ISBHPK ABQ: Leaven!

The ancient Egyptians are credited with making the first leavened bread. Perhaps a batch of dough was allowed to stand before it was baked. Wild yeast cells settled in and grew, producing tiny bubbles of carbon dioxide and making the dough rise. The bread was softer and more palatable, so it became the custom to let the dough stand for some time before baking. This technique was hit or miss, however, because on some days, the air bore no suitable yeast.

Later, a baker discovered that a little dough raised in this manner could be used as a starter for the next batch of bread. The portion of bread kept to start the next batch was called leaven; it was the forerunner of today`s sourdough bread.

https://www.chicagotribune.com/news/ct-xpm-1986-02-09-8601100787-story.html