Ancient Europeans may have developed the ability to digest milk due to periodic famines and disease.
Europeans’ greed for drinking milk began around 9,000 years ago, when the first dairy herds reached the southern corner of the continent, researchers reported on July 27. nature. It wasn’t until several thousand years ago that a large number of Europeans developed the genes to digest lactose, which researchers say is the sugar in milk.
These findings — based on animal fat residue samples from hundreds of archaeological sites and DNA annotation data — undermine the influential idea that milk use increased dramatically as the product’s nutritional and health benefits drove the evolution of lactose tolerance, say biogeochemist Richard Evershed of the University of Bristol in England and colleagues.
Milk drinkers who cannot digest lactose experience diarrhea, gas, bloating and intestinal cramps. Those emotional discomforts were too mild to trigger the development of lactose tolerance in his group, Evershed’s group says. However, due to periodic famines and outbreaks of infectious diseases, the lactose-induced flow became fatal, because the educated men in the rural communities were severely diminished. The most common threats tend to fuel the development of lactose intolerance.
Evershed’s report “comprehensively rules” widespread milk consumption as the evolutionary force behind lactose tolerance, says bioarchaeologist Oliver Craig of the University of York in England. Further research should clarify the scale and extent of famine or infectious disease episodes that influenced how early Europeans brewed milk, adds Craig, who was not involved in the new study. Researchers should also keep in mind that cheese and other dairy-based dairy products date as early as about 7,400 years ago in Europe (SN: 12/12/12). If these foods were widely available, it’s unclear why lactose-intolerant Europeans didn’t survive times of famine or disease, Craig says.
Evershed’s team estimated the frequency of milk use in Europe from about 9,000 to 500 years ago by publishing data from animal fat extracted from the remains of more than 13,000 pottery fragments from around 550 archaeological sites.
At the beginning of that time period, migrant farmers introduced dairy farms to Eastern Europe’s Balkan Peninsula, where regular residents embraced milk drinking, the researchers say. The consumption of milk then fluctuated in different parts of the continent. After about 7500 years, relatively heavy use of milk in western France, northern Europe and the British Isles. It is less common in central Europe.
The Eversion team also tracked the origin and spread of the main gene responsible for lactose tolerance using ancient DNA data published by nearly 1,800 Europeans and Asians. The first European evidence of a gene variant in adults responsible for boosting the activity of lactase, the enzyme that breaks down lactose and confers chemical tolerance, dates back to about 6,650 years ago, researchers say. However, these features, known as lactase persistence, did not become common in Europe until about 3,000 years ago.
Before that time, increasing levels of lactation continued to align with images of people linked to hunger in particular regions, the researchers report. Between 8,000 and 4,000 years ago, agricultural sites excavated across Europe show signs of periodic population declines that were affected by severe food shortages, researchers say (SN: 10/1/13).
Estimates of settlement density, a measure of how closely people lived, also tended to decline at times as density increased. The spread of animal-borne infections such as salmonella lowered the density of the compound and residents could not digest the excess lactose, leading to deaths, scientists suspect. In those periods of malnourishment and illness, the Evershed group speculates that lactase persistence is an approach to the poor nutrition in milk.
But archaeologist Ron Pinhasi of the University of Vienna is not convinced that the theory supports hunger and disease. Rainfall death is more frequent in malnourished children, he says, so he questions whether an adult-enough slaughter may have triggered the development of milk tolerance. No current proposal explains how lactase persistence occurs, he says.
In other parts of the world, and for equally mysterious reasons, regular milk consumption does not necessarily lead to the development of lactose tolerance. For example, lactose intolerance rarely occurs among milk-drinking Central Asian pastoralists, but biological signs of lactose tolerance often appear in East African Hadza hunter-gatherers who do not drink milk.
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