I've been rummaging through cookbooks that used to be my grandmother's, and among the treasures is Any one Can Bake - a 1929 promotional publication from 'the Educational Department of the ROYAL BAKING POWDER CO.'
You might get a laugh from their version of the origin of leavened bread - from a longer article entitled, 'The Evolution of Baking Powder'.
The Royal Baking Powder Co tells us ...
Yeast came first Very early in written history we come upon mention of both 'leavened' and 'unleavened' bread, so the actual origin of raised bread is obscure. Some prehistoric matron, perhaps, was not too careful about washing out the vessel in which she mixed the grain and water for her baking. A stray yeast cell lodged in the scrapings and developed in the next moist mixing so that the loaf grew astonishingly. It was porous and softer than her ordinary loaves.
She tasted this unusual mass - and found it good. She added a bit of the magic dough to her fresh mixture in the hope that it would impart its characteristics to the new loaves - and of course it did.
Thus began the leavening of grain mixtures, and for generations yeast in some form or another was the only leavening agent known.
What can I say? Let's all pay homage to that prehistoric slattern whose naughty lapse in hygiene turned out to be a blessing for us all. And thank heavens that a single yeast cell just happened to come along at the right time! :P