More than a quarter-billion dairy cows help supply the demand for milk worldwide. Scientists had long thought the popularity of milk helped explain the evolution of the genetic feature allowing many people to digest lactose, the sugar found in milk, as adults. However, a new study published July 28 in the journal Nature suggests prehistoric people in Europe were drinking milk thousands of years before they evolved this trait. These new findings suggest the popularity may not have driven the rise of this variation—famine and disease may have instead.
HERE'S THE BACKGROUND — Dairying proved a useful advance because milk is highly nutritious, boosting the number of people one can support without slaughtering prized animals for meat. Although most European adults can drink milk without problems, two-thirds of the adults in the world today (and nearly all adults 5,000 years ago), can experience discomfort if they drink too much milk. The reason — lactose, which can lead to cramps, diarrhea, and flatulence for those that don’t have the proper enzymes to digest it.
Nearly all babies produce lactase, an enzyme found in the small intestine that breaks down lactose. For most people worldwide, lactase production declines rapidly between weaning and adolescence. However, a genetic trait known as lactase persistence, in which the body keeps producing lactase in adequate quantities into adulthood, lets about one-third of adults in the world drink milk without problems. This variation has evolved multiple times in the past 10,000 years and has spread to various groups in Europe, central and southern Asia, the Middle East, and Africa.
Previous research suggested milk consumption in ancient humans helped drive the evolution of lactase persistence. However, much about this idea stood untested because much remained unknown about when and where both milk drinking and lactase persistence first emerged.
WHAT DID THE SCIENTISTS DO? — To investigate the emergence of dairy farming, scientists examined 13,181 fragments of pottery from 554 archaeological sites. By analyzing 6,899 residues of animal fat on those artifacts, the researchers could reconstruct when and where prehistoric groups drank milk.
To see how dairy farming developed alongside evolving human genetics, the scientists assembled a database of the presence or absence of the lactase persistence genetic variant from ancient DNA sequences from 1,786 prehistoric European and Asian individuals.
"The extent of milk use does not help to explain patterns of natural selection for lactase persistence," study co-author Mark Thomas, an evolutionary geneticist at University College London, tells Inverse.
WHAT DID THEY FIND? — The new study found that milk was consumed extensively in European prehistory, dating from the earliest farming there from about 7000 BC, nearly 9,000 years ago. However, lactase persistence was not common until about 1000 BC, nearly 4,000 years after it was first detected about 4600 to 4700 BC.
In other words, milk consumption was common in Europe when prehistoric people there were still mostly lactose intolerant. This raised questions as to whether milk consumption is the key driver for lactase persistence.
Indeed, when the scientists analyzed UK Biobank genetic and medical data of more than 300,000 living individuals, they found only minimal differences in milk-drinking behavior between genetically lactase persistent and non-persistent people. Moreover, the large majority of lactase non-persistent individuals experienced no short or long-term negative health effects when they consumed milk. In addition, statistical models they developed suggested that changes in milk use over time would likely not prove an evolutionary force strong enough to drive the rise of lactase persistence.
"The extent of milk use does not help to explain patterns of natural selection for lactase persistence," study co-author Mark Thomas, an evolutionary geneticist at University College London, tells Inverse.
Still, these findings do suggest that some powerful evolutionary force likely did help make the lactase persistence trait widespread. The researchers suggested that famine or disease or both might have been these evolutionary factors. If lactase non-persistent people are healthy, drinking milk can lead to discomfort but not death. However, if they are malnourished or have a diarrheal disease or both, the dehydration they can experience after drinking milk could prove life-threatening.
As human populations grew in prehistory, the poor sanitation that would have proved common in early settlements would have proven risky to health. During such times, drinking milk may have increased death rates among people without lactase persistence. This may in turn have increased the prevalence of lactase persistence among the survivors.
WHAT'S NEXT? — "It would be great in the future, when the data is available, to test these models in Africa, Arabia, and Southern Asia, where lactase persistence levels are also high," Thomas says.
Future research can also apply the research team's statistical methods "to other genes that have been under strong natural selection, and even to traits that are shaped by many genes that have been under strong natural selection — for example, height," Thomas says. "This would help us better understand human evolution and why we have certain disease vulnerabilities today."
