Let's not forget the Chenille Sisters!

But there's still another source for harmony that hasn't been mentioned: work songs. Coonjine, railroad, chain gangs, chanteys -- they all contributed. While their roots might be in religious, art, or other sources, they evolved into harmonies in their own right. And a stomp-and-go chantey is more akin to something like "Make Me Mean" than to a congragation lining out a song.