Brewers were the original suppliers of yeast in pure enough form and sufficient quantity for large scale use by bakers, in the foam skimmed off the top of the vat. I think most commercial beers have been heat-pasteurized, and yeast dies if it gets over 138 degrees, so adding beer to a recipe adds flavor but probably no active yeast.
Beer brewing and bread baking are both skills that have been around for thousands of years, nobody really knows which came first.
I think one of the microbrewery/restaurants in Lincoln may use some unpasteurized beer in a beer bread that also uses some spent grain, but they've got access to the raw products. I don't know if all the yeast comes from the beer or if they supplement it. I've never been all that fond of their bread, the residual beer taste is too strong for this non-beer-drinker.