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

  • 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.

  • So what's to account for that difference?

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

  • 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 and 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.

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

  • 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.

  • 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 skinmelanin.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

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

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B2 TED-Ed skin melanin uv light skin color sun

【TED-Ed】The science of skin color - Angela Koine Flynn

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    Sh, Gang (Aaron) posted on 2016/03/11
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