Perhaps they stopped making unnecessary melanin in order to conserve energy. Or, people with lighter skin may have had a fitness advantage because they were more efficient at harnessing the weaker sunlight of northern climes to make vitamin D. Ongoing studies are searching for evidence that could settle the question.
Humans are continuing to evolve in response to diseases, diet, climate and other factors. But technological advances have made natural selection "a much less potent force on us in the present than it was in the past," said Noah Rosenberg, a human geneticist at the University of Michigan.
Today, lactose-intolerant kids can compensate by drinking soy milk and eating a variety of readily available nutritious foods. People deficient in vitamin D can take a supplement.
Modern medicines also may have reduced the pressure for the gene pool to create and spread mutations that would protect against new diseases.
But without a time machine, all science can do is make an educated guess as to where the human genome is heading, said Jerry Coyne, an evolutionary biologist at the University of Chicago:
"There are some things we're never going to know."
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karen.kaplan@latimes.com
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We're still evolving
Humans may like to think of themselves as a fully evolved species, but natural selection is continuing to shape our gene pool. Anytime there is a mismatch between our DNA and our environment, favorable mutations that fill the gap are bound to spread. Here are some examples:
DISEASE
Mutations in about two dozen genes have proved useful in fighting malaria. Most of those mutations are concentrated in people of African descent, since the disease is most widespread on that continent.
DIET
Changes in a gene called LCT allowed adults to continue producing an enzyme crucial for metabolizing milk -- giving carriers of the gene a distinct dietary advantage. The gene variant spread rapidly among societies that herded cattle, sheep and goats.
CLIMATE
As humans migrated out of Africa into northern latitudes, mutations for lighter skin spread rapidly. Scientists aren't sure why; the leading theory is that pale skin makes it easier to synthesize vitamin D in regions with less sunlight.
Los Angeles Times
