Why aren’t all adults lactose intolerant?

The continued ability to digest lactose is due to a relatively new mutation in a single gene called LCT. Roughly 10,000 years ago, mutations emerged that allowed for increased LCT gene activity in adulthood,^[1] which is technically called ‘lactase persistence’. People with this mutation, which is most common among people of Northern European descent, can digest milk sugar into adulthood. Mutations persist when they confer benefits. Some of the ancestral humans with this mutation benefited from their mutant superpower ability to digest milk sugar into adulthood. Why would that be? In a word, pastoralism. Raising livestock can provide for a steady supply of meat and milk, and certain geographies were set up well for animal husbandry.^[2]

One of those regions, Sweden, provides some interesting data. Nearly all modern Swedish people can digest lactose, but as little as 4,000-5,000 years ago, there was a stark divergence in that area: DNA from Swedish hunter-gatherer populations of that era show only 5% to have had lactase persistence, while DNA from cattle-raising populations of the same era show 75% with lactase persistence.^[3] If you’re a modern-day Swede, you probably owe your milk-digesting ability to the success of the latter population.