毛发。

A few million years ago, there were several hominin species roaming the Earth. These included the famous fossil known as "Lucy", an Australopithecus afarensis from 3.2 million years ago.

数百万年前,几个人族族群在地球上迁徙漫游。它们包括320万年前的阿法南方古猿(Australopithecus afarensis),也就被称为著名的化石“露西”的族群。

These hominins were ape-like. Lucy was a lot like a chimpanzee, except that she could walk upright and had a slightly bigger brain. Her skin was not preserved, but she was probably covered with fur.

这些人族长得像猿类。露西非常像黑猩猩,除了她可以直立行走,脑子也稍微大一点。她的皮肤没有能够保存下来,但是她很可能毛发浓密。

However, between two and three millions years ago our ancestors began to inhabit more open savannahs. This meant they were out in the Sun's glaring heat for many more hours each day.

不过,在两三百万年前,我们的祖先开始在空旷的平原上居住。这就意味着每天他们要经受数小时炽烈的阳光。

Around the same time, they also started to hunt and eat more meat – and game animals were more abundant in the open. This move into open spaces offered an explanation for our lack of hair.

大约同时,他们开始捕猎,吃更多的肉类——而空旷地的猎物更多。人类迁移至空旷地区可以解释人类的毛发为什么会减少。

In the 1990s, Peter Wheeler of Liverpool John Moores University in the UK came up with a mathematical model showing how much excess heat hominins would need to lose in open habitats in order to function. If their brains got too hot, their thought processes would be impaired.

20世纪90年代,英国利物浦约翰摩尔斯大学(Liverpool John Moores University)的彼得·惠勒(Peter Wheeler)提出了一个数学模型,它可以计算出人族在空旷地带生存需要减少多少毛发。如果大脑的温度过高,人族的思维过程会受损。

If the hominins were covered in hair, they could not have lost heat fast enough. Wheeler reasoned that two related changes happened that allowed our ancestors to keep cool.

如果人族身上长满毛,他们就无法快速散热。惠勒的推理是发生了两个相关的变化,让我们的祖先保持较低的体温。

One was an upright gait. Walking on two legs meant that only the tops of their bodies were under direct sunlight.

其中之一是直立行走。这意味着只有上半身被阳光直接照射到。

But the hominins also started running long distances. This meant they could bring down large game animals by running them to exhaustion, but it also put them at risk of over-heating. To cope with that, they needed to lose their fur – in order to help them sweat.

但是,人族也开始长距离奔跑。这意味着他们可以追逐大型猎物直至它们筋疲力尽。但是这也可能导致他们体温过高。为此,他们需要减少毛发——以帮助排汗。

Hominins as hairy as chimpanzees could not cope with blazing midday sunlight. Unable to hunt or forage, they would have to hide in the shade, wasting hours of precious time. Similarly, modern chimpanzees stay in shady forests.

人族与黑猩猩一样体毛繁盛,无法承受中午强烈的日照。因为无法打猎或觅食,他们就只能躲在树荫下,浪费数小时宝贵的时间。同样道理,现代的黑猩猩也躲在森林的树荫里。

In contrast, early humans could keep going because of their ability to "dump the heat" via sweating, says Tamás Dávid-Barrett of the University of Oxford in the UK.

而与之相反,英国牛津大学的塔玛斯·大卫·贝雷特(TamásDávid-Barrett)说,原始人可以通过排汗来“散热”,这一能力帮助他们在中午也可以活动。

"It would be [an] enormous advantage to be able to spend the entire midday foraging, finding mates or fighting enemies," he says. "Sweating allows that, and for sweat to be efficient you need to be mostly hairless. That is the reason why sweating is a useful thing and hence why hair loss is a useful thing."

“能够花整个中午寻找伴侣或与敌人战斗成为他们的巨大优势。”他说,“排汗是前提,而高效率的排汗就需要身体大部分地方没有毛发。因此,排汗成为一种有用的功能,而减少毛发也变得有用。”

Our sweaty hairlessness, the theory goes, allowed us to hunt for longer, chasing nutritious large game that eventually helped give us the energy we needed to fuel growing brains.

按照理论,人类依靠体毛少和排汗旺盛的优势可以延长捕猎和追逐富含营养的大型猎物的时间,这最终帮助我们获得所需能量,促进大脑的发展。

Today, humans are the sweatiest primates alive. We have up to five million sweat glands, called eccrine glands. They produce a maximum of about 12 litres of the stuff per day, according to estimates made by anthropologist Nina Jablonski of Pennsylvania State University in University Park, US.

如今,人类是现存排汗最旺盛的灵长类动物。我们有500万条汗腺,又名外分泌腺。根据美国宾夕法尼亚州立大学(Pennsylvania State University)帕克校区(University Park)的人类学家妮娜·加布隆斯基(Nina Jablonski)的估计,人类一天最多出汗12升。

While other primates sweat, they do not have such high-density sweat glands, says Jablonski. The few that approach human levels of sweatiness seem to have evolved that way for similar reasons.

加布隆斯基说,虽然其他灵长类动物也会出汗,但是它们的汗腺密度比人类低很多。接近人类汗腺水平的少数几种动物也是因为同样原因进化的结果。

For instance, the tropical and fast-running patas monkey has lots of eccrine glands. "They almost certainly evolved this relatively high density of sweat glands for the same reason that humans did, to stay cool," says Jablonski. "They did not go as far as we did, however, in losing all body hair."

比如,热带地区身手敏捷的赤猴就拥有大量汗腺。“它们之所以拥有相对高密度的汗腺也是基于同样的理由,为了保持凉爽。”加布隆斯基说,“不过,它们并没有达到人类的程度,并没有失去大部分体毛。”

However, since Wheeler proposed his ideas we have learned that hominins were upright walkers long before they began to inhabit more open environments. Lucy could walk on two legs, but she lived long before climatic changed pushed later species into less forested environments.

然而,在惠勒提出了他的观点后,我们发现人族早在进入空旷环境生存之前就已经学会了直立行走。露西可以直立行走,很长一段时间以后才出现气候变化,迫使人族进入森林较少的地区生存。

This means walking upright may not have been a major contributing factor for hairlessness. The hominins were simply not running all that much yet.

这就意味着直立行走可能并不是毛发减少的主要原因。人族尚未开始大量奔跑。

However, our upright gait did come in handy when early humans moved into a hotter, more open lifestyle.

然而,直立行走在人类向炎热、空旷地区迁徙的过程中成为一个便利条件。

Their ability to walk upright and, eventually, run, would have been a considerable advantage, both to hunt prey and avoid predators. At this point, walking upright could drive the loss of more hair, and vice versa.

直立行走以及最终的跑步能力后来成为人捕猎和躲避肉食动物的重要优势。此时,直立行走有可能促进了毛发减少,反之亦然。

"The less hairy you are, the more advantage you get from being bipedal, and the more time you spend [upright], the more advantage you get from losing hair," says Dávid-Barrett. "These two could have been co-evolving."

“你的毛发越少,双足站立的优势就越大。而你直立的时间越长,你减少毛发所得的优势就越大。”大卫·贝雷特说,“这两个因素可能是互相促进的。”

The key question, then, may be when hominins began running in earnest. One extinct hominin had the perfect anatomy for running.

那么,关键问题就是人族何时开始认真跑步的。一支已经灭绝的人族的身体结构表明他们非常适合奔跑。

Homo erectus first appeared on Earth about 1.8 million years ago. They stood upright and had larger brains than several of their forebears. H. erectus was also the first early human to venture out of Africa, and is believed to be our direct ancestor.

180万年前,地球上首次出现直立人(Homo erectus)。他们双足站立,脑容量超过了他们的祖先。直立人也是首批冒险离开非洲的人类,他直接是我们的祖先。

This fits with Jablonski's ideas. "We lost our heavy coat of hair and gained more sweat glands probably at about the same time, during the course of the early evolution of the genus Homo," she says.

这符合加布隆斯基的观点。“毛发减少和汗腺增多差不多在同一个时候开始进行,都发生在人属(Homo)早期的进化过程中。”

Crucially, H. erectus had a body heavily adapted for running. They had long Achilles tendons, which chimps and gorillas do not have, and narrow waists and shoulders – allowing their bodies and heads to rotate independently.

关键在于直立人的身体为奔跑进行了大量的调适。他们的跟腱很长,这是黑猩猩和大猩猩所没有的,细腰和窄肩让他们的身体和头部可以独立摆动。

At this point the story starts to look quite neat. However, there has long been a flaw in the sweat-cooling hypothesis that did not sit well with Dávid-Barrett.

到此为止,情况开始变得明朗。不过,排汗降温的假设还存在一个不符合大卫·贝雷特的观点的瑕疵。

Wheeler's early models did not account for the fact that hominins had to keep warm at night, when the temperature dropped considerably. Without fur, they would have had little protection from the cold.

惠勒早期的模型并不能解释夜间温度骤降时人族如何保暖。假如没有毛发,他们将无法抵御寒冷。

Dávid-Barrett and his colleague Robin Dunbar have tried to iron out this wrinkle their ideas were published in the May 2016 Journal of Human Evolution.

2016年5月,大卫·贝雷特和他的同事罗宾·邓巴(Robin Dunbar)在《人类进化期刊》(Journal of Human Evolution)上发表论文,试图解释这一问题。

They argue that, in order to survive without fur at night, hominins needed to burn many more calories. That means our ancestors needed to fuel their bodies with calorie-rich food during the day.

他们认为,没有毛发的人族需要通过燃烧更多的卡路里来熬过晚上。这就意味着我们的祖先需要在白天吃卡路里丰富的食物来支持身体。

Dávid-Barrett argues that we could only do this when we started regularly eating cooked food.

大卫·贝雷特认为要做到这一点,人类必须开始规律食用煮熟的食物。

According to a much-discussed hypothesis first put forward by Richard Wrangham of Harvard University, hominins began cooking their food as much as two million years ago. Wrangham was trying to explain how hominins acquired such big brains, but his idea could also explain how our ancestors survived cold nights without fur.

根据哈佛大学理查德·兰厄姆(Richard Wrangham)首先提出的讨论较多的假设,人族在200万年前就开始烹煮食物。兰厄姆试图通过这一假设解释人族获得这么大的脑容量的方法,但是他的观点同样也可以解释我们的祖先在没有毛发的情况下如何熬过寒冷的夜晚。

Dávid-Barrett points out that, if hominins were cooking food, they must have been using fire. As well as providing them with a better diet, the heat from the fires would have kept them warm at night.

大卫·贝雷特指出,假如人族已经开始烹饪,他们就肯定已经会使用火。火不仅可以改善他们的饮食,还可以提供热量,让他们在夜间取暖。

However, this idea only holds up if cooking really is as ancient as Wrangham suggested. There is no archaeological evidence that hominins were using fire two million years ago, and a study published in March 2016 argued that cooking only became common about 500,000 years ago.

不过,这一观点成立的前提是烹饪的出现确实如兰厄姆所认为的那么早。没有考古学证据证明人族在200万年前就开始使用火。2016年3月发表的一个研究认为50万年前烹饪才普及。

Despite these difficulties, the sweating hypothesis remains the leading explanation for why we lost our hair. However, there are other ideas out there.

尽管存在这些挑战,排汗假设仍然是解释人类失去毛发的首选。不过,也存在其他一些解释。

One suggestion was that early humans spent much of their time in water, so like walruses and whales they needed hair-free skin to swim effectively. However, this "aquatic ape" idea is largely discredited. If nothing else, plenty of aquatic animals have fur and swim just fine: fur seals and otters are but two examples.

一种观点认为人类早期很多时候在水中活动,他们像海象和鲸鱼一样需要无毛的皮肤以便提高游泳的效率。不过,这一“水中猿类”的观点基本不可信。一个漏洞就是水中有毛皮的生物也可以正常游泳:比如带有毛皮的海豹和水獭。

Another idea was that we lost our hair to rid ourselves of parasites. The trouble with this is that does not explain why other primates kept their hair.

另一种观点认为,毛发减少是为了消除寄生虫。这个观点的问题在于它并未解释为什么其他灵长类动物的毛发得以留存。

However, over the last 20 years scientists have begun to turn to a new source of evidence: genetics.

然而,在过去的20年间,科学家开始把目光转向一个新的证据来源:遗传学。

A 2004 study found that a variant of the MC1R gene, which is known to be important for darker skin colour, was already present 1.2 million years ago.

2004年的一项研究发现,MC1R基因的一种变异早在120万年前就已经存在。已知该变异与深色皮肤有重要关系。

This is telling, because naked skin would only get darker after it was repeatedly exposed to heat. The first patches of fur-free skin were probably pink, after which the tropical sun quickly pushed the evolution of dark skin. The presence of the MC1R variant suggests that our ancestors were on the path to dark skin, and therefore hairlessness, by 1.2 million years ago.

这一点很能说明问题,因为裸露的肌肤只有在频繁暴露在光热下才会变黑。第一批无毛皮肤很可能是粉色的,后来热带的阳光迅速将其变成深色皮肤。MC1R变异的存在说明我们的祖先在120万年前就能够开始皮肤变深和毛发减少的过程。

Futher evidence, this time from lice, backs this up. A study published in 2004 examined the evolution of the lice that sometimes infest our hair. Different species live in our pubic hair and head hair, and the researchers found that the two diverged 1.18 million years ago.

进一步的证据来自虱子。2004年发表的一项研究对寄生在人毛发中的虱子的进化过程进行探究。他们发现在头发和阴毛里生存的虱子分属两类,它们的分化时间是118万年前。

"The assumption is that there is an ocean of bare skin between the two," says Dave Wilkinson of Liverpool John Moores University. So the split in the lice reflects the loss of the hair on our chests. "It's a good a guess as any," he says. These two lineages could also be explained by "direct physical contact between modern and archaic forms of Homo," the authors propose.

“我们的假定是这两种虱子之间隔着大片的裸露皮肤。” 利物浦约翰摩尔斯大学(Liverpool John Moores University)的戴夫·威尔金森(Dave Wilkinson)说。所以,虱子的分化反映了人类胸毛减少的现象。“这一推测并不比其他推测更有可能。”他说。虱子分为两种也可以是因为“远古人类与较新近人类发生了直接的身体接触。”

A big problem for all studies of our hair loss is that the genetic instructions that caused it remain mysterious. Geneticists do not fully understand how our eccrine glands are made.

所有关于毛发减少的研究所存在的一大难题是其基因根源仍未解开。遗传学无法彻底理解外分泌腺是如何形成的。

They are now getting closer to finding out. In 2015, by tweaking genes in mice, scientists showed that the production of sweat glands is closely connected to the production of hair.

他们现在正在一步步靠近真相。2015年,科学家通过对老鼠基因的细微调整表明汗腺的产生与毛发的产生有密切关系。

"Developmentally, in the embryo, these organs are both derived from the skin," says lead author Yana Kamberov of the University of Pennsylvania. "Both are hearing signals from the same source, the skin is deciding which one to make, and they are being made in the same shared space."

“从发育过程来看,在胚胎期,这些组织都来自皮肤”宾夕法尼亚大学的第一作者亚那·坎贝罗夫(Yana Kamberov)说,“它们接受的指令来自同一个源头。皮肤可以决定生长汗腺或是毛发,它们的生长空间也是一样的。”

Crucially, her team found that sweat and hair gland production are inversely related. When a certain gene was highly active, the mice had more sweat glands than hair, but if the gene was inactive, the mice were hairier and had fewer sweat glands.

亚那的团队作出了关键的发现,汗腺和毛囊的生长呈负相关。当某一种基因高度活跃时,老鼠的汗腺就多于毛发,而当这一基因不活跃时,老鼠的毛发较多,汗腺较少。

The details are still hard to come by. "We don't know when the changes occurred," says Kamberov. "We don't know if humans lost their fur first and then there was an expansion of sweat glands, or whether it was concurrent."

其中的细节仍未确定。“我们不知道这些变化是在什么时候发生的。”坎贝罗夫说,“我们不知道如果是毛发先减少,随后汗腺才开始扩展;还是两者同时进行。”

Nevertheless, Kamberov's work suggests that Wheeler was right all along, at least in outline. The loss of our hair is tied up with our propensity to sweat, even at the genetic level – and that in turn is bound up with our ability to run and catch big game, and thus to feed our big brains.

不论如何,坎贝罗夫的研究说明惠勒一直都是正确的,至少在总体上是这样。毛发减少与我们的排汗功能有密切关系,甚至在基因层面也是这样。这又关系到我们的奔跑能力和捕猎大型动物的能力,并最终与大脑的进化有关。

Now take another look at your strangely hairless friends. It is undoubtedly odd that we are so short of fur. But it seems this is not just an accident of evolution: it is tied up with the evolutionary changes that make us who we are.

现在可以换个角度来看待你身边毛发较少的朋友了。毫无疑问,人类的毛发出奇的少。但是似乎这不只是进化中的意外:它关系到我们成为人的进化过程。
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