"Lactase persistence (LP), the dominant Mendelian trait conferring the ability to digest the milk sugar lactose in adults, has risen to high frequency in central and northern Europeans in the last 20,000 years. This trait is likely to have conferred a selective advantage in individuals who consume appreciable amounts of unfermented milk. Some have argued for the "culture-historical hypothesis," whereby LP alleles were rare until the advent of dairying early in the Neolithic but then rose rapidly in frequency under natural selection. Others favor the "reverse cause hypothesis," whereby dairying was adopted in populations with preadaptive high LP allele frequencies."
Weighing in on this debate, the authors of the above excerpt sequenced the lactase locus in the DNA samples of 9 different 7,500 year-old human skeletons from Eastern Europe. Although the skeletons date to well after the domestication of cattle, their latest results, just published in PNAS, show that these individuals did not possess the lactose tolerance allele. These findings therefore lend support to the hypothesis that the domestication of cattle was a driving force behind the evolution of lactose tolerance in human adults. Sadly, we still know very little about what drove the evolution of chocolate milk-producing cows.