What are the origins of leaven? 

For a long time, the addition of leaven was the only way to make the bread rise. It is very difficult to date the discovery of leaven, but the first representations of it date from the Ancient Egyptian Empire. Depending on the version, leaven was discovered by the Babylonians or by the Hebrews. But the most frequently cited origin is that of Egypt: a person would have been late in baking his cereal dough, and the dough, under the effect of fermentation, would have begun to swell, thus creating the first leavened bread. The Egyptians and before them the Sumerians mastered the fermentation process: they made beer and bread together.