My first-year textbook has this to say:
"Skin colour is influenced by three substances: hemoglobin, carotene, and
most important, the pigment melanin. Melanin is a granular substance
produced by specialized cells in the epidermis called melanocytes. all
humans appear to have approximately the same number of melanocytes. It is
the amount of melanin and the size of melanin granules that vary.
Melanin has the capacity to absorb potentially dangerous UV rays that are
present . . . in sunlight. Because of this ability, melanin provides
protection from overexposure to UV radiation, which can cause . . . skin
cancer . . .
Natural selection appears to have favoured dark skin in the areas nearest
the equator, where the sun's rays are most direct and thus, where exposure
to UV light is most intense. In considering the potentially harmful
effects of UV radiation from an evolutionary perspective, three points must
be kept in mind. (1) Early hominids lived mostly in the tropics, where
solar radiation is more intense than in temperate areas like northwestern
Europe; (2) unlike modern city dwellers, most earlier hominids spent the
majority of time outdoors; (3) early hominids did not wear clothing that
would have provided some protection from the sun. Given these conditions,
UV radiation could indeed have been a powerful agent selecting for optimum
levels of melanin production in early human ancestors.
As hominids migrated out of Africa into Europe and Asia, selective
pressures changed. In particular, those populations that eventually
occupied northern Europe encountered cold temperatures and cloudy skies,
sometimes during the summer as well as in winter. Winter also meant fewer
hours of daylight, and with the sun well to the south, solar radiation was
indirect. Moreover, since physiological adaptations were not sufficient to
meet the demands of living in colder climates, these populations had
undoubtedly adopted certain cultural practices, such as using fire and
wearing animal skins and other kinds of clothing. As a consequence of
reduced exposure to sunlight, the advantages provided by deeply pigmented
skin in the tropics were no longer important, and selection for melanin
production may have been relaxed (Brace and Montagu, 1977).
However, relaxed selection for dark skin is probably not [alone] adequate
to explain the very depigmented skin seen in some northern Europeans. More
than likely, another factor, the need for adequate amounts of vitamin D,
was also critical. The theory concerning the possible role of vitamin D,
known as the *vitamin D hypothesis*, offers the following explanation.
Vitamin D plays a vital role in the mineralization and normal growth of
bone during infancy and childhood and is available in a few foods,
including fish oils, egg yolk, butter, cream, and liver. But the body's
primary source of vitamin D is its own ability to synthesize it through the
interaction of UV light and a cholesterol-like substance found in skin
cells. Therefore, if normal bone growth is to occur, adequate exposure to
sunlight is essential. Insufficient amounts of vitamin D during childhood
result in rickets, which leads to bone deformities throughout the skeleton,
especially the weight-bearing bones of the lower extremity and pelvis.
Thus, people with rickets frequently have bowed legs and pelvic
deformities. Pelvic deformities are of particular concern for women, for
they can lead to a narrowing of the birth canal, which, in the absence of
surgical intervention, frequently results in the death of both mother and
fetus during childbirth. This example illustrates the potential for
rickets as a significant selective factor favouring less pigmented skin in
regions where climate and other factors operate to reduce exposure to UV
radiation."
-"Introduction to Physical Anthropology, 7th ed.", by Robert Jurmain, Harry
Nelson, Lynn Kilgore, and Wenda Trevathan. pp. 140-141.
The text offers further substantiation of the vitamin D hypothesis, but
it's probably redundant for our purposes. In any case, there would seem to
be at least a great possibility of accounting for skin colour in this way.
Regards,
Mike Hardie
<hardie@globalserve.net>
http://www.globalserve.net/~hardie/dv/