All I know is that it started west in 1847 from Missouri. I would guess with the family of Dr. John Savage as one of his daughters (my great grandmother) was the cook. It came on west and settled near Salem Or. Doc. Savage’s daughter met and married my great grand father on the trail and they had 10 children. It was passed on to me though my parents when they passed away. I am 76 years old so that was some time ago. I first learned to use the starter in a basque sheep camp when I was 10 years old as we were setting up a homestead on the Steens Mountains in southeastern Oregon. A campfire has no oven, so the bread was baked in a Dutch Oven in a hole in the ground in which we had built a fire, placed the oven, scraped in the coals from around the rim, and covered with dirt for several hours. I used it later making bread in a chuck wagon on several cattle drives - again in southeastern Oregon.Link (via Making Light)
Update: Mike sez, "Sourdoughs International has a great collection of sourdough starters from some of the oldest bakeries in the world. I'm not much of a sourdough fan, myself, but I love the romance of the sourdough descriptions on their site. Alas, not as free as your pioneer starter."