Human Biological Adaptability: Skin Color as an Adaptation
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 20� of the equator.  Most of the lighter pigmented people lived in the   northern hemisphere north of 20� latitude,   where ultraviolet radiation is much less intense on average.
  (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.