This was discovered in prehistoric days. People lived closer to Nature in those days. They did not even use baby bottles for their own babies. It was not hard for them to notice that a lot of animals have nipples, and that the baby animals nurse from them just as human babies do from their mothers. And by the way, women can milk their own breasts too.

And once people started keeping and breeding domestic animals, it was convenient enough to get milk from goats, sheep, horses, cattle, or whatever, especially since their teats are so convenient to the hand. They just had to tame the animal enough that it would cooperate, more or less. Even so, they might hobble the animal's hind legs while milking her, if she is known to kick.

Now, to city folk this whole process of lactation is a great mystery. Because you get milk from the store, right? And you never see any cows there.

And by the way, it was probably from breeding livestock that people discovered that the male has a substantive role in producing babies.

You don't think that people have always been aware that all mammals nurse their young with milk that comes out of teats?

By watching calves nurse.

I came across this question in my head a few weeks ago and it kept on messing with my head. Was a guy like "I dare you to squeeze that pink thing" or was a guy sitting in a meadow and came across a cow and thought in his head "I'm thirsty. How about I start squeezing that pink thing? It's my only hope." Seriously, help me find the answer!!! I'm DYING to know. Was it a creepy old man? Was it a person stalking a cow and seeing the mother cow feeding it's child? WHAT IS THE AAANNNSSSWWWEEERRR? And, I bet the first person to do it was freaked out when they saw what came out -,-