how come a lot of people are lactose intolerant if everyone started off by drinking milk as a baby?
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Most animals that drink milk are lactose intolerant as adults.
Mothers only produce milk for a short time after having a baby, which means that being able to drink milk is kind of useless a year or two after being born (since your mom isn’t feeding you milk anymore).
It takes energy to generate lactase, the enzyme that digests lactose. But, if you’re not drinking milk, there’s no reason to generate lactase, so doing that just decreases your chances of surviving because you’re spending energy on something useless.
As a consequence, most mammal children grow out of producing lactase as they get older.
Humans did something weird though. Rather than relying on their mothers for milk, people decided to start stealing milk from mother cows, and drinking that.
Now, there’s a way for humans to drink milk at any age, not just as children, so it’s advantageous to produce lactase as an adult, which allows natural selection to produce humans who can drink milk.
There hasn’t been enough time for this adaptation to spread to the entire human population though, so only some people with origins in specific areas of the world where drinking milk was common produce lactase as adults. People who haven’t developed this adaptation, just like every other mammal, are called lactose-intolerant.
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