How was beer brewed before the discovery of Yeast? Yeast was identified as the cause of fermentation in the 1800s, but beer has been around since long before then.  How was fermentation started before the intentional introduction of yeast?  Was it all lambic-style ambient yeast, or what?
You don't need to know what something is to use it effectively.  Though yeast was only identified as a microorganism recently, it has been known as the cause of fermentation for many centuries.  It's easy to underestimate how sophisticated people throughout history were.

Before yeast was monocultured in labs it was actively cultured by brewers.  They would transfer the yeast cake from a batch that's finishing into a recently-brewed batch.  This cake contained a whole ecosystem of yeasts and bacterias, but a few strains were usually dominant.

By applying artificial selection brewers developed different strains of yeast in different areas.  The yeast culture was the make-or-break characteristic of a brewery, and defined the styles of beer that could be produced more than any other factor (other than water in Burton).

Evidence of this process comes from the history of lager: bottom-fermenting yeast (Saccharomyces carlsbergensis) emerged in the fifteenth century as a direct result of the cold storage regime of northern brewers such as Carlsberg.

They may not have known it was a microorganism, but middle ages brewers most certainly knew that yeast was the cause of fermentation.  That's why they called it "Godisgood".