People in Europe consumed milk thousands of years before they developed the genetic ability to tolerate it. This apparently contributed to the development of lactose tolerance, which is particularly widespread in Europe, as an international research team writes in the journal "Nature". Accordingly, times of crisis played an important role in the development of the ability to digest milk. Those who could do that were more likely to survive famine and disease.
Humans are the only mammal that continues to drink milk beyond infancy. This is made possible by a gene mutation that also allows adults to digest milk. About a third of people worldwide have this ability, in Europe it is even more than 85 percent. Without this mutation, milk sugar (lactose) cannot be broken down completely and can cause cramps, diarrhea and flatulence in the large intestine - symptoms of lactose intolerance, which affect around 15 percent of adults in Germany.
"In order to digest lactose, we need to produce the enzyme lactase in our gut," explains co-author George Davey Smith of the University of Bristol. Almost all infants produce lactase, but globally, most people's ability to do so declines rapidly between weaning and adolescence. "However, a genetic trait called lactase persistence has evolved multiple times over the past 10,000 years and spread to different milk-drinking populations in Europe, central and southern Asia, the Middle East and Africa," he explains.
To find out how this lactase persistence came about, Richard Evershed's team from Bristol created a database with information on milk residues on old potsherds. For this purpose, data on almost 7,000 animal fat residues on more than 13,000 ceramic fragments from 554 localities were analyzed.
The results suggest that milk consumption was widespread in Europe as early as 9000 years ago, but varied by region and time. The genetic mutation that leads to lactase persistence, on the other hand, first appeared about 5,000 years ago and was present in appreciable frequency 3,000 years ago. This is indicated by DNA sequences from more than 1700 prehistoric Europeans and Asians.
So, did lactose tolerance spread because more and more people were consuming milk? The modeling of genetic and archaeological data does not clearly confirm this connection. An examination of the genetic and medical data of more than 300,000 modern-day Europeans also revealed only minimal differences in milk consumption between people with and without lactase persistence.
Even lactose-intolerant people don't actually get sick from drinking milk, says Smith. They can get dehydrated in connection with diarrhea – but that is not fatal in healthy people. "However, if you are severely malnourished and have diarrhea, then you have life-threatening problems," explains the epidemiologist. "When their crops failed, prehistoric humans were more likely to consume unfermented milk with a high lactose content - exactly when they shouldn't." "
To test this, the researchers fed their statistical models with information about past famines and pathogen loads. Indeed, the lactase persistence gene variant appears to have been subject to greater natural selection at such times.
The authors write: "Our study shows that in later prehistory, as population and settlement size grew, human health was increasingly compromised by poor sanitation and increased incidence of diarrheal diseases, particularly of animal origin."
Under these conditions, consumption of milk would have led to an increase in mortality, with people without lactase persistence being particularly at risk. Famine, which increased disease and malnutrition, would have exacerbated this situation: "As a result, individuals who do not carry a copy of the lactase persistence gene variant are more likely to die before or during their reproductive years." the proportion of people with lactase persistence in the population has increased.
