Ultraviolet rays do two things to you:
cause skin cancer (as you probably already know),
but they also make vitamin D from other
in your skin. A few minutes of direct, full-body
sunshine every day will give the average
light-skinned person enough vitamin D to survive.
After that, any more UV rays will do more harm
So, the body needs a balancing
mechanism which will keep UV rays to a minimum
while still letting enough through to make the
vitamin D. This mechanism is the skin pigment
melanin, which darkens the skin and blocks UV
rays. Long, frequent exposure to the sun triggers
increased production of melanin, because the body
figures it is getting plenty of vitamin D, so it
might as well avoid getting skin cancer too.
Cover up, and your body reduces the pigment
because it figures there is a greater risk of
rickets (caused by not getting enough vitamin D).
This mechanism doesn't work as well as it
could- it has long response times, it can't
drastically change someone's skin color, and it is
tied to sunlight instead of vitamin D levels.
Ideally we would change color like chameleons in
response to our vitamin D levels, but that
mechanism hasn't evolved yet. So, to cope with
varying degrees of sunlight around the world,
humans have evolved different base levels of
Africa and Australia have plenty of
sun- enough to give white people skin cancer.
Northern Europe is often cloudy, and people must
cover most of their bodies to stay warm, and the
sun circles closer to the horizon so the UV rays
must pass through more blocking ozone on their way
through the atmosphere.
Before the days of
vitamin D milk, Africans would have died of
rickets if they stayed in Finland too long. So,
each place has a different skin color for optimal
survival- people who are too dark or too light
gradually die off. If there aren't any major
migrations, evolution will cause skin color to
gradually converge to the locally optimal value
for each place.
If Santa Barbara were
isolated for thousands of years, the people would
surely become naturally dark. In fact, if
everyone drinks vitamin D milk for the next few
thousand years, then everyone will have skin like
the Africans- because nobody would get rickets,
but light skin would still carry a risk of skin
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