Yeast is not required for bread baking. Think flat bread.
But somewhere along the line someone did use yeast to make bread, and alcohol would have been smelled in the process. The Egyptians were making beer thousands of years befor Christ. Yeasts were known to exhibit the fermentation processes. Some smart gut, possibly and Egyptian, must have put two and two together and came up with BEER! Thus the phrase, "Walk like an Egyptian!"..........
Also, you don't need to "add" yeast - yeast floats around in the air as a naturally occuring substance. Leave some soaked grains around long enough and yeast will find it and grow on it. I would imagine that's where beer got it's start.