When ultraviolet sunlight hits our skin, it affects each of us a little differently.
当紫外线照射到皮肤上时，它对我们的影响因人而异。
Depending on skin color, it will take only minutes of exposure to turn one person beetroot-pink,
取决于肤色，一个人只在阳光下暴露几分钟，就会晒成甜菜根般的红色，
while another requires hours to experience the slightest change.
而另一个人，需要几小时的暴露，才能体现轻微的肤色改变。
So what's to account for that difference and how did our skin come to take on so many different hues to begin with?
到底怎样解释这种差异呢，我们的皮肤又是怎样呈现出如此多不同的肤色呢？
Whatever the color, our skin tells an epic tale of human intrepidness and adaptability,
无论怎样的肤色，我们的皮肤都在讲述一段人类无惧和适应性的史诗般的故事，
revealing its variance to be a function of biology.
肤色的多变性是一项生物学功能，
It all centers around melanin, the pigment that gives skin and hair its color.
这一切都围绕着黑色素，这种颜料赋予皮肤和头发的颜色。
This ingredient comes from skin cells called melanocytes and takes two basic forms.
这种成分来源于被称为黑素细胞的皮肤细胞，它具有两种形式。
There's eumelanin, which gives rise to a range of brown skin tones, as well as black, brown, and blond hair,
其中一种叫真黑色素，它产生一系列的褐色皮肤，以及黑色、棕色以及金色的头发，
and pheomelanin, which causes the reddish browns of freckles and red hair.
另一种是伪黑色素，则生成褐色雀斑和红色头发。
But humans weren't always like this.
但人类并不总是这样。
Our varying skin tones were formed by an evolutionary process driven by the Sun.
我们不同色调的皮肤是受太阳驱动的进化过程中形成的。
In began some 50,000 years ago when our ancestors migrated north from Africa and into Europe and Asia.
它大约起源于五万年前，当我们的祖先从非洲北上，迁移到欧洲与亚洲时。
These ancient humans lived between the Equator and the Tropic of Capricorn,
这些远古人类生活在赤道和南回归线之间，
a region saturated by the Sun's UV-carrying rays.
这部分地域充满了紫外线。
When skin is exposed to UV for long periods of time,
当皮肤被长期暴露于紫外线中，
the UV light damages the DNA within our cells, and skin starts to burn.
紫外线会损坏细胞中的基因，皮肤开始烧伤。
If that damage is severe enough, the cells mutations can lead to melanoma,
如果损伤足够严重，细胞突变可导致黑素瘤，
a deadly cancer that forms in the skin's melanocytes.
这是一种由皮肤中的黑色素细胞形成的致命癌症。
Sunscreen as we know it today didn't exist 50,000 years ago.
我们今天所知的防晒霜，在五万年前并不存在。
So how did our ancestors cope with this onslaught of UV?
那么我们的祖先是如何应对大量的紫外线照射的呢？
The key to survival lay in their own personal sunscreen manufactured beneath the skin: melanin.
生存的关键在于他们自身的防晒霜，也就是皮下产生的黑色素。
The type and amount of melanin in your skin determines whether you'll be more or less protected from the sun.
你们皮肤中黑色素的种类和数量，决定了你在受太阳照射时，是多还是少的受保护程度。
This comes down to the skin's response as sunlight strikes it.
这取决于阳光照射时皮肤的反应。
When it's exposed to UV light, that triggers special light-sensitive receptors called rhodopsin,
当暴露在紫外线下时，将触发被称为紫膜质的特殊感光受体，
which stimulate the production of melanin to shield cells from damage.
它促进黑色素的产生，以保护细胞免受损伤。
For light-skin people, that extra melanin darkens their skin and produces a tan.
对于肤色偏浅的人来说, 额外的黑色素会使肤色加深变为黄褐色。

Over the course of generations, humans living at the Sun-saturated latitudes in Africa
经历无数代后，生活在非洲阳光饱和的纬度的人们，
adapted to have a higher melanin production threshold and more eumelanin, giving skin a darker tone.
已经进化到拥有更高的黑色素的生产阈值和更多真黑素，使皮肤拥有更深的色调。
This built-in sun shield helped protect them from melanoma,
这种自身的防太阳光紫外线保护层可防止黑素瘤的形成，
likely making them evolutionarily fitter and capable of passing this useful trait on to new generations.
他们为适应环境而进化，并且将这种特性代代相传。
But soon, some of our Sun-adapted ancestors migrated northward out of the tropical zone, spreading far and wide across the Earth.
但是不久，部分适应日晒的祖先们离开了热带区域，往北上移居，他们广泛的迁移到世界各地。
The further north they traveled, the less direct sunshine they saw.
他们越往北，看见到的阳光就越少。
This was a problem because although UV light can damage skin, it also has an important parallel benefit.
这有一个问题，因为尽管紫外线会损伤皮肤，同时它对人体有一个重要的益处。
UV helps our bodies produce vitamin D, an ingredient that strengthens bones and lets us absorb vital minerals,
紫外线帮助身体产生维素D，这种物质可以强化骨骼，并帮助我们吸收重要的矿物质，
like calcium, iron, magnesium, phosphate, and zinc.
比如钙，铁，镁，磷酸盐，锌。
Without it, humans experience serious fatigue and weakened bones that can cause a condition known as rickets.
没有它，人体会感到极为疲乏且骨骼脆弱，这种状况被称为佝偻病。
For humans whose dark skin effectively blocked whatever sunlight there was,
对于那些深色皮肤有效地阻止了任何阳光的人，
vitamin D deficiency would have posed a serious threat in the north.
在北方维生素D缺乏将造成严重威胁。
But some of them happened to produce less melanin.
但是他们中的一些人正好产生较少黑色素。
They were exposed to small enough amounts of light that melanoma was less likely,
他们暴露在少量的阳光下，是不大可能产生黑色素瘤的，
and their lighter skin better absorbed the UV light.
并且他们的浅色皮肤能更好地吸收紫外线。
So they benefited from vitamin D, developed strong bones, and survived well enough to produce healthy offspring.
因此他们得益于维生素D，发育出强健的骨骼，并能更好的生存、繁衍健康的后代。
Over many generations of selection, skin color in those regions gradually lightened.
经过很多代的自然选择后，居住在那些区域的人的肤色逐渐淡化。
As a result of our ancestor's adaptability, today the planet is full of people with a vast palette of skin colors,
作为我们祖先适应的结果，如今我们的地球上充满了各种肤色的人，
typically, darker eumelanin-rich skin in the hot, sunny band around the Equator,
通常，黑色素丰富的深色皮肤的人生活在炎热的、阳光充足的赤道带；
and increasingly lighter pheomelanin-rich skin shades fanning outwards as the sunshine dwindles.
从赤道呈扇形越往外阳光越少，含有伪黑色素的皮肤的人肤色也越来越浅。
Therefore, skin color is little more than an adaptive trait for living on a rock that orbits the Sun.
因此，肤色仅仅是生活在围绕太阳运行的地球上的人们的适应特征。
It may absorb light, but it certainly does not reflect character.
它可以吸收阳光，但肯定不反应品格。