The science of skin color - Angela Koine Flynn

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When ultraviolet sunlight hits our skin, it affects each of us 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. What’s to account for that difference, and how did our skin come to take on so many different hues to begin with? Angela Koine Flynn describes the science of skin color.

Lesson by Angela Koine Flynn, animation by Tomás Pichardo-Espaillat.

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?

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.

In 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 skin: melanin.

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 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.

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.