As for milk from animals, someone must have connected babies suckling from their mothers with small mammals such as goat kids suckling from the nannies, and wondered if they could try ( say) goat milk. This might have become quite important for human babies if their mothers ran out of milk, perhaps if they could not get enough water to drink in a drought. Children would probably have been able to digest animal milk, and then when humans began to herd goats, sheep, etc, they forced themselves to drink the milk when there was nothing else, and thus their digestive systems evolved to be lactose-tolerant.
Actually, there is one simple - if crude - test - if something tastes bitter, it is probably not good for you. A tiny taste might be enough to warn you without killing you.
Thinking of rhubarb, I expect some stone-age cavedweller picked a stalk to poke into a beehive to get honey out, and exclaimed "I say, chaps, this combination tastes rather smashing" - between stings, of course.