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The evolution of race was as simple as the politics of race is complex
By Gina Kirchweger
Ten years ago, while at the university of Western Australia,
anthropologist Nina Jablonski was asked to give a lecture on human
skin. As an expert in primate evolution, she decided to discuss the
evolution of skin color, but when she went through the literature on
the subject she was dismayed. Some theories advanced before the 1970s
tended to be racist, and others were less than convincing. White skin,
for example, was reported to be more resistant to cold weather, although
groups like the Inuit are both dark and particularly resistant to cold.
After the 1970s, when researchers were presumably more aware of the
controversy such studies could kick up, there was very little work at
all. "It's one of these things everybody notices," Jablonski says, "but
nobody wants to talk about."
No longer. Jablonski and her husband, George Chaplin, a geographic
information systems specialist, have formulated the first comprehensive
theory of skin color. Their findings, published in a recent issue of the
Journal of Human Evolution, show a strong, somewhat predictable
correlation between skin color and the strength of sunlight across the
globe. But they also show a deeper, more surprising process at work:
Skin color, they say, is largely a matter of vitamins.
Jablonski, now chairman of the anthropology department at the
California Academy of Sciences, begins by assuming that our earliest
ancestors had fair skin just like chimpanzees, our closest biological
relatives. Between 4.5 million and 2 million years ago, early humans
moved from the rain forest and onto the East African savanna. Once on
the savanna, they not only had to cope with more exposure to the sun,
but they also had to work harder to gather food. Mammalian brains are
particularly vulnerable to overheating: A change of only five or six
degrees can cause a heatstroke. So our ancestors had to develop a
better cooling system.
The answer was sweat, which dissipates heat through evaporation.
Early humans probably had few sweat glands, like chimpanzees, and those
were mainly located on the palms of their hands and the bottoms of their
feet. Occasionally, however, individuals were born with more glands than
usual. The more they could sweat, the longer they could forage before the
heat forced them back into the shade. The more they could forage, the
better their chances of having healthy offspring and of passing on their
sweat glands to future generations.
A million years of natural selection later, each human has about
2 million sweat glands spread across his or her body. Human skin, being
less hairy than chimpanzee skin, "dries much quicker," says Adrienne
Zihlman, an anthropologist at the University of California at Santa Cruz.
"Just think how after a bath it takes much longer for wet hair to dry."
Hairless skin, however, is particularly vulnerable to damage from
sunlight. Scientists long assumed that humans evolved melanin, the main
determinant of skin color, to absorb or disperse ultraviolet light. But
what is it about ultraviolet light that melanin protects against? Some
researchers pointed to the threat of skin cancer. But cancer usually
develops late in life, after a person has already reproduced. Others
suggested that sunburned nipples would have hampered breast-feeding.
But a slight tan is enough to protect mothers against that problem.
During her preparation for the lecture in Australia, Jablonski
found a 1978 study that examined the effects of ultraviolet light on
folate, a member of the vitamin B complex. An hour of intense sunlight,
the study showed, is enough to cut folate levels in half if your skin
is light. Jablonski made the next, crucial connection only a few weeks
later. At a seminar on embryonic development, she heard that low folate
levels are correlated with neural-tube defects such as spina bifida and
anencephaly, in which infants are born without a full brain or spinal
cord.
Jablonski and Chaplin predicted the skin colors of indigenous
people across the globe based on how much ultraviolet light different
areas receive. Graphic by Matt Zang, adapted from the data of N.
Jablonski and G. Chaplin
Jablonski later came across three documented cases in which
children's neural-tube defects were linked to their mothers' visits
to tanning studios during early pregnancy. Moreover, she found that
folate is crucial to sperm development -- so much so that a folate
inhibitor was developed as a male contraceptive. ("It never got
anywhere," Jablonski says. "It was so effective that it knocked out
all folate in the body.") She now had some intriguing evidence that
folate might be the driving force behind the evolution of darker skin.
But why do some people have light skin?
As far back as the 1960s, the biochemist W. Farnsworth Loomis
had suggested that skin color is determined by the body's need for
vitamin D. The vitamin helps the body absorb calcium and deposit it
in bones, an essential function, particularly in fast-growing embryos.
(The need for vitamin D during pregnancy may explain why women around
the globe tend to have lighter skin than men.) Unlike folate, vitamin D
depends on ultraviolet light for its production in the body. Loomis
believed that people who live in the north, where daylight is weakest,
evolved fair skin to help absorb more ultraviolet light and that people
in the tropics evolved dark skin to block the light, keeping the body
from overdosing on vitamin D, which can be toxic at high concentrations.
By the time Jablonski did her research, Loomis's hypothesis
had been partially disproved. "You can never overdose on natural
amounts of vitamin D," Jablonski says. "There are only rare cases
where people take too many cod-liver supplements." But Loomis's
insight about fair skin held up, and it made a perfect complement
for Jablonski's insight about folate and dark skin. The next step
was to find some hard data correlating skin color to light levels.
Until the 1980s, researchers could only estimate how much
ultraviolet radiation reaches Earth's surface. But in 1978, NASA
launched the Total Ozone Mapping Spectrometer. Three years ago,
Jablonski and Chaplin took the spectrometer's global ultraviolet
measurements and compared them with published data on skin color in
indigenous populations from more than 50 countries. To their delight,
there was an unmistakable correlation: The weaker the ultraviolet
light, the fairer the skin. Jablonski went on to show that people
living above 50 degrees latitude have the highest risk of vitamin D
deficiency. "This was one of the last barriers in the history of human
settlement," Jablonski says. "Only after humans learned fishing, and
therefore had access to food rich in vitamin D, could they settle
these regions."
Humans have spent most of their history moving around. To do
that, they've had to adapt their tools, clothes, housing, and
eating habits to each new climate and landscape. But Jablonski's
work indicates that our adaptations go much further. People in the
tropics have developed dark skin to block out the sun and protect
their body's folate reserves. People far from the equator have
developed fair skin to drink in the sun and produce adequate amounts
of vitamin D during the long winter months.
Jablonski hopes that her research will alert people to the
importance of vitamin D and folate in their diet. It's already
known, for example, that dark-skinned people who move to cloudy
climes can develop conditions such as rickets from vitamin D
deficiencies. More important, Jablonski hopes her work will begin
to change the way people think about skin color. "We can take a
topic that has caused so much disagreement, so much suffering,
and so much misunderstanding," she says, "and completely disarm it."
(From Discover, Vol. 22, No. 2, February, 2001. Gina Kirchweger © 2001. Reprinted with permission of Discover. )
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