The Biology of Skin Color — HHMI BioInteractive Video


[MUSIC PLAYING] 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. [MUSIC PLAYING] Biological anthropologists
like myself spend our lives studying how humans evolved
and why we differ from one another physically. [MUSIC PLAYING] 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. [MUSIC PLAYING] 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. [MUSIC PLAYING] 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. [MUSIC PLAYING] 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. [MUSIC PLAYING] 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. [MUSIC PLAYING] 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. [MUSIC PLAYING] 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. [MUSIC PLAYING] 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. [MUSIC PLAYING] 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. [MUSIC PLAYING] 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. [MUSIC PLAYING] 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. [MUSIC PLAYING]