Why most people lactose intolerant?
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Lactose is a sugar that looks different than other sugars, and so it needs a special protein (think biological/chemical machine) to break it down. In most mammals, the gene that produces that protein gets turned off when you get older to save energy. This means that the default in mammals is adult lactose intolerance – they can’t digest lactose as adults.
At some point, likely connected to agriculture, humans mutated to *not* turn off that gene. Humans with that mutation could digest milk from other animals as an adult, adding another food source – which is an advantage as long as you get more calories from milk than you spend making the protein. As such, it’s common in areas where humans kept large numbers of cattle or other milk-producing livestock.
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