Prehistoric Europeans were drinking milk as adults thousands of years before most of them evolved the gene necessary to digest lactose, a study carried out at University College London and the University of Bristol has found.
By poring over DNA and archaeological records dating back 9,000 years, the researchers believe that it was occasional bouts famine and disease periodically killing off the lactose intolerant that drove the protective gene’s evolution. Not simply that a gene arose after Europeans began dairy farming that allowed them to drink milk into adulthood without ill effects, as previously thought.
In order to digest lactose, we need to produce an enzyme known as lactase in our guts. Almost all babies can do this but the ability declines in the majority of the global population as they mature into adulthood.
The ability to continue producing the enzyme is known as lactase persistence and exists in around one third of adults in the world, including most of the population of Europe.
When those without the enzyme drink milk, the lactose passes into their large intestine where it can cause cramps, diarrhoea, and flatulence – a condition known as lactose intolerance.
While this condition is unpleasant, it is rarely fatal. However, its effects can be magnified in those who are malnourished in times of famine or disease, the researchers say. This is what likely drove the evolution of lactose tolerance.
“If you’re healthy and lactase non-persistent, and you drink lots of milk, you may experience some discomfort, but you’re not going to die because of it,” said co-author Prof George Davey Smith of the University of Bristol.
“If you’re severely malnourished and have diarrhoea, however, 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 doing so poses the greatest risk to them.”
The discovery depended on several different teams working on different pieces of the puzzle. First, a team led by Prof Richard Evershed from Bristol’s School of Chemistry put together a database of nearly 7,000 organic animal fat residues taken from fragments of pottery from more than 500 archaeological sites to find out where and when people were consuming milk.
They found that milk was regularly consumed since the earliest dairy farming began around 9,000 years ago but fluctuated across different areas at different time periods.
Secondly, a team led by Prof Mark Thomas of UCL looked for the presence of the lactose persistence genetic variant in the DNA sequences of 1,700 prehistoric Europeans and Asians.
They found it first emerged around 5,000 years ago and reached appreciable levels 2,000 years later. They also found that the gene was no more common during periods of greater milk consumption, overturning the long-held view that this was a factor in driving the evolution of lactase persistence.
Finally, Prof Thomas’s team compared the presence of the lactase persistence gene variant in times of suspected famine and disease. They found that the gene became more widespread during these times, indicating that famine and disease played a key part in its evolution.
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