Did they look at cow udders and say "That looks delicious" or what? Why did they start drinking milk from other species?
You know what, I've been asking myself this for a very long time too. I'll stick around to see an answer :p
They knew milk was potable because almost every human child they had ever known drank human milk. They additionally could see the animals drink there own species' milk and live off of it. I mean it is really pretty obvious to anyone observing that it is potable.