Musically, this song was inspired by Bulgarian folk music. Really.
In 1955, the Bulgarian composer Philip Koutev assembled a 24-voice choir to record songs culled from the villages of his country. His recordings found their way to Elektra Records, which released them in America in 1966 on an album called Music Of Bulgaria
, credited to the Ensemble Of The Bulgarian Republic.
The album found a following on the West Coast, and it enthralled Jerry Garcia, who called the voices "exceptionally pure." It's influence is most apparent on "Uncle John's Band."