The gene that makes it possible to digest milk became dominant in Europeans during periods of disease and famine thousands of years ago. Milk was already consumed on a large scale in Europe when the population was still lactose intolerant, but tolerance for milk sugars did not arise because of that consumption. Only when external factors put pressure on the health of early herders in Europe did natural selection take place. People who could tolerate lactose lived longer and their genes became dominant as a result.
That is the conclusion of a study led by the University of Bristol that involved dozens of scientists from all over Europe. Their article was published on Wednesday in Nature. For this study, they mapped out traces of milk consumption over the past 9,000 years and combined this with research into the DNA of Europeans who died long ago.
All babies have the ability to produce lactase. This enzyme digests the lactose in breast milk. When infancy is over, most of the world’s population no longer produces this enzyme, making them lactose intolerant. In Europe, however, the majority of the population is still able to digest milk, while in the past the gene variant that makes this possible was also missing.
Remnants of milk fat
To find out what caused this beneficial mutation, the researchers first mapped European milk consumption between 7000 BC and 1500 AD. They did this by analyzing potsherds from 554 archaeological sites. In a large part of these shards they found remnants of milk fat, in fairly constant amounts over the period studied.
This means that milk has been consumed on a large scale in Europe for 9,000 years. Research into the DNA of the remains of 1,700 people who lived during this period, it became clear that they were initially mostly lactose intolerant, but that the gene modification that changed this was advancing.
Thus, while milk consumption was constant and substantial, strong selection took place. If it wasn’t the simple fact that people drank a lot of milk, why? The researchers suspected that disease and famine had an influence. The symptoms of lactose intolerance – especially diarrhea – are tolerated if you are otherwise healthy, but for those already sick or starving, watery stools can be disastrous. Then you survive better with lactase than without.
To test this hypothesis, the researchers mapped periods of epidemics and crop failures. They did this by looking at population density and contact with wild animals in the settlements studied (both affect the risk of disease) and fluctuations in population numbers (sudden declines can indicate famine). When the researchers crossed these elements with their database of ancient DNA, they found that these variables explained the advancement of lactose tolerance well. In calculation models even 284 (disease) and 689 (hunger) times better than can be expected under a regular pattern of natural selection.