If you look at a grape, you will notice a white haze around the globe. It looks almost like dust. When you touch a grape you can see your fingerprints.
That is yeast. Yeast occurs naturally. All fruits collect yeast that is constantly floating around, looking for a sugar source to feed on. One of the reasons the Romans made good wine is because they used grapes. I'm not sure if the Romans knew about yeast as we see it, but they knew alot about Wine Making. Unless you use commercially prepared grape juice, natural yeast will ferment your grapes into wine.
We have all seen wild animal shows where the Elephants and Monkeys get drunk eating fruits at certain times of the year. That is because the fruits have been fermented by the yeast that naturally occurs in the air.
I dont know about you, but I wouldn't want to be anywhere near a drunken Elephant.