TED-ED-01 关于肤色

The science of skin color

肤色的科学常识

When ultraviolet sunlight hits our skin, it affects each of us a little differently. Depending on skin color, it will take only minutes of exposure to turn one person beetroot-pink, while another requires hours to experience the slightest change. So what's to account for that difference and how did our skin come to take on so many different hues to begin with?

当紫外线照射我们的皮肤，它对每个人的影响是不同的。基于不同的肤色，紫外线会花一段时间摄入一些人的皮肤，并将其变成粉菜色，而另一部分照射紫外线后肤色不会有明显变化。所以究竟是什么导致这个不同，我们的皮肤又是如何开始应对这些不同的因素的呢？

00:31

Whatever the color, our skin tells an epic tale of human intrepidness and adaptability, revealing its variance to be a function of biology. It all centers around melanin, the pigment that gives skin and hair its color. This ingredient comes from skin cells called melanocytes takes two basic forms. There's eumelanin, which gives rise to a range of brown skin tones, as well as black, brown, and blond hair, and pheomelanin, which causes the reddish browns of freckles and red hair. But humans weren't always like this. Our varying skin tones were formed by an evolutionary process driven by the Sun. In began some 50,000 years ago when our ancestors migrated north from Africa and into Europe and Asia.

不管是什么肤色，我们的皮肤会呈现出其顽强和可适应性，展示其多变的生物特性。期中黑色素是最重要的成分，它决定皮肤和头发的颜色。来自皮肤细胞的成分叫做黑色素细胞，它主要有两种形态。一种是真黑素，它能够增加你皮肤的深色度，并且给予你乌黑的、褐色的或者金色的一头秀发。另一种是褐黑素，它能产生红褐色的雀斑和红色头发。但是人们通常不喜欢这种。我们多样的肤色其实是太阳光的照射慢慢演变的成果。它起源于50000年前，当我们的祖先从非洲移民到北部的欧洲和亚洲。

01:22

These ancient humans lived between the Equator and the Tropic of Capricorn, a region saturated by the Sun's UV-carrying rays. When skin is exposed to UV for long periods of time, the UV light damages the DNA within our cells, and skin starts to burn. If that damage is severe enough, the cells mutations can lead to melanoma, a deadly cancer that forms in the skin's melanocytes.

先人们住在赤道和南回归线间的区域，这块区域饱受含UV的太阳光射线的照射。的那个皮肤过久暴露在UV的照射中，UV射线会损害我们细胞中的DNA，紧接着皮肤会被晒伤。如果晒伤很严重的话，细胞的突变可能会导致黑素瘤，它会在皮肤的黑色素中形成并最终致命。

01:49

Sunscreen as we know it today didn't exist 50,000 years ago. So how did our ancestors cope with this onslaught of UV? The key to survival lay in their own personal sunscreen manufactured beneath the skin: melanin.

我们现在的防晒霜在50000年前并不存在。所以我们的祖先是如何应对UV射线的攻击呢？最关键的其实是他们自身皮肤里的防晒成分：黑色素。

02:05

The type and amount of melanin in your skin determines whether you'll be more or less protected from the sun. This comes down to the skin's response as sunlight strikes it. When it's exposed to UV light, that triggers special light-sensitive receptors called rhodopsin, which stimulate the production of melanin to shield cells from damage. For light-skin people, that extra melanin darkens their skin and produces a tan.

皮肤里面的黑色素的种类和数量决定了你是否或多或少地避免太阳光的伤害。这取决于当皮肤被晒时的反应。当皮肤暴露在UV光下，这会触发特殊的视网膜紫质这个感光感受器，而它会促进黑色素的产生来防御细胞受损。对于皮肤白的人来说，这种额外的黑色素会加深他们的肤色，变成古铜色。

02:30

Over the course of generations, humans living at the Sun-saturated latitudes in Africa adapted to have a higher melanin production threshold and more eumelanin, giving skin a darker tone. This built-in sun shield helped protect them from melanoma, likely making them evolutionarily fitter and capable of passing this useful trait on to new generations.

在人类的不断进化中，住在阳光饱和的非洲的人们已经适应了，他们的身体会产生更多的黑色素和真黑素，因此他们的肤色会呈现更黑的颜色。这种内在的阳光防御屏障帮助他们远离黑素瘤的风险，这可能也让他们在进化过程中将这种特征遗传至子子孙孙。

02:55

But soon, some of our Sun-adapted ancestors migrated northward out of the tropical zone, spreading far and wide across the Earth. The further north they traveled, the less direct sunshine they saw. This was a problem because although UV light can damage skin, it also has an important parallel benefit. UV helps our bodies produce vitamin D, an ingredient that strengthens bones and lets us absorb vital minerals, like calcium, iron, magnesium, phosphate, and zinc. Without it, humans experience serious fatigue and weakened bones that can cause a condition known as rickets.

但是不久，一部分祖先继续北迁远离了热带地区，零零散散分散在全球各地。越往北的祖先们，他们直接晒到的太阳光就越少。这也带来一个问题，UV光可以损害皮肤，但它也有一定的好处。UV可以帮助人体合成维他命D，维他命D是帮助强化骨骼的重要成分，它同时帮助人体吸收矿物质，像钙、铁、镁、磷、锌。没有了它，人会感到严重的疲乏，身体骨骼也会变得脆弱甚至得软骨病。

03:33

For humans whose dark skin effectively blocked whatever sunlight there was, vitamin D deficiency would have posed a serious threat in the north. But some of them happened to produce less melanin. They were exposed to small enough amounts of light that melanoma was less likely, and their lighter skin better absorbed the UV light. So they benefited from vitamin D, developed strong bones, and survived well enough to produce healthy offspring. Over many generations of selection, skin color in those regions gradually lightened.

对于那些肤色比较黑，能够有效防御太阳光的人，缺乏维他命D会给生活在北部的人带来严重威胁。但有些人碰巧产生的黑色素更少一些。他们接触的太阳光不足以导致黑色瘤，他们的浅肤色也能更好地吸收UV光。所以他们能更好地吸收维他命D，骨骼更强健，进而他们的下一代也更健康。在多年的物种选择后，这些区域的人们肤色会慢慢变得更浅。

04:05

As a result of our ancestor's adaptability, today the planet is full of people with a vast palette of skin colors, typically, darker eumelanin-rich skin in the hot, sunny band around the Equator, and increasingly lighter pheomelanin-rich skin shades fanning outwards as the sunshine dwindles.

得益于我们祖先强大的适应能力，如今地球上的人们有各种各样的肤色。典型的，在赤道附近的阳光带、热带地区有真黑素比较多的黑人，还有随着暴露太阳光越少肤色余额浅的其他肤色的人们。

04:26

Therefore, skin color is little more than an adaptive trait for living on a rock that orbits the Sun. It may absorb light, but it certainly does not reflect character.

因此，肤色不过是一个决定于接收太阳光多少而可调节的特征。它能够吸收光纤，但是他不能反映人的个性。

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