Question 1

Why can humans drink milk as adults when no other animal can?

Accepted Answer

Most adult humans cannot. Only those carrying a recent mutation called lactase persistence, which keeps the lactose-digesting enzyme switched on past childhood, can comfortably drink fresh milk. It arose within the last several thousand years in dairy-keeping populations and is absent in most other mammals.

Question 2

Is being lactose intolerant normal or a health problem?

Accepted Answer

It is the normal, ancestral human condition. Roughly two-thirds of adults worldwide stop producing much lactase after early childhood, just as nearly every other mammal does. Lactose intolerance reflects the original human setting, not a defect or disease.

Question 3

Why did the lactase gene spread so fast in some populations?

Accepted Answer

A 2022 study found the mutation spread fastest during famine and disease, not during ordinary times. In crises, malnourished lactose-intolerant people who drank fresh milk could suffer fatal diarrhea, so those able to digest milk safely were more likely to survive.

human-biology

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The Mutation in Your Coffee Cup