Milk consumption was prevalent in prehistoric people in Europe, thousands of years before humans developed the genetic trait that allowed them to digest the milk sugar lactose as adults. In other words, prehistoric people in Europe consumed milk even when they were lactose intolerant. When humans evolved a particular genetic trait that allowed them to digest lactose as adults, they were no longer lactose intolerant.
According to a new study published in the journal Nature, famine and exposure to infectious disease best explains the evolution of the ability of humans to consume milk and other non-fermented dairy products. This contradicts the belief that lactose tolerance emerged because it allowed people to consume more milk and dairy products.
Researchers Mapped Patterns Of Milk Consumption Over Last 9,000 Years
Researchers from the University of Bristol and University College London, who led the study, mapped prehistoric patterns of milk consumption over the last 9,000 years to offer insights into milk consumption and the evolution of lactose tolerance.
5,000 Years Ago, Almost All Adults Faced Problems Upon Excessive Milk Consumption
In the present day, most European adults can drink milk without discomfort. However, two-thirds of adults in the world experience problems if they consume too much milk. About 5,000 years ago, almost all adults faced problems if they drank too much milk. The reason why some people experience problems when they consume too much milk is that it contains lactose, and if this unique sugar cannot be digested, it will travel to the large intestine where it can cause cramps, diarrhoea, and flatulence. This condition is known as lactose intolerance. The new study conducted in the United Kingdom suggests that the effects of lactose intolerance are rare.
How Is Lactose Digested?
In a statement released by University of Bristol, George Davey Smith, a co-author on the paper, said to digest lactose, one needs to produce the enzyme lactase in the gut. He added that almost all babies produce lactase, but in the majority of people globally, the production of lactose declined between weaning (getting a child or young animal used to food other than mother's milk) and adolescence.
Evolution Of Lactose Persistence
Over the last 10,000 years, a genetic trait called lactase persistence has evolved multiple times and spread in various milk-drinking populations in Europe, central and southern Asia, the Middle East and Africa, Smith said. In the present day, around one third of adults in the world are lactose persistent.
Lactase Persistence Genetic Trait Was Not Common Until 1,000 BC
The researchers mapped patterns of milk use over the last 9,000 years, probed the UK Biobank, a large-scale biomedical database and research resource, and combined ancient DNA and archaeological data using new computer modelling techniques. By doing so, the researchers showed that the lactase persistence genetic trait was not common until around 1,000 BC. This was nearly 4,000 years after the lactase persistence genetic trait was first detected around 4,700 to 4,600 BC.
Natural Selection Drove Lactase Persistence To High Frequencies
Professor Mark Thomas from University College London, who co-authored the study, said the lactase persistence genetic variant was pushed to high frequency by some sort of turbocharged natural selection, and that such a strong natural selection is hard to explain.
Milk Consumption Patterns Varied In Different Regions At Different Times
Professor Richard Evershed from Bristol's School of Chemistry, the lead author on the paper, assembled an unprecedented database of nearly 7,000 organic animal fat residues from 13,181 fragments of pottery from 554 archaeological sites to establish how lactose persistence evolved. This helped him find out where and when people were consuming milk.
Milk was used extensively in European prehistory, dating from the earliest farming nearly 9,000 years ago, Evershed found. Consumption of milk increased and decreased in different regions at different times.
When Was Lactase Persistence Genetic Variant At Appreciable Frequencies?
The team from University College London, led by Professor Mark Thomas, assembled a database of the presence or absence of the lactase persistence genetic variant using published ancient DNA sequences from more than 1,700 prehistoric European and Asian individuals.
The team did so in order to understand how the variation in the consumption of milk patterns in different regions at different times relates to the evolution of lactase persistence.
These variations first occurred around 5,000 years ago.
The researchers found that by 3,000 years ago, the lactase persistence genetic variant was at appreciable frequencies, and is very common today.
After this, the researchers developed a new statistical approach to analyse how well changes in milk use through time explain the natural selection for lactase persistence. However, the researchers found no relationship.
Minimal Differences Found Between Genetically Lactase Persistent And Non-Persistent People
The team led by George Davey Smith had been probing the UK Biobank data, which comprises genetic and medical data for more than 300,000 living individuals. The researchers found only minimal differences in milk-drinking behaviour between genetically lactase persistent and non-persistent people. According to the study, the large majority of people who were genetically lactase non-persistent experienced no short or long-term negative health effects when they consume milk.
Milk Use Was Widespread In Europe Even In The Non-Lactase Persistent
Davey Smith said the findings show milk use was widespread in Europe for at least 9,000 years, and healthy humans, even those who are not lactase persistent, could happily consume milk without getting ill. He added that drinking milk in lactase non-persistent individuals does lead to a high concentration of lactose in the intestine, which can draw fluid into the colon, and dehydration can result when this is combined with diarrhoeal disease.
Severely Malnourished People Had Life-Threatening Problems Upon Milk Consumption
Davey Smith explained that if one is healthy and lactase non-persistent, and drinks lots of milk, they may experience some discomfort, but they are not going to die of it. However, if one is severely malnourished and has diarrhoea, then they will have life-threatening problems.
He said that prehistoric people would have been more likely to consume unfermented high-lactose milk when their crops failed.
The team led by Professor Thomas applied indicators of past famine and pathogen exposure into their statistical models. The explanation their results supported was that the lactase persistence gene variant was under stronger natural selection when there were indications of more famine and more pathogens.
Conclusions
The authors concluded that the study demonstrates how, in later prehistory, as populations and settlement sizes grew, human health would have been increasingly impacted by poor sanitation and increasing diarrhoeal diseases, especially those of animal origin. They explained that under these conditions, consuming milk would have resulted in increasing death rates, with individuals lacking lactase persistence being especially vulnerable.
According to the authors, this situation would have been further exacerbated under famine conditions, when disease and malnutrition rates are increased. As a result, individuals who did not carry a copy of the lactase persistence gene variant were 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," the authors concluded.