How was the usage of yeast for bread discovered?
The first yeast was &quot;just there&quot; - in the environment, everywhere. People discovered very early on that leaving the dough (or just a flour-water slurry) out would lead to it getting &quot;sour&quot; and &quot;bubbly&quot;, thus leavening the bread: What we today call sourdough is in fact a mixture of yeasts and bacteria (lactobacillae).

The origins of bread-making are so ancient that everything said about them must be pure speculation. (M.G. Gaenzle: Encyclopedia of Food Microbiology)

Early sources are the Bible (-&gt; the explicit demand for of &quot;unleavened&quot; or &quot;unsoured&quot; bread during Passover suggesting that otherwise was the norm) and sources from Ancient Egypt.
The &quot;modern&quot; yeast that was explicitly added was mentioned by Pliny the Elder in his Naturalis historia as a foam skimmed from beer:

In Gaul and Spain, where they make a drink by steeping corn in the way
that has been already described - they employ the foam which
thickens upon the surface as a leaven: hence it is that the bread in
those countries is lighter than that made elsewhere.
Source: The Natural History, Chapter 12: Wheat. Pliny the Elder.
John Bostock, M.D., F.R.S. H.T. Riley, Esq., B.A. London. Taylor and
Francis, Red Lion Court, Fleet Street. 1855. 

Sources from the 18th century describe how the yeast from the brewing process (Saccharomyces cerevisiae) was used for baking, leading to the &quot;specialized strains&quot; of bakers' yeast cultivated separately when the brewers switched to bottom-fermenting yeast (S. pastorianus) in the 19th century, which preferred cooler temperatures and was not as easy to harvest as the top-fermenting types.
The use of separately cultivated yeast allowed to bake &quot;milder&quot; breads than the sourdough types with their typical acidic undertones but is restricted to certain flour types.