Hm, I'm going to give a very anecdotal, old-wive's-tale answer that gets passed around in the brewing world. I don't know how much help it will be, but hopefully it's better than the empty comment section you have now!

Beer (or some version of it) has been brewed for a long time, longer-than-we've-had-writing kind of long (so at least 5000 years). And, as you might expect, people had no idea what yeast was then, but it is essential for beer since it's what actually causes fermentation. Now a days you can just run down to your local beer store and pick up a vial of whatever strain you want, but back then people often thought it was an act of god and would just pray for their pots of wort (sugar water from soaked cereal) to start fermenting (i.e. that wild yeast would randomly contaminate their wort and start reproducing, eating the sugars and shitting CO2 and alcohol). The wives tale part is that it's said that the vikings would have "magical" stir sticks passed down from generation to generation that they would use to stir the wort during beer making and that these were semi-sacred because without the stir stick the beer would not ferment. It's thought that these stir sticks basically had a bunch of dormant yeast cultures on them and introducing the cultures to the sugar water would start the fermentation process, so they were basically saving their own yeast generation after generation.

An older method also used chewed-up bread as part of the mash (the soaking of the cereal to make the wort) which may also have introduced some yeast, though I'm not sure how well that would work out if the bread has already been even partially baked . . .

To come a little closer to the present, the Germans have something called reinheitsgebot which is basically a beer purity law which says that only 4 ingredients are allowed in German beer: barley, water, hops, and yeast. However, when the law was first made back in the 1400s they only listed the first three, because it took until Louis Pasteur discovered how microorganisms affected fermentation to figure out that yeast even played a role!

I suppose what I'm trying to say is that, at least in the world of beer, people were entirely unaware of yeast. They didn't think it was alive, or dead, or anything; they thought yeast was pretty much magic.