Here are some interesting thoughts where the sourdough came from one of my vintage books by Jeff Smith “The Frugal Gourmet Cooks American”.
“Sourdough is probably the oldest form of “raised” or “leavened” bread that we know. While the Egyptians are generally given credit for baking the first raised bread, we must also remember that their claim to this fame occurred just after the Jews had fled Egypt, leaving their leavening behind. They were taught to bake unleavened loaves of bread in the desert. Most likely from the historical events that it was the Jews who brought a wild leavening from the desert, a sourdough, into Egypt, and the Hebrew people taught the Egyptians how to bake raised bread in the first place.
In any case, we are all familiar with the biblical lines about a little leaven leavening the whole. The Bible is referring to sourdough, and we have been using these wild yeast forms ever since. There is some evidence that Columbus brought a sourdough starter with him on the ships when he set out for the New World. In the time of Columbus, sourdough was the only kind of yeast available. It was used in this country from the very beginning.”