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Copyright 2001, all rights reserved
WONDER QUEST with April Holladay, A Weekly Column * August 15, 2001* Albuquerque
Skin, eye, and hair color may have evolved as beauty traits
Q: I would like to know why human skin color evolved to lighter shades as the early humans migrated northward out of
Africa-- Bill F.
A: The genetics of skin color is complex, says Luca Cavalli-Sforza in Genes, Peoples, and Languages. It invo lves at least four
genes and has not been fully worked out so tracing its history is difficult.
Right: [SARC, Univ of Oslo] Prehistoric hunt.
By the way, some people disagree with your out-of-Africa statement. Another theory on how humans populated the globe holds
that, in many regions, we evolved into modern humans at the same time.
We clearly inherit such traits as skin color, eye color, and hair color. If I, a blue-eyed woman, marry a blue-eyed man, then our
children will be blue eyed. It's easy to see how a region can end up populated with blue-eyed people.
But what's the competitive advantage to have blue or brown eyes-or white or black skin? Let's consider skin first. People have considered several theories,
including the ultraviolet (UV) theory, Vitamin D, and skin cancer. It seems reasonable on the face of it: dark skin predominates in equatorial Africa and
elsewhere where the Sun's rays are direct. The dark skin protects against skin cancer and ensures you don't produce too much Vitamin D. Whereas, up in
Sweden where the Sun's rays slant in and skin cancer is a slight threat, people's skin is light to let in the UV and stimulate more production of Vitamin D.
Jared Diamond (1999 winner of the Medal of Science award and UCLA evolutionary biologist) points out flaws in these theories.
"Among tropical peoples," he writes "anthropologists love to stress the dark skins of African blacks, people of the southern Indian peninsula, and New
Guineans and love to forget the pale skins of Amazonian Indians and Southeast Asians living at the same latitudes." [Emphasis mine.]
He notes that dark peoples of equatorial West Africa and the New Guinea mountains get no more UV radiation than the light-skinned folk in Switzerland, if
you take cloud cover into account.
Diamond brings up Darwin and advances another theory. Nearly 125 years ago the discover of natural selection dismissed natural selection as an explanation of
geographic variation of beauty traits, such as skin color.
Traits vary geographically because of
- Natural selection
- Sexual selection-meaning those traits that serve as arbitrary signals by which individuals of one sex attract mates of the opposite sex while intimidating
rivals-like the mane of a mature male lion.
- No particular reason, mutations that happened to arise and spread in one area. Neutral in effect--perhaps as much as 90% of DNA evolution is this kind.
Diamond eliminates natural selection as a contender for the cause of variation in skin color. How about sexual selection?
"...there's an overwhelming importance to skin, eye, and hair color that is obvious to all of us-[in] sexual selection," Diamond says. "Before we can reach a
condition of intimacy permitting us to assess the beauty of a prospective sex partner's hidden physical attractions, we first have to pass muster for skin, eyes,
and hair."
"Even the briefest personal ad in a newspaper mentions the advertiser's skin color, and the color of skin that he or she seeks in a partner," he points out. Sexual
selection may well account for how skin color changed as people populated the globe.
Charles E. Taylor, UCLA evolutionary biologist, thinks so, too. "Diamond argues for sexual selection because nothing else seems to fit," Taylor says. "This is
a cop-out, of course, but it makes sense to me."
But, honestly, we don't understand skin-color variation yet. It could be sexual, environmental, or random. The debates still rage.
"It is not impossible that white skin color originated in Northern Africa," says Taylor.
(Answered by April Holladay, science correspondent, August 15, 2001)
Further Surfing:
Discover: Race without color
Anthro: Eye-color calculator
Arizona State U: Becoming human
PBS: Human evolution connections
Washington State U: Human prehistory
Books:
Genes, Peoples, and Languages by Luca Cavalli-Sforza, March 2000.
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