From: ph111-bx@violet.berkeley.edu (;;;;7202)
Newsgroups: talk.origins,soc.culture.african.american,alt.rap
Subject: Re: White skin-how did it develope?
Date: 21 Aug 1992 05:22:22 GMT
Organization: University of California, Berkeley
NNTP-Posting-Host: violet.berkeley.edu

In article <1992Aug20.223509.7577@msuinfo.cl.msu.edu> writes:
>
>     I have thought over the evolution of skin color, and have run into the
>following questions:
>
>1) Following that the darker colors for humans are generally dominant vis-a-vis
>the lighter colors
>2) That the darker the skin color, the better the protection from UV radiation
>
>    Then what would be the evolutionary mechanism that would select for lighter
>skin towards the northern climates?  There's little I can see that would
>actively select against the darker skins/for the lighter skins, and I do not
>believe that a lack of evolutionary pressure is evolutionary pressure in, by,
>and of itself.

One theory relates skin color to ultraviolet light and its ability to
stimulate production of vitamin D in the deep layers of the skin.  In
tropical climes, dark skin evolved to filter out excessive ultraviolet
light and prevent hypervitaminosis, which forms calcium deposits in soft
tissues and can lead to fatal kidney dysfunction.  When people migrated
out of the warmer regions, selection favored those with less pigmentation
to facilitate greater absorption of the lesser amount of ultraviolet light
and keep up vitamin D production.  Insufficient vitamin D prevents enough
calcium from being absorbed in the intestines, causing the body to draw
it out of the bones for use by the nervous system.  This can cause the
symptoms of rickets:  deformed legs, spine, and pelvis, and, if it is
severe enough, death from a malfunctioning nervous system.

There are also other similar theories involving other light sensitive
vitamins.  Others have also found that a swarthy complexion is best
adapted to a hot, dry climate since a medium pigmentation averts
severe sunburn and does not absorb the heat that could lead to heat
stress and death.  Similarly, darkly pigmented skin is more susceptible
to frostbite, thus partially explaining the presence of light skin
colors in cold areas.  However, many other factors enter the picture,
such as heat loss by sweating, ionizing radiation, amount of body fat,
stature, and cultural factors.  The wide variation in Africa alone
indicates that skin color is not the only factor providing adaptation
for a climatic zone.

It has been suggested that the genes controlling melanin production may
have more evolutionary importance for some unknown effect than for
pigmentation itself; that is, selection may favor the genes that produce
dark pigmentation in equatorial regions not because dark pigmentation
is important but because some other effect of the same genes is beneficial
there.  This would fit with a proposal that heavy pigmentation is a
secondary effect of genes that provide increased resistance to disease.
As the tropics are conducive to growth of microbes, selection, it is
argued, would favor genes that provide resistance to disease.  Besides
providing for a powerful immune system, these genes also cause production
of a lot of melanin.