Darwin's theory of natural selection evolves

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.