In part one of Jazz, a DVD collection by Ken Burns, it is stated that the original blues were slow. When the Black Codes were introduced and classically trained Creole musicians (Creoles of Color) began working with Black descendants of slaves who used the blues and church music in combination, musicians began using the open spaces in the music to improvise, between the lines, so to speak. These improvisations gave rise to what was often called ragtime, a popular form already extant, but came increasingly to be called jazz. Musicians began competing with each other in these improvisations.
I had never heard that before and found an instant comparison with the way musicians in the Afro-Cuban tradition would take some part of a standard form, e.g. the son, and elaborate that part until it became a new form in itself.
Addendum same date: This blog entry of Dec. 30, 2014:
Listening to a very old tape of the program Afro-Pop with George Collinet, I heard his interview with a Congolese key boardist who had invited an elderly balafon player to his home where his Fender Rhodes electric piano sat. Eventually, the old musician touched the piano and began playing a familiar piece which the interviewee played for us. He noted how his people did not like chords. Why? This is fascinating: because each note must be separate so it can fit into the rhythmic pattern. If chords are used, they crowd out other contributions and African music is above all social, everyone adds something in and room must be available.
I can sense intuitively what he is saying and it was a revelation to hear that. But I don’t know how that would be expressed in terms of musical theory.