Skin color is
due primarily to the presence of a pigment called melanin
,
which is controlled by at least 6 genes. Both light and dark complexioned
people have melanin. However, two forms are produced--pheomelanin
,
which is red to yellow in color, and eumelanin
,
which is dark brown to black. People with light complexioned skin mostly
produce pheomelanin, while those with dark colored skin mostly produce
eumelanin. In addition, individuals differ in the number and size of
melanin particles. The latter two variables are more
important in determining skin color than the percentages of the different
kinds of melanin. In lighter skin, color is also affected by red cells in blood
flowing close to the skin. To a lesser extent, the color is affected by the presence
of fat under the skin and carotene
,
a reddish-orange pigment in the skin. Hair color is
also due to the presence of melanin.
Melanin is
normally located in the epidermis
,
or outer skin layer. It is produced at the base of the epidermis by specialized
cells called melanocytes
.
These cells have photosensitive receptors, similar to those in the eye, that
detect ultraviolet
radiation
from the sun and other sources. In response, they
produce melanin within a few hours of exposure.
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Cross section of
human skin
(colors are not true to life in
this illustration) |
Nature has selected for people with darker skin in tropical latitudes,
especially in nonforested regions, where ultraviolet
radiation
from the sun is usually the most intense.
Melanin acts as a protective
biological shield against ultraviolet radiation. By doing this, it helps to prevent sunburn damage that could
result in DNA changes and, subsequently, several kinds of
malignant skin cancers. Melanoma
in particular is a serious
threat to life. In the United States, approximately 54,000 people get this
aggressive type of skin cancer every year and
nearly 8,000 of them die from it. Those at highest risk
are European Americans. They have a 10 times higher risk than African
Americans.
Ultraviolet radiation
reaching the earth usually increases in summer and decreases in winter. The skin's
ability to tan in summertime is an acclimatization to this seasonal change. Tanning
is primarily an increase in the number and size of melanin granules due to the stimulation
of ultraviolet radiation.
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Irish boy who
is essentially
unable to tan |
While skin tanning is often
most noticeable on light complexioned people, even those with very dark brown skin
can tan as
a result of prolonged exposure to the sun. Some
Northwest Europeans have
substantially
lost the ability to tan as a result of relaxed natural selection. Their skin burns and peels rather than tans.
This is due to the fact that they produce a defective form of a skin protein
Mc1r (melanocortin-1 receptor) which is necessary for the production of
melanin. They are
at a distinct disadvantage in tropical and subtropical environments. Not only
do they suffer the discomfort of readily burning, but they are at a much higher risk for
skin cancer. The same is true of
albinos.
It would be harmful if
melanin acted as a complete shield. A certain amount of
shortwave ultraviolet radiation (UVB) must
penetrate the outer skin layer in order for the body to produce
vitamin D.
Approximately 90% of this vitamin in people normally is
synthesized in their skin and the kidneys from a cholesterol-like precursor
chemical with the help of ultraviolet radiation.
The
remaining 10% comes from foods such as fatty fish and egg
yolks. Simple vitamin D is
converted by our bodies into two sequential forms. The last form, commonly referred to as vitamin D3, is needed for the intestines to absorb calcium and
phosphorus from food for bone growth and repair. Calcium is also
necessary in adults to maintain normal heart action, blood clotting, and a
stable nervous system. New evidence suggests that
vitamin D may help prevent a wide range of cancers, including those of the
colon and breasts. Vitamin D plays an additional
important role in promoting the production of cathelicidin, which
helps to defend our bodies against fungal, bacterial, and viral infections,
including the common flu.
Too much
ultraviolet radiation penetrating the skin may cause the breakdown of folate in the body, which can cause anemia.
Folate is derived from folic acid (one of the B vitamins)
in our food. Pregnant women who are deficient in
folate are at a higher risk of having
miscarriages and
babies with neural tube defects.
Because folate is needed for DNA replication in dividing cells, its
absence can have an effect on many body processes, including the production of
sperm cells. It may be that the ability to produce melanin
was selected for in our early human ancestors because it helped preserve the
body's supply of folate in addition to reducing the chances of developing
skin cancer.
People who live in far
northern latitudes, where solar radiation is relatively weak most of the year, have an
advantage if their skin has little shielding pigmentation. Nature selects for less
melanin when ultraviolet radiation is weak. In such an environment, very dark skin
is a disadvantage because it can prevent people from producing enough vitamin
D,
potentially resulting in rickets disease
in children and osteoporosis in adults.
Contributing to the development of osteoporosis in older people is the fact
that their skin generally loses some of its ability to produce vitamin D. Women who had prolonged vitamin D deficiencies
as girls have a higher incidence of pelvic deformities that prevent normal
delivery of babies.
The Inuit
people of the American Subarctic are an exception. They have moderately
heavy skin pigmentation despite the far northern latitude at which they live. While
this is a disadvantage for vitamin D
production, they apparently made up for
it by eating fish and sea mammal blubber that are high in
vitamin D.
In addition, the Inuit have been in the far north for only about 5,000 years.
This may not have been enough time for significantly lower melanin production
to have been selected for by nature.
In the United
States and other developed nations, milk is now
usually fortified with vitamins D and A in order to prevent
developmental problems such as those described above.
However, the popularity of carbonated soft drinks and other alternatives to milk along with a decrease in the amount of time spent outdoors has led to a considerable rise in the rate of rickets disease.
Not surprisingly, vitamin D deficiency is most acute in the winter in
temperate and colder zones, especially among people of
African ancestry. There is a growing
epidemic of vitamin D deficiency in the United States. The 2007-2008
"National Government Health and Nutrition Examination Survey" found that
only 23% of teens and adults had a sufficient amount of this essential
vitamin in their blood. It was 45% a decade earlier. The rate for
African Americans in particular was far worse. It has dropped from 12%
to only 3% having what doctors consider to be an adequate amount of vitamin
D for overall good health. Given this difference, it is not surprising
that 15% African American women have pelvis deformities, while the rate
among European Americans is only 2%.
There is also a strong
correlation between the amount of sunlight that children are exposed to and
whether or not they will develop
multiple sclerosis as adults. Most cases of this degenerative neural
disorder are in the temperate regions of the world where the sunlight is
rarely intense. Children growing up in tropical and subtropical regions
rarely develop MS regardless of where their ancestors came from.
This protection apparently continues for those who move to far northern or far
southern regions after 16 years of age. What processes are responsible
for this protection from MS and its possible relationship to skin color are
unknown.
New
research by Nina Jablonski and George Chaplin has led to the discovery that
women generally produce 3-4% less melanin in their skin than do men in all
populations of the world. They suggest that this is probably due to the
fact that women have far higher calcium requirements during their reproductive
years. Mate selection preference and other cultural practices may also
be partly responsible for this gender difference in skin coloration.
Nina Jablonski breaks the illusion of skin color--explanation
of why humans have a wide variation
in skin color around the world.
This link takes you to an external website.
To return here, you
must click the "back" button on your browser program.
(length = 14 mins,
46 secs) |
Skin Color
Distribution Around the World
Before the mass global
migrations of people during the last 500 years, dark skin color was mostly concentrated
in the southern hemisphere near the equator and light color
progressively increased farther away, as illustrated in the map below.
In fact, the majority of dark pigmented people lived within 20º
of the equator. Most of the lighter pigmented people lived in the
northern hemisphere north of 20º latitude,
where ultraviolet radiation is much less intense on average.
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|
(Data for
native populations collected by R.
Biasutti prior to 1940.) |
Such a non-random
distribution pattern of human skin color was predicted by
Constantin
Wilhelm
Lambert Gloger, a 19th century
German zoologist. In 1833, he observed that
heavily pigmented animals are to be found
mostly in hot climates where there is intense sunshine. Conversely, those in cold
climates closer to the poles commonly have light pigmentation.
Presumably, the relative
intensity of solar radiation is largely responsible for this distribution pattern.
There are exceptions to
Gloger's rule
in the animal kingdom.
In
some species the survival value of having a camouflaged body
can be more important than the selective pressures of ultraviolet radiation.
This is the case with arctic hares, which are white like snow in the winter
and mottled black, brown, and gray similar to the rocky ground in their
region after the snow melts in the summer. These color changes make it
more difficult for arctic foxes to prey on them. Among
humans, mate selection preferences may counter some of the evolutionary trend in skin
color predicted by Gloger. Ultraviolet radiation is
also blocked to some degree by hair, clothing, smog, fog, smoke, clouds, and
trees. The Inuit case described earlier suggests
that diet may additionally be a significant factor in some societies.
NEWS: In the February
2007 issue of the Journal of Nutrition, a research team led by Lisa
Bodnar of the University of Pittsburgh School of Public Health reported that
among 400 first-time pregnant women from that city who participated in their
study, only 16.1% of African Americans and 52.9% of European Americans had
sufficient amounts of vitamin D in their blood to be considered healthy at
the end of their pregnancies. This was despite the fact that more than
90% of them had regularly taken standard doses of prenatal vitamin
supplements. Tests of umbilical cord blood indicated that only 7.6% of
the African American infants and 33.9% of the European American ones
had adequate amounts of vitamin D at birth. The authors concluded that
prenatal vitamins should include higher doses of vitamin D, especially in
the northern regions of the U.S.
