Why milk?

Humans keep three types of domestic animals, cows, sheep and goats, in part for their ability to make large amounts of milk we can harvest. We regard milks as such a good, that we give its name to liquids made from plants, like soy milk and coconut milk. Humans are a rare mammal that can keep processing milk and dairy, well after weaning age (I think dogs and cats may be able to as well).

But, did you ever wonder what possessed several primitives, perhaps just getting the hang of herding animals rather than hunting them, to drink the stuff coming off an animal's udders?

It probably didn't happen like that.

Humans, as noted above, are mammals. Women produce milk much like other mammal species do. This would be something even primitives would know well. They would reason that cows, sheep, dogs, and other animals they might watch nursing their young were doing the same thing.

Sometime women don't produce breast milk, or they produce too little, especially when faced with multiple births. Lack of sufficient food is one cause, but there are others. Death at childbirth is a persistent human curse, which was much worse for nomadic or semi-nomadic primitives in the stone age. If the mother died and the child survived, they had to be fed somehow.

It doesn't take much of a stretch to see that a tribe or clan with one or more hungry children, not enough breast milk, and access to cows, sheep or goats, would try to get milk from these animals and feed it to the baby. We know now this is far from ideal (see the baby formula industry), but it works after a fashion, and it's better than starvation.

Any adult and even most youngsters who then tried the cow milk, however, would not have developed lactose tolerance, and would have grown ill if they drank it. How, then, did our species go on to develop lactose tolerance?