Making bread is a fairly complex process - how long have we been doing it this way? Do we know who started putting yeast in bread and worked out the need to leave it to rise before cooking? Do we know what led to someone thinking "I have some of this yucky, grey mold stuff. I know! I'll put it in my food!" or was it likely an accident?
Yeast is pretty much everywhere, just floating around in the air. There are beers nowadays made using wild yeast and naturally occurring fermentation, and even one released a couple years back fermented with yeast found in the brewmaster's beard. So, it was almost certainly an accident caused by someone leaving flatbread dough out for too long. Actually isolating some of the stuff in the form of a "yucky, grey mold stuff" likely occurred later, after people acquired a taste for rising bread and wanted an easier, more reliable way to make it.
Sorry, in advance if this is breaking the rule against conjecture.