Re: The role of radioactivity in evolution

an.rasmussen@xxxxxxxxx wrote:
Tim Tyler wrote:

Radiation is the main reason people whose ancestors lived in Africa
look so different from northerners.

Thank you Tim Tyler,

This sounds interesting, what is the evidence that this is
indeed the case?

High levels of melanin in equatorial folk is near-universally
recognised as an adaptation to deal with the radition levels
there.

Albino skin cancer rates, suntans, vitamin D deficiency,
and the properties of melanin represent the main evidence
that this is the case - e.g.:

``The melanosomes in each recipient cell accumulate atop the
cell nucleus, where they protect the nuclear DNA from
mutations caused by the ionizing radiation of the sun's
ultraviolet rays. People whose ancestors lived for long
periods in the regions of the globe near the equator
generally have larger quantities of eumelanin in their
skins. This makes their skins brown or black and protects
them against high levels of exposure to the sun, which more
frequently results in melanomas in fairer-skinned people.''

- http://en.wikipedia.org/wiki/Melanin#Melanin_and_human_adaptation

Studies have been performed to see if genes have undergone
positive selection to produce this result - as opposed to
the theory that the result is due to a relaxation of the
selection favouring the genes for melanin production:

``The most recent scientific evidence indicates that all
humanity originated in Africa. It is most likely that the
first people had relatively large numbers of eumelanin
producing melanocytes and, accordingly, darker skin (as
displayed by the indigenous people of Africa, today). As
some of these original peoples migrated and settled in areas
of Asia and Europe, the selective pressure for eumelanin
production decreased in climates where radiation from the
sun was less intense. Thus variations in genes involved in
melanin production began to appear in the population,
resulting in lighter hair and skin in humans residing at
northern latitudes. Studies have been carried out to
determine whether these changes were due to genetic drift or
positive selection, perhaps driven by requirement for
vitamin D. Of the two common gene variants known to be
associated with pale human skin, Mc1r does not appear
to have undergone positive selection, while SLC24A5 has.''

- http://en.wikipedia.org/wiki/Melanin#Melanin_and_human_adaptation
--
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