Milk is easy. Gelatin isn't so hard, because by the point it was discovered we already had lots of weird food. Most foods were invented with a thought process like, "Take parts of different animals and plants that haven't killed us yet, and toss them together in different ways. See if it tastes good."
What confuses the hell out of me is bread.
Once you know how to make it, it's not so hard. But I can't help but imagine that bread was the most difficult food item to invent. You take wheat, ground it up into flour, and it ain't so tasty by itself. You set it on fire and get nothing. You bake it and get crisp flour. You mix it with eggs and fat, and you end up with a weird sludge that's maybe palateable. You bake that sludge, and you get a cracker. And people like crackers.
Then, some day, some poor sod finds out his wheat-egg sludge is moldy, but he bakes it anyway. Miraculously, it smells good, and when he actually tries to eat it, it's a hell of a lot better than a cracker.
Don't get me started on pasta.