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Dairy farmers living in Central Europe around 7,500 years ago may have been the first human adults to drink cow's milk—at least comfortably.
Integrating genetic and archaeological data, Mark Thomas and colleagues at University College London were able to trace down the first evidence of lactase—the enzyme that allows us to digest the complex milk sugar lactose—persisting beyond the weaning years into adulthood to "exactly when you see the beginning of Linearbandkeramik culture [considered the first Neolithic society in Europe]," Thomas says. "When that started, you saw a change from a mixed economy to one based primarily on cattle." And, with this revolution, came a strong evolutionary advantage for people able to consume milk and its nutrients without digestive discomfort.
Before the evolution of lactase persistence, humans typically lost their ability to digest lactose around the age of five. (This is thought to have helped motivate weaning.) Still today, most of the world's population can only tolerate milk for the first few years of life. But, through at least four parallel evolutions starting several thousand years ago, lactase persistence spread throughout human populations. One of these, the earliest, is known to have originated in Europe.
The genetic mutation conferring this advantage—shared by most lactose tolerant Europeans—was commonly thought to have occurred first in the northern part of the continent, where the sun shines less and people may be in greater need of the vitamin D found in cow's milk. (Sunlight is human's main source of vitamin D, which is necessary for the body's uptake of calcium.) But Thomas's new research published today in the journal PLoS Computational Biology begs to differ.
Thomas and his colleagues note the trait started farther south before spreading to the north, according to the results of their computer simulation model. "I suspect there are two important factors [triggering the evolution of lactase persistence]: consistency in supply and contaminated fluids," Thomas says.
Pioneer farmers made their way north with domesticated crops from the Near East, he explains, but these crops were not necessarily well suited for the new environment. So, as the pioneers found themselves isolated with only feeble crops and cattle as well as parasite-ridden water sources, cow's milk may have become an increasingly important staple for survival. "Seasonal crops are boom and bust, but cattle provide food even when crops are failing," Thomas says. "The only problem is you must be able to drink it." Those on the brink of starvation, he notes, would not have been able to survive the diarrhea that lactose intolerance brings.
Of course, there are other (less probable) ideas still being milked: Thomas points out one example—that tolerance evolved as "prestige" via "Neolithic drinking games": men battling it out to drink a quantity of milk without getting sick. "Like football players, maybe they screamed at each other while downing one," Thomas says. "Then some guy comes along, doesn't get sick, and everyone thinks he's cool." One has to wonder, did the macho milk drinker then rub it in his opponents' faces by chanting, "Got milk?"