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This is Scientific American's 60-Second Science. I'm Steve Mirsky. Got a minute?

"The ability to digest milk as adults, and as infants, actually, is due to the expression of an enzyme¹ called lactase." That's the University of Pennsylvania's Sarah Tishkoff at the AAAS conference in Washington DC on February 20th, talking about the recent evolution of lactose tolerance² in different human populations within the last 9,000 years. "Individuals who don't express this enzyme can't digest that complex sugar."

Can we put a number on the evolutionary³ importance of lactose tolerance? "The selective pressure is quite remarkable⁴. We actually estimated it to be about 10 percent. You're more likely to have 10 percent more offspring essentially⁵."

How strong is that? Look at population genetics to see how fast a trait will spread that develops in one individual in a population and that confers a 10 percent reproductive advantage. A mathematical analysis reveals that in just 100 generations, such a trait can be found in 95 percent of the individuals in the population.

That time period could be less than 2,000 years for humans. "I'm often asked the question, are humans still evolving? I would say the answer is absolutely yes."