Title: Why are some people lactose intolerant? - Quora
Headings: Why are some people lactose intolerant?
Why are some people lactose intolerant?
Anonymous

Content: But the domestication of animals changed things. Humans had access to animals which would sit still for being milked, which wild animals would not. That gave at least some of us access to a regular supply of lactose, which we didn’t have previously. At that point, it became advantageous for those populations not to be lactose intolerant. A mutation for adult lactase persistence did, indeed, appear about 8000 years ago, probably somewhere in western Asia and, since it was useful, it spread through the population. So, then, most humans are lactose intolerant because for most of human history, they had no reason not to be. When conditions changed, we started moving in the direction of not being lactose intolerant, but evolution is slow and the advantage is slight, so only about a third of us aren’t lactose intolerant now. Prabha Karan
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Answered 2 years ago · Author has 403 answers and 646.9K answer views
Originally Answered: How can someone become lactose intolerant all of a sudden? There are so many reasons for it, not a single reason.