Interesting thread. There is a natural close relationship between certain pitches, notably thirds, sixths, and octaves. This is unlikely to have escaped the notice of our forefathers who, in their spare time, built pyramids etc. So I imagine harmony singing is very old. Those who explored the possibilities produced something which is generally pigeonholed today as art music. Not quite of the soil. Those who preferred solo are more likely to be associated with folk or popular traditions. There are 'between' areas, probably resulting from people with different voice ranges singing together. The important things are does it work, and is it expressive. All other divisions in music are bogus. I wouldn't fret too much about the origins. Best wishes.