From their mother. Human breast milk is very bitter and I'm sure protein wise very different than cow milk. I doubt scientists have really studied this. Humans are not supposed to be drinking bovine milk. As a visitor to this planet I find it strange. Milk has a lot of lactose and will have interesting affects on adults including but not limited to insulin resistance whereas babies are developing very fast and need simple quick energy.
Humans aren't "supposed" to eat anything. You think we're supposed to eat flour or sausage or arugula or lentils?
But because we like to survive, we eat anything and everything that gives us nutrition and helps us live. Also, there are pastoral tribes like the Maasai in East Africa that historically have lived on bovine milk as a staple food. Is that authentic and traditional enough for you that you might no longer consider it "strange", but rather as traditional as it gets?
We are not. The dumb grazing animals are supposed to eat the various forms of grass. Their guts and stomachs are designed to convert those to energy correctly. Humans since time immemorial have eaten mostly vegetables and meats when they can catch them. Only recently did we start poisoning ourselves for the profits.
This is just not true. Since time immemorial, humans have eaten meat and fish constantly as a main part of their diet. Catching fish and animals isn't that much different from digging up roots. We've eaten everything we can, not just vegetables.
And humanity has been drinking bovine milk for many millennia. Long before even the concept of capitalist "profits" existed. Again, see the Maasai for example.
And it's still the wrong thing to do. Humans got that wrong and stuck with it. They will argue till they are blue in the face and so will I.
Humans are not "supposed" to eat and drink most of what we do (maybe fruits are an exception). However, we have evolved to consume a lot of things - including, if we have the right genes, milk.
As many people do and get a fatty liver and ultimately Cirrhosis.
That said, we've coevolved with technology of one sort or another (the broadest definition, to include cooking, plant breeding, hunting with weapons, domestication and animal husbandry) ever since we began to master fire, a million years ago give or take.
Aggressively incorrect.