Early Europeans could not stomach milk, but drank anyway

Carl Zimmer

In many ways, humans are weird mammals. And our relationship with milk is especially weird. In every mammalian species, females produce milk to feed their young. The nursing babies digest the milk with the help of an enzyme called lactase, which cuts milk sugar into easily absorbed fragments. When the young mammals are weaned, they stop making lactase. After all, why waste energy making an enzyme you no longer need?

But it is common for our species to keep consuming milk into adulthood. What’s more, about one-third of people carry genetic mutations that allow them to produce lactase throughout their lives, making it easier to digest milk. Scientists have long suspected that dairy consumption and the persistence of lactase rose together in human history. When people started herding cattle and other livestock some 10,000 years ago, the theory went, those with a mutation for lactase persistence gained a new source of calories and protein. People without the mutation, in contrast, became sick when they tried to consume milk and so did not take advantage of the new milk supply.

But a new study of ancient human DNA and milk-drenched pottery shards suggests that the traditional story does not hold up. “Something was not quite right with the received wisdom,” said Richard Evershed, a biogeochemist at the University of Bristol in England, and an author of the study.

Dr. Evershed and his colleagues found that Europeans were consuming milk without lactase for thousands of years, despite the misery from gas and cramping it might have caused. The scientists argue that the lactase mutation only became important to survival when Europeans began enduring epidemics and famines: During those periods, their poor health would have exacerbated gastric distress, leading to life-threatening diarrhea.

The study, published in the journal Nature, emerged out of a collaboration among more than 100 scientists with very different kinds of expertise, including genetics, archaeology and epidemiology. For his part, Dr. Evershed pioneered methods in the 1990s to detect traces of milk fat left behind on ancient pots.

Over the years, he and his colleagues have found milk on thousands of pottery fragments across Europe and neighboring regions. For the new study, the scientists used this database to create a map of milk consumption over the past 9,000 years.

The oldest evidence of milk came from Turkey, which was home to some of the world’s first agrarians. Those farmers then moved across Europe, taking their cattle and other livestock with them. By 6,000 years ago, they had arrived with their milk in England and Ireland. Dr. Evershed and his colleagues found that some societies took up milk while neighboring ones did not. They also found that milk production went through boom-and-bust cycles over the centuries.

Mark Thomas, a geneticist at University College London, led the team’s analysis of lactase persistence. He and his colleagues analyzed DNA harvested from 1,786 ancient skeletons found across Europe and neighboring regions. They looked for a mutation that kept the lactase gene switched on during adulthood. The oldest mutation they found dated back about 6,600 years ago. But in their collection of ancient remains, it stayed rare until 4,000 years ago. For those 2,600 years, in other words, Europeans were consuming milk despite almost none of them being able to make lactase as adults.