>But I do remember from the
>lecture (mentioned above) that it had to do with the differences in pigment
>quality or quantity (I'm not sure, but I think the latter) in different
>skin-types that are responsible for the ability to deal with a certain
intensity
>of Ultra Violet radiation in relation to the vit. D synthesis.
As a physical anthropologist, this is one of the topics I lecture about in
depth to my college students in several different classes, including
Introduction to Physical Anthropology, Human Variation and Adaptation, etc.
People differ in the amount of melanin (dark pigment) their bodies produce
as a baseline level (the color on the inside of your upper arm), as well as
in the additional melanin they produce in response to exposure to UV
radiation (tanning). There are at least 5-6 different genes that contribute
to baseline skin color, and several more that contribute to tanning ability.
It is a finely tuned system that, as a result of natural selection over
hundreds of thousands of years, matches one's skin color to the amount of
ultraviolet A and B radiation you are ordinarily exposed to.
In areas of high ultraviolet radiation year round (near the Equator), it is
best to be dark-skinned, as the melanin in the skin filters out the UVA and
UVB which can cause damage of several kinds -- sunburn, skin cancer, and
most important, destruction of the circulating folic acid in your
bloodstream. If you have too little pigment, so much of your folic acid can
be destroyed that you become deficient, and deficiencies in folic acid can
lead to neural tube defects, which in turn can cause miscarriages and/or
early deaths of children born with such neural tube defects. Despite the
dark skin, people who live in areas of high UVA and UVB get enough sunlight
to produce the proper amount of Vitamin D, necessary for absorbing calcium
from the diet to make strong bones and prevent rickets.
In areas of low ultraviolet radiation year round (far from the Equator,
especially in the north), it is best to be light-skinned, as folic acid
destruction is much less likely, and you need to absorb as much of the
meager sunlight as you can in order to make sufficient Vitamin D, so you
don't get rickets. Rickets affects reproductive success directly, because
is you have rickets as a child, you can grow up with a deformed pelvis, and
then both you and child may die during childbirth (we are speaking in
evolutionary terms -- obviously in modern medical settings, you can have a
C-section).
In areas with moderate levels of UV radiation, which fluctuate across the
seasons, it is best to have intermediate levels of pigmentation and have a
good tanning ability, so that one can temporarily have darker-skin when
needed in the summer. This way, you have enough melanin to protect your
folic acid, but also let in enough sunlight to make appropriate amounts of
Vitamin D.
People in the last several hundred years have moved around all over the
planet, thus moving out of the zones where their ancestors evolved for
thousands and thousands of years. Thus you can get into trouble with
rickets if you are dark-skinned and live where there is little sunlight
(African-Americans in Detroit and Buffalo, South Indians in London, etc.).
Likewise, you can get into trouble with folic acid deficiency if you are
light-skinned and live where there is intense UV radiation (read early
accounts of English, French, German, and Belgian colonial administrators'
wives in Africa, who report miscarriage after miscarriage -- some due to
malaria, no doubt, but some also due to folic acid deficiency). Of course,
you can also get into trouble with skin cancer if you are light-skinned and
live where there is intense UV radiation, but that doesn't usually affect
reproduction.
Kathy Dettwyler
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