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NINA JABLONSKI: Human brains are gray.
Human blood is red.
Our bones are off-white.
Doesn't matter where you're born or to whom.
But human skin is different.
Some of us have rich, dark brown skin.
Some of us have pinkish white skin.
Most of us are somewhere in between.
For the longest time, why this variation exists
was a real scientific mystery that opened the door for some
to invest this biological trait with moral value,
and then use that to justify the suffering of others.
But biological traits aren't good or bad.
They're features that have evolved because they
enhance an organisms odds of surviving and passing
on its genes.
Like other animal traits, the sepia rainbow
of human skin color evolved through natural selection.
Now, thanks to advances in anthropology and genetics,
exactly how and why it did is no longer a mystery.
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Biological anthropologists like myself spend our lives
studying how humans evolved and why we differ from one
another physically.
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Our skin provides one of the most visible markers
of human variability.
It's something that sets us apart
from our closest animal relatives.
Under their dark fur, chimpanzees have pale skin.
And millions of years ago, that was probably also
the case for the primates that were our common ancestors.
So where did humanity's range of skin colors come from?
From physics, we know that the color of any object
comes from the wavelengths of light
that it reflects back to an observer's eye.
We see leaves as green because they reflect back
the wavelengths our eyes see as green, absorbing
the wavelengths we see as other colors, like blue or red.
In humans, different wavelengths of light
are reflected or absorbed by a pigment
in the top layer of our skin.
That pigment's called melanin.
It sits inside what looks like tiny grains, the melanosomes,
that are produced by cells called melanocytes.
Our individual genetic inheritance
determines the type of melanin inside our melanosomes.
The reddish-yellow pheomelanin is
more abundant in lightly pigmented people.
More darkly pigmented people have more
of the brown-black eumelanin.
And the more eumelanin, the darker the skin.
Melanin also colors human and animal hair
and the feathers of many birds.
Interestingly, the wavelength of light that melanin reflects
are far less important biologically than the ones
it actually absorbs.
And of the ones that absorbs, the ones
that are the most important are those that we can't even see.
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Much of the light given off by the Sun
is invisible to our eyes.
Some of that is what's called ultraviolet radiation, which
is highly energetic.
So much so it can actually penetrate living cells.
When it does, it can wreak havoc within them.
It can even cause mutations in skin cell DNA.
What stands between us and that threat
is the melanin in our skin.
ZALFA ABDEL-MALEK: Melanin is kind of like the sensor,
it's kind of like a guardian molecule.
And its main job is protection.
NINA JABLONSKI: For instance, it protects skin cell DNA
by forming what are called supranuclear caps
and absorbing UV.
ZALFA ABDEL-MALEK: They're like little parasols
around the nucleus.
And UV cannot penetrate these to go and attack the DNA.
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NINA JABLONSKI: That's just one of the things
molecular biologist Zalfa Abdel-Malek finds
remarkable about melanin.
Another is the broad range of benefits
it provides a diverse collection of species.
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ZALFA ABDEL-MALEK: We know that melanin in lower vertebrates
is important for regulating body temperature.
It can also give animals camouflage,
and allows them to recognize other members of the species
to propagate the species.
NINA JABLONSKI: In humans, one of melanin's functions
is clearly to protect cells from UV damage.
As we evolved, we lost hair and increased melanin production
in our skin.
So is there a connection between the intensity of UV
radiation and skin color?
Hi, Tess.
I first became fascinated with UV and skin color in the 1990s.
But as I searched for information
about the global distribution of solar UV,
I discovered the available data was in fact quite spotty.
I began casting a wider net.
And almost by accident, found exactly the raw data
needed to fix that.
It hadn't been collected by anyone interested in
my questions, but rather, by NASA.
SPEAKER: Ignition.
And lift off.
NINA JABLONSKI: In the 1980s, concern about the health risks
posed by the depletion of UV-blocking atmospheric ozone
led NASA to take millions of UV measurements from space.
I asked NASA to send me the data,
and then asked my geographer husband, George Chaplin,
to try to visualize it.
It turned out to be a bigger request than I'd realized.
But he found a way to turn all those data points into a map.
A map that showed for the very first time
exactly how UV exposure varies throughout the world.
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Most striking was the clear gradient
between the equator and the poles,
which was interrupted only in places where
altitude increased UV exposure.
GEORGE CHAPLIN: This is actually in the Tibetan Plateau.
NINA JABLONSKI: And persistent cloud cover decreased it.
GEORGE CHAPLIN: Congo basin, so it's
full of humidity and moisture, which is blocking the UV.
NINA JABLONSKI: Solar energy is a fundamental attribute
of any environment.
And it's a well-established fact that organisms
living at different latitudes adapt in some way
to their local solar conditions.
To see how closely humans skin color correlates with UV
exposure, I collected skin pigment measurements
made by anthropologists studying indigenous peoples.
For many years, anthropologists have
faced the challenge of how to accurately measure skin color.
We now use this little device called a reflectometer.
Basically, it sends out light of specific colors,
and then it measures the amount of light
that is reflected back.
This tells us what color Tess's skin is,
and we can then compare this to people all over the world.
George then created a second map using measured skin
colors and environmental data.
It showed UV intensity does indeed predict skin color.
Wherever UV is strong, skin is dark,
like it is near the equator or at high altitude.
At the poles, the skin of indigenous people
is almost always lighter.
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That suggests that variation in human skin melanin production
arose as different populations adapted biologically
to different solar conditions around the world.
As we've noted, our early ancestors probably
had full body hair covering pale skin, just like other primates.
So when did the darker shades of human skin begin to evolve?
DNA sequencing has made it possible to find evidence that
can help answer that question.
Rick Kittles is a geneticist who's
skilled at deciphering such clues.
RICK KITTLES: Whenever a species undergoes
some form of selection, some form
of natural selection, evidence of that selection
is found in the genome.
And so as a geneticist, we get really
excited when we explore the genome for these signatures.
One way in which that's done is by sampling
worldwide populations and looking
throughout the genome at variation
and comparing across populations.
And it's a very exciting process.
I feel like a detective when I go through that process.
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NINA JABLONSKI: One of the many genes
that genetic detectives have linked to human pigmentation
is called MC1R.
Sampling from around the world indicates
there's a fair amount of variation
in the DNA sequence of that gene,
but not from every corner of the globe.
RICK KITTLES: When we look at MC1R
within African populations, we don't see a lot of diversity.
And the particular allele that they
have in those African populations
is the one that codes for darker skin.
MC1R codes for a protein which is involved in the switch
from the production of pheomelanin to eumelanin.
And we know pheomelanin is the reddish-yellow pigment,
and then the eumelanin is the brown-black pigment.
NINA JABLONSKI: The absence of MC1R diversity
in African populations indicates that, in that part
of the world, there is strong negative selection
against any alleles that would alter dark skin.
And how long has this allele been
fixed in African populations?
Other genetic studies have calculated
that it has been as much as 1.2 million years.
Since our species evolved in equatorial Africa,
it's reasonable to conclude that by that time,
all humans were dark skinned.
The fossil record supports what we've
gleaned from genetic evidence.
But here's where we confront what was, for me,
the heart of the mystery.
The evolution of dark skin in humans
suggests that under strong UV light,
that trait provided a survival advantage.
So what exactly was that advantage?
It's certainly true UV damage to skin cell DNA
can lead to cancer, and skin cancer can be fatal.
For a long time, that seemed the likeliest explanation.
Except skin cancer generally develops after a person's
peak reproductive years.
For that reason, though it might cut your life short,
it's unlikely to affect your ability to pass on your genes.
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As I was struggling to conceive of an alternative explanation,
I happened to attend a lecture on severe birth defects.
That talk was about a research project that had found evidence
that certain birth defects are far more
common among pregnant women with diets deficient in a B
vitamin called folate.
Only weeks before, I'd come across a paper that
described how strong sunlight breaks down folate circulating
in skin blood vessels.
Here was a direct link between UV radiation, skin color,
and reproductive success.
It was a small eureka moment for me.
In the years since, we've learned that folate is not
only essential for normal embryonic development,
it's even needed for healthy sperm production in males.
Folate is biological gold.
It is an essential nutrient.
And it needs to be protected from UV radiation
as it circulates in the blood vessels in the skin.
That is what melanin does.
I felt I was halfway home on my quest to understand human skin
color variation.
But if big question remained, why aren't we all dark skinned?
It turns out there's another side to our relationship
with UV light.
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UV light is not all bad.
In fact, a small portion of it known as UVB
is critical for the synthesis in our bodies
of vitamin D, a process that starts in the skin.
Without vitamin D, humans cannot absorb calcium from our diet
to build our bones and for a healthy immune system.
Back when all our ancestors lived close to the equator,
there was no problem getting enough UVB through dark skin
to make the vitamin D needed.
But then some populations started moving north,
where the UV striking Earth's surface is much weaker.
In northern latitudes, dark skin makes
it hard to produce the vitamin D that human bodies really need.
The consequences of vitamin D deficiency
include rickets, a bone development disease
that can cripple the young.
In higher latitudes with less UV,
the selective pressure on MC1R that
produced dark skin in our ancient ancestors
began to abate.
RICK KITTLES: When we look at the early movement out
of Africa when that constraint was relaxed,
we then see a plethora of variation.
NINA JABLONSKI: In European and Asian populations,
geneticists have discovered greater variation in the MC1R
gene, but less variation in several other genes, ones
associated with lighter skin types.
RICK KITTLES: Different environments
lead to other genes being selected for
and being important for those populations
in terms of skin color.
NINA JABLONSKI: Selection for light skin gene variants
occurred multiple times in different groups
around the world, some of it in just the last 10,000 years.
Support for the idea that the UV vitamin D connection helped
drive the evolution of paler skin comes from the fact
that indigenous peoples with diets
rich in this essential vitamin have dark pigmentation.
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The tension between these two aspects
of our biological inheritance, on the one hand, the need
to protect ourselves from most ultraviolet radiation,
and on the other, the need to use some ultraviolet radiation
for our own benefit, these forces
drove the evolution of the wonderful variation
in human skin color that we see around us today.
It's the legacy of an evolutionary balancing
act, necessitated by the different environmental
conditions people have faced around the globe.
thing is, where once human migrations took
many generations, we now move about the planet
at the speed of sound.
That means increasing numbers of us
have pigmentation that's not a good match with where we live.
ZALFA ABDEL-MALEK: People with fair skin
and red hair, your phenotype is telling you
you have a high risk of skin cancer
if you're out in the Sun.
If you're a dark skinned individual living,
for example, in Scandinavia or in Minnesota,
you're not going to have optimal exposure
to UV for optimal vitamin D synthesis,
and you need to take supplements.
NINA JABLONSKI: We now know that we
need to make cultural adaptations like these
to stay healthy.
But that's not all we've learned.
With the knowledge we now have about evolution,
we also know that skin color is a flexible trait that
has changed through time as various groups of people
moved to sunny or less sunny parts of the world.
And we know that skin color is inherited independently
of other traits, and is not associated
with other aspects of a person's appearance or behavior.
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Skin color is a product of evolution
and should never have been judged
as something good or bad.
We are a very clever and adaptable species.
And we are one under the Sun.
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