Persisting lactase into adulthood is the unusual state — roughly what share of adults do?

About a third. Every mammal produces lactase as an infant and switches it off after weaning; continuing to make it as an adult is the derived, unusual condition, and calling the majority of humanity intolerant gets the biology backwards.

It arose independently at least four times - once in northern Europe, and separately in East Africa, West Africa and the Middle East, each time through a different mutation near the same gene, and each time in a population that kept cattle, sheep or camels. That is one of the clearest examples of convergent evolution in humans, and one of the strongest and most recent signals of natural selection in the genome, appearing within roughly the last ten thousand years.

Why the advantage was so large is not settled. Milk as calories is the obvious answer, but people who cannot digest lactose can still eat yoghurt and hard cheese, where bacteria have already broken it down - so fermentation solves the problem without any genetics. Current suggestions lean on famine and disease: fresh milk as a fallback when crops fail, or as a clean fluid during outbreaks, in which case the selection happened in the worst years rather than the ordinary ones.