Nobody knows. Bread predates writing. Likely scenario: someone leaves out their cereal based porridge/gruel for too long and wild wind blown yeast colonized it. The mixture began to bubble and someone was brave/stupid enough to eat it. They lived (this being the ultimate evolutionarily selection tool). Eventually heating the mixture was struck upon by someone, or it inadvertently feel into a fire and smelled good. Yeast is everywhere so, "by accident".