The strain of yeast in IDY is Saccaromyces cervisiae. Recall that in spanish beer is cerveza. It comes from beer. Beer is very, very old, going back to Egyptian times. Most western and middle eastern cultures drank beer because the fermentation killed the other contaminants. Drinking water in many parts of the world was considered dangerous. In eastern cultures they drank tea and other boiled drinks to make them safe.
Anyway, before IDY most bread used natural starters and since many don't rise that much, bread tended to be denser. Also, the milling didn't allow for very fine grinds. So breads tended to be made of whole grains, incompletely milled, using weak yeast. Welcome to the fun middle ages....
My guess is that modern beers would have no live cultures, but you might be able to get a culture going from beer that came from a brewhouse and not a major factory.