Prehistoric tolerance to the milk sugar lactose is likely to have evolved due to the impact of famine and infectious diseases rather than a high-dairy diet, according to a study published in the journal Nature.
Researchers mapped milk use in Europe over the past 9,000 years by combining ancient and modern DNA analysis with studies of animal fat residue from thousands of pottery fragments at hundreds of archaeological sites.
This allowed them to trace the development of a genetic trait called lactase persistence, in which people retain production of the gut enzyme lactase that is found in almost everyone as babies, thereby allowing them to digest lactose.
The team found that most early Neolithic people did not have lactase persistence and that it only became appreciably present in the Bronze and Iron Ages, suggesting evolutionary selection of the trait.
“The lactase persistence genetic variant was pushed to high frequency by some sort of turbocharged natural selection. The problem is, such strong natural selection is hard to explain,” said researcher Mark Thomas, professor of evolutionary genetics at University College London, UK.
Analysis of 6,899 samples of animal fat residues from broken pottery at 554 archaeological sites revealed that milk was widely used in Europe from the Neolithic period onwards. However, its consumption varied across regions and throughout history, suggesting fluctuating local food production and cultural changes in dietary preferences.
When the researchers studied DNA from half a million modern Europeans, they found that genetic lactase persistence was also only weakly associated with milk consumption. Lactase persistence was neither consistently linked with health nor fitness, suggesting other factors were responsible for its rapid rise and that its absence did not lead to significant ill effects.
Nonetheless, study co-author Prof. George Davey Smith, director of the MRC Integrative Epidemiology Unit at the University of Bristol, UK, pointed out that drinking milk leads to high intestinal lactose concentrations in people who are not lactase persistent. This can draw fluid into the colon, and dehydration can result when this is combined with diarrheal disease.
“If you are healthy and lactase non-persistent, and you drink lots of milk, you may experience some discomfort, but you not going to die of it,” he added. “However, if you are severely malnourished and have diarrhea, then you’ve got life-threatening problems. When their crops failed, prehistoric people would have been more likely to consume unfermented high-lactose milk – exactly when they shouldn’t.”
Indeed, the researchers found that selection of the lactase persistence variant was driven by famine and pathogen exposure.
The authors conclude: “Our study demonstrates how, in later prehistory, as populations and settlement sizes grew, human health would have been increasingly impacted by poor sanitation and increasing diarrheal diseases, especially those of animal origin. Under these conditions consuming milk would have resulted in increasing death rates, with individuals lacking lactase persistence being especially vulnerable. This situation would have been further exacerbated under famine conditions, when disease and malnutrition rates are increased.
“This would lead to individuals who did not carry a copy of the lactase persistence gene variant being more likely to die before or during their reproductive years, which would push the population prevalence of lactase persistence up.
“It seems the same factors that influence human mortality today drove the evolution of this amazing gene through prehistory.”