Why most people lactose intolerant?
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Because adult animals are not meant to drink milk. It’s for nourishing babies until they can move onto to more complex sources of nutrition.
Most mammals don’t produce the enzyme to break it down in adulthood.
Normally it’s only expressed as a very young child.
Some groups of humans mutated to express the enzyme throughout adulthood. So by default humans are lactose intolerant, those who can digest lactose are mutants who express the childhood enzyme as adults.
Because most people stop drinking milk after infancy. This means that we no longer need to be able to digest it, so the cells turn off the gene that lets us digest it so as to not waste energy and materials making an enzyme we don’t need.
The ancestors of Europeans and some other groups however found alternate sources of milk and continued to drink it, as well as make it into cream, cheese, butter and many other delicious things, and over time we evolved to turn that enzyme off later and later, and eventually not turn it off at all, because it was still being useful.
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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