When ultraviolet sunlight hits our skin, it affects each of us a little differently.

陽光的紫外線照射到我們的皮膚時，它對每個人的影響有點不同。

Depending on skin color, it'll take only minutes of exposure to turn one person beetroot-pink, while another requires hours to experience the slightest change.

根據膚色的不同， 有些人曝曬幾分鐘後，皮膚就變得像甜菜根一樣紅，但有些人經過數小時卻只有輕微的變化。

So what's to account for that difference?

那麼，是什麼造成這種差異呢？

And how did our skin come to take on so many different hues to begin with?

而我們的皮膚又為什麼有這麼多不同的色調呢？

Whatever the color, our skin tells an epic tale of human intrepidness and adaptability, revealing its variance to be a function of biology.

不論膚色，我們的皮膚述說了關於人類的無畏與適應性的史詩故事，顯示其多樣性是生物的一個功能。

It all centers around melanin, the pigment that gives skin and hair its color.

其核心是「黑色素」，賦予皮膚與頭髮顏色的色素。

This ingredient comes from skin cells called melanocytes and takes two basic forms.

此要素來自名為黑素細胞的皮膚細胞，有兩種基本型態。

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.

其一是「真黑素」，它會導致各種棕色調皮膚及黑色、棕色和金色的頭髮，另外就是「褐黑素」，它會導致紅棕色的雀斑和紅髮。

Our varying skin tones were formed by an evolutionary process driven by the Sun.

我們不同的皮膚色調是受太陽的影響，經演化而形成。

It began some 50,000 years ago when our ancestors migrated north from Africa and into Europe and Asia.

始於 5 萬多年以前，當時我們的祖先從非洲往北遷移到歐洲和亞洲。

These ancient humans lived between the Equator and the Tropic of Capricorn, a region saturated  by the Sun's UV-carrying rays.

這些古代人類住在赤道與南迴歸線之間，這是太陽的紫外線達飽和的地區。

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.

當皮膚長期暴露於紫外線下，紫外線會損害我們的細胞內的 DNA，而皮膚就會開始灼傷。

If that damage is severe enough, the cells' mutations can lead to melanoma, a deadly cancer that forms in the skin's melanocytes.

如果皮膚損傷夠重，細胞突變會導致「黑素瘤」，它是由皮膚的黑素細胞形成的一種致命癌症。

Sunscreen as we know it today didn't exist 50,000 years ago.

現今眾所周知的防曬產品在 5 萬年前並不存在。

So how did our ancestors cope with this onslaught of UV?

那麼我們祖先怎麼應付紫外線的攻擊呢？

The key to survival lay in their own personal sunscreen manufactured beneath the skin—melanin.

生存關鍵在於他們的皮膚下製造的防曬──黑色素。

The type and amount of melanin in your skin determines whether you'll be more or less protected from the sun.

你皮膚裡黑色素的種類和數量決定了你的防曬能力。

This comes down to the skin's response as sunlight strikes it.

簡單來說就是太陽照射時皮膚的反應。

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.

皮膚暴露於紫外線下時會觸發特殊的感光受體「視紫質」，它會刺激黑色素的產生， 以避免皮膚受到傷害。

For light-skin people, that extra melanin darkens their skin and produces a tan.

多餘的黑色素會使淺膚色的人膚色變深，變成棕褐色。

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.

在世代繁衍的過程中，生活在陽光充足的非洲緯度地區的人類為適應環境，能生產較多的黑色素及較多真黑素，使其膚色較深。

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.

這與生俱來的遮陽罩使他們不易罹患黑色素瘤，使他們演化以更適應居住環境，並將這個有用的特質傳給後代子孫。

But soon, some of our Sun-adapted ancestors migrated northward out of the tropical zone, spreading far and wide across the Earth.

但很快的，一些適應太陽的祖先向北遷移，離開了熱帶區，遍佈地球各個角落。

The further north they traveled, the less direct sunshine they saw.

他們越往北移動， 直射的陽光就越少。

This was a problem because although UV light can damage skin, it also has an important parallel benefit.

這是一個問題， 因為雖然紫外線會損傷皮膚，但它也帶來同等重要的好處。

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.

紫外線可使我們身體產生維生素D，這種成分可以強化骨骼，並幫助我們吸收重要礦物質的成分，如鈣、鐵、鎂、磷酸鹽和鋅。

Without it, humans experience serious fatigue and weakened bones that can cause a condition known as rickets.

少了它，人類會感到極度疲勞，並造成骨骼脆弱，導致「佝僂病（軟骨病）」。

For humans whose dark skin effectively blocked whatever sunlight there was, vitamin D deficiency would have posed a serious threat in the north.

對那些能有效阻擋所有陽光的皮膚黝黑者而言，到了北方，缺乏維生素 D 會造成嚴重的健康威脅。

But some of them happened to produce less melanin.

但他們之中有些人，恰好擁有較少的黑色素。

They were exposed to small enough amounts of light that melanoma was less likely, and their lighter skin better absorbed the UV light.

他們暴露較少的陽光下，所以罹患黑色素瘤的可能性較小，而他們較淺的皮膚更容易吸收紫外線。

So they benefited from vitamin D, developed strong bones, and survived well enough to produce healthy offspring.

於是他們受益於維生素 D，擁有強健骨骼，存活了下來， 以繁育健康的後代。

Over many generations of selection, skin color in those regions gradually lightened.

經過數代的物竸天擇，那些地區的人膚色逐漸變淡。

As a result of our ancestors' 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.

由於我們祖先的適應能力，今天地球充滿了各種不同膚色的人，通常富含真黑素的深膚色者居住在炎熱、陽光充足的赤道附近，而隨著陽光的減少，富含褐黑素且膚色漸淺者則由此區呈扇型向外擴散。

Therefore, skin color is little more than an adaptive trait for living on a rock that orbits the Sun.

因此，膚色可不只是為了生活在繞太陽運行的岩石上的適應特性。

It may absorb light, but it certainly does not reflect character.

它可以吸收陽光，但它肯定不代表個性。

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