Early Europeans drank milk for thousands of years before they evolved the ability to fully digest it as adults, scientists say.

New results published in the journal Nature suggest that being able to digest the lactose in milk wasn’t usually much of an advantage for ancient people in Europe. Instead, the new study suggests that famine and disease made lactose intolerance deadly.

The new discovery challenges the long-standing assumption that dairy farming spread through ancient populations alongside the genetic quirks that prevent adults from losing the ability to digest lactose.

Like other young mammals, human children produce an enzyme called lactase that breaks down lactose. The gene for lactase usually turns off in adulthood because aside from humans, adult mammals don’t drink milk.

Without lactase, lactose from milk ends up feeding gut microbes that produce gas, which can cause uncomfortable digestive problems.

“You’ll get some cramps. You’ll get some diarrhea. Might fart a bit more. It might be unpleasant for you,” said geneticist Mark Thomas of University College London, who led the genetics work for the new study. “It might be embarrassing, but you’re not going to die.”

But when our ancient ancestors suffered through plagues or famines, getting diarrhea from drinking milk was probably more than just uncomfortable, the authors suggest.

“Then we’re talking about a life-threatening condition,” Thomas said.

About one-third of people alive today have a genetic variant that keeps their lactase gene from turning off. This trait has evolved independently multiple times in the ancestors of people now living in parts of Africa, the Middle East, South Asia and Europe.

Scientists long assumed that lactase persistence evolved alongside the spread of dairy farming, which happened over a few thousand years beginning around 7000 BC.

However, earlier studies revealed that lactase persistence was vanishingly rare in Europe until about 3,000 years ago. But after that, it took only a few thousand years for the trait to become widespread — the blink of an eye in evolutionary time.

Why this trait would evolve so quickly was a mystery.

“Lactase persistence has been under enormous amounts of natural selection over the last eight to ten thousand years … more than any other part of the genome in Europeans,” said Thomas. “It was, for a very long period of time, the one trait upon which life and death pivoted more than any other. … It’s insane. It just defies explanation.”

Searching for an explanation, the authors sought to reconstruct the history of milk use in the region over the past 9,000 years. They examined fat residues left on more than 7,000 pottery shards collected at 550 archaeological sites across Europe.

“When people were cooking … fat liquefies and then penetrates into the pores of the pottery,” said organic geochemist and study co-author Mélanie Roffet-Salque of the University of Bristol. “It’s quite stunning, really. But thousands of years later when archaeologists excavate a piece of pottery that had been discarded and then we analyze the pottery, it’s still there.”

The pottery shards showed that milk consumption was widespread across most of Europe for thousands of years before most Europeans became lactose tolerant.

Studying health data on modern Britons, the researchers didn’t find any evidence that drinking milk hurts the health of modern adults who don’t produce lactase.

Surprisingly, using data on ancient population fluctuations to approximate when and where ancient Europeans dealt with famine and disease, the researchers found that sickness and hunger might explain the evolution of lactase persistence better than milk consumption.

Famine could have forced ancient people to drink more milk than usual as other food sources ran out. And both malnutrition and disease could have made lactose-induced diarrhea very dangerous. Severe diarrhea can kill — it is still the second leading cause of death for children under 5 worldwide.

Shevan Wilkin, a biomolecular archaeologist at the University of Zurich who reviewed the new paper, said the research was an important step forward but that she’s not necessarily convinced that famine and disease alone can explain the evolution of lactase persistence.

“The reason I don’t know if I think they’re right, I also don’t know if I think they’re wrong, is before 2,000 years ago, there were absolutely times of famine,” Wilkin said.

Thomas said he’d like to see similar studies done in Africa, where lactase persistence evolved independently three different times. Wilkin agreed, noting that Europe is over-studied, and that future research should focus on other regions, including central Asia, where people drink lots of milk despite lacking a genetic variant that keeps lactase from turning off in adults.

“I think it’d be really interesting to apply this [in] multiple places,” said Wilkin. “It’s just such a cool and ambitious undertaking, and I think it’s really going to spur a ton of new studies.”

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