in the "encyclopedia of food microbiology", michael gaenzle writes: "the origins of bread-making are so ancient that everything said about them must be pure speculation. one of the oldest sourdough breads dates from 3700 bce and was excavated in switzerland, but the origin of sourdough fermentation likely relates to the origin of agriculture in the fertile crescent several thousand years earlier ... bread production relied on the use of sourdough as a leavening agent for most of human history; the use of baker's yeast as a leavening agent dates back less than 150 years."