LACTOSE TOLERANCE EVOLVED IN EUROPEANS THANKS TO FAMINE AND DISEASE
Wide-ranging evolutionary study overturns the long-held idea that the gene for lactose tolerance spread because it allowed dairy-farming humans to consume more milk
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 bouts famine and disease periodically killing off the lactose intolerant that drove the protective gene’s evolution. This overturns the widely held model that as we began to farm dairy, a novel genetic mutation occurred that allowed some to be lactose tolerant as adults, and from there this became positively selected for and spread throughout the population.
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 gene to produce lactase switches off 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 people 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.
“When their crops failed, prehistoric people would have been more likely to consume milk”
“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 teams working on different pieces of the puzzle. First, researchers led by Prof Richard Evershed from the University of Bristol 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 discovered that milk was regularly consumed since the earliest dairy farming began around 9,000 years ago, but fluctuated across regions at different time periods.
A second team led by Prof Mark Thomas from University College London looked for the presence of the lactase-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, 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.