The Egyptians were making bread by methods similar in principle to the sourdough baking of today. Brewing and baking were closely connected in early civilizations. For all but a fraction of this time, bread was leavened naturally with airborne spores of sourdough culture. With what seemed a divine and invisible hand the sourdough culture miraculously created digestible bread. Sourdough starter is made from water and grain flour, which creates the environment and food for these microorganisms to perform their magic.
Wild yeast metabolizes complex sugars and starches from carbohydrates.
This produces the food that the bacteria and yeast need to survive and multiply. Grains are complex carbohydrates not easily digested by bacteria. Imagine a carbohydrate as a solid brick wall. It would be impossible for the bacteria to bite this food, it's simply too large. The airborne wild yeast creates enzymes needed to break down this wall into manageable bitesize pieces for the bacteria to eat.