The best-known example involves the gene that regulates a person's ability to make an enzyme required to digest lactose, the sugar in milk. Historically, the LCT gene shut down in early childhood as babies were weaned off breast milk. But after cows, sheep and goats were domesticated, people with a mutation that allowed them to drink milk as adults had a nutritional advantage that made it easier for them to propagate their genes.
DNA analyses have shown that the mutation cropped up in Europe about 8,000 years ago, and quickly spread all the way to India. Today, it is carried by more than 95% of people of Northern European descent.
A 2007 study bolsters the theory that the rise of pastoralism prompted the gene's spread. Using new techniques to analyze ancient DNA, German and British researchers checked the genes of eight European farmers who lived 7,000 to 7,800 years ago, before the widespread adoption of a herding lifestyle. None of those early farmers had the mutation for lactose tolerance.
The adaptation was so important that it happened at least five times. Hawks and colleagues have recently discovered LCT variants that arose independently over the last 5,000 years among herders living in the Arabian Peninsula and sub-Saharan Africa.
The human genome is still adapting to our relatively new agricultural diet, based on starches and sugars.
Type 2 diabetes may be one of the consequences. Scientists have compared the genetic profiles of diabetes patients with those of healthy controls and found some recently spreading genes that seem to protect against diabetes by affecting the body's ability to digest starches. That may explain why Native Americans, who came to farming relatively recently, have a higher risk of diabetes, Hawks said.
The usefulness of blue eyes is far less clear. In his 1871 book "The Descent of Man, and Selection in Relation to Sex," Darwin proposed that blue eyes spread among Europeans simply because they were sexually desirable.
Some scientists find that theory plausible. Others propose that blue eyes are a side effect of some other trait that is evolutionarily useful -- though as yet unidentified.
Pale skin is a leading contender. The earliest humans in Africa had dark skin to protect against the damaging effects of solar radiation. But as people migrated farther from the equator, the melanin required to make their skin dark became less necessary.
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."
(BEGIN TEXT OF INFOBOX)
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:
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.
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.
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