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.

根據皮膚的顏色， 有的人只要曝曬幾分鐘，皮膚就變得像甜菜根一樣紅，而有的卻需要數小時才有絲毫的改變。

So what's to account for that difference?

那是甚麼造成這種差異呢？

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

首先，我們的皮膚是如何呈現這麼多不同顏色呢？

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

不管是甚麼膚色，我們的皮膚述說人類剛毅性與適應性的史詩故事，顯示其變異是生物的一個功能。

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

主要是圍繞在「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.

這種要素來自皮膚細胞 ─ 稱為「黑素細胞」，黑色素有兩種基本型。

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」，會產生一系列的棕色皮膚色調以及黑色、棕色和金色的頭髮，另一是「褐黑素」，它會導致紅棕色的雀斑和紅髮。

But humans weren't always like this.

但人類並不一直是這樣的。

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影響，經演化而形成。

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

5 萬多年前開始， 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合的地區。

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.

當皮膚長時間暴露於紫外線下，紫外線會損害我們的細胞內的 DNA，皮膚開始曬傷。

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.

現今我們所知道的防曬霜 在5 萬年前並不存在。

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

那麼我們祖先是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.

生存關鍵在於他們自己產生的防曬，就是在皮膚下製造的：黑色素。

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

皮膚裡黑色素的種類和數量決定你防曬程度的多寡。

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

這涉及到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.

當皮膚暴露於紫外線下，會觸發特殊的感光受體 ─ 稱為「視紫質」，它會刺激黑色素的產生， 以免細胞遭受破壞。

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

對於淺膚色的人，額外的黑色素會使膚色變深而曬黑。

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.

在世代繁衍過程中，住在非洲陽光飽和緯度地區的人類適應成為有較多黑色素生產量，及較多的真黑素，使皮膚呈現較深色。

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，很可能使他們進化成為更適合及更能夠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，遍佈整個地球。

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

他們越往北移動， 就越少見到陽光的直射。

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，但它同時也有重要的好處。

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.

紫外線可幫助我們身體產生維生素D ─ 一種強化骨骼並讓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.

沒有它，人類會感到嚴重疲勞且骨骼脆弱，這會造成一種疾病， 稱為「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，當他們到北方時，維生素D缺乏會造成嚴重的威脅。

But some of them happened to produce less melanin.

但他們之中有些人， 恰好產生較少的黑色素。

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

由於他們暴露在量夠少的陽光下， 所以罹患黑色素瘤的可能性較小，且他們皮膚較白，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，他們從維生素 D 中受益，形成強健骨骼，而且存活良好， 足以繁育健康的後代。

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

經過數代的物竸天擇，在那些地區的人膚色逐漸變淡。

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.

由於我們祖先的演化適應，今天地球充滿了許多不同膚色的人，通常富含真黑素的深膚色者住在炎熱多陽的赤道附近地帶，而富含褐黑素且膚色漸淺者則由此區呈扇型向外擴散，因為那兒陽光逐漸減少。

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

因此，膚色只不過是生活在環繞太陽的一個岩石地球上的適應特徵而已。

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。