If the germ theory is relatively new, how do they think fermentation was happening (like wine, ale, yogurt etc.) thousands of years ago?
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As others have said, yeast is everywhere and fermentation could be spontaneous. However, people have been keeping yeast for a long time even before they knew it was yeast or that they were keeping it.
One great example is the “yeast ring” or “yeast logs”. The basic concept is that wood has a lot of surface area, so pieces of wood would be placed in with a fermenting beverage.
By retrieving the wood from the batch, letting it dry, and using the same wood again next time, they were preserving a colony of yeast from batch to batch.
Did they know what yeast was? Not really, but they knew that reusing the same wood over and over made their fermentation work a lot better than leaving it to chance.
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