Smithsonian.com

Key lime pie ice cream cone, courtesy of Flickr user kern.justin

Anyone who enjoys ice cream can thank evolution. Just 10,000 years ago, no one past infancy could digest milk sugar, called lactose. Babies always made lactase, the enzyme that breaks down this sugar, but after weaning lactase production would stop.

Then along came livestock. Sometime in the past 10,000 years, several different populations—all raising cattle or camels in Northern Europe, East Africa and the Middle East—gained the ability to digest milk for life. Certain gene variants became prevalent that caused lactase production to continue into adulthood.

Lactose tolerance offered these populations a crucial advantage, says anthropologist Henry Harpending, co-author of a recent book called The 10,000-Year Explosion: How Civilization Accelerated Human Evolution. Before the gene variants arose, people had to remove the sugar from of cow or camel milk by fermenting it, but that eliminated between 20 to 50 percent of its calories.  With the ability to digest milk, humans could access this additional energy.

In The 10,000-Year Explosion, Harpending and co-author Gregory Cochran, both at the University of Utah, argue that the ability to digest lactose shaped human history. Lactose-tolerant populations, they claim, could better survive famines, and may also have been better conquerors, aiding the spread of their civilizations and cultures. “The European and maybe Arab expansions that whacked the Byzantine Empire may have been outcomes of this new ability to digest food,” Harpending said in an interview.

The ability to digest lactose is also evidence that humans are still evolving. In those 10,000 years, it arose independently in at least four places around the globe. Today, more than 90 percent of all people have some degree of lactose tolerance. How much tolerance people have depends on which gene variants and the number of copies of those genes they posses. About a third of the population digests lactose imperfectly and experiences some symptoms of lactose intolerance, and some people, mostly of African, Asian or Mediterranean descent, are not able to digest lactose at all.

The rapid selection for lactose tolerance raises an interesting question.  Were people who already had the gene variant motivated to domesticate animals, or were people who domesticated animals more likely to benefit from having a lactase-producing gene variant? “Which came first, the cattle or the mutation, you can’t tell,” Harpending says. “If the mutation had not occurred, there wouldn’t be so much dairying. But if people who could digest lactose didn’t have cattle, the mutation would have had no advantage.”

– Joseph Caputo