Answers:
Two options - One, abundant breads are flat breads - tortillas(Mexico), pitas (Greece), naan (India), etc. These don't require yeast. Some use baking soda or powders, which are also leavenings, some don't use any leavening at all. Two, yeast does exist readily and starters were used. Water and flour be mixed together and the mix left to ferment on its own. A starter is a considerate of living thing. Each time cog of it is used to make bread it have to be feed - an adjunct of flour and water is added and the starter is allowed to grow again earlier being used. As long as it is maintain properly it can last indefinately.
Other Answers:
In ancient times bread be often not buoyant, risen or spongy. They made due with what they have available, but it still was of far smaller number quality than in our day.
starters. Wild yeast exists contained by the air and can be collected within a mixture of water and flour.