More people, mutations seem to boost evolution

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 percent 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, it happened at least five times. Hawks and colleagues have recently discovered LCT variants that arose independently over the past 5,000 years among herders living in the Arabian Peninsula and sub-Saharan Africa.