I never realized what a world unto itself sourdough was. First part of a documentary above, and a good article on the world sourdough library here.
All modern recipes begin with a starter, basically a flour-and-water slurry colonized by bacteria and wild, airborne yeasts that eat, breed and exhale carbon dioxide, which helps the bread rise. The tangy taste and brightly acidic smell derive from lactobacilli, cousins of the bacteria that curdle milk into cheese and yogurt. Starters are bespoke to the environments in which they were created; no two sourdoughs taste exactly alike.