The New Orleans players took Brass Band music and specifically Marches used in Parade and Funeral Processions and started "Jazzing Up" the Melody-improvising around it.
In the case of our instrument, Pops Foster was one of the first to use the String Bass, primarily relegated to String Bands and play some of the lines that Tuba players had played, dropping bowing in favor of "plucking" and even "slapping" the instrument.
Pianists like Jelly Roll Morton and Scott Joplin were introducing syncopations into piano Rags.
At some point, this evolved into Dixieland Jazz, and then some of those early players moved through the US to places like Kansas City and started Big Bands (Fletcher Henderson, Count Basie, on down to Paul Whiteman's Orchestra-the "smooth Jazz" of it's time).
This evolved into the dominant form of Popular Music from the 1920s-1940s.
The economic and social disruption caused by World War II and the Musician's Union Strike coupled with musicians wanting to explore new territory led to Charlie Parker, Dizzy Gillespie and others pioneering "Bebop". Louis Armstrong, instrumental in the development of early Jazz, called it "Chinese Music". Other players and critics called it "anti-Jazz".
Enter Miles Davis, John Coltrane, Dave Brubeck, Jerry Mulligan and "Modal" and "Cool School" styles. Players, fans and critics think that Coltrane can't play at all when he initially introduces "sheets" of sound playing.
Miles plugs in.....his 60s bands contain Chick Corea, Herbie Hancock, Joe Zawinul, Wayne Shorter, John McLaughlin and the founding fathers of the Fusion Movement. Players and critics call it "anti-Jazz" and "sellout music". Players combine musical influences from Coltrane's "sheets of sound" and Miles modal experiments with Hendrix and Sly Stones Rock and Funk Electric Instrumentation to create Fusion.
This is all refuted in the 80's by Wynton Marsalis....

New traditionalism and the return of the Great American Songbook as a resource for players on CD and live performance returns.
I could go on....What has always happened is that Music and most Art combines Thesis and Antithesis to create Synthesis.
I personally think everyone should listen to the entire Smithsonian Collection of Classic Jazz at least once in their lives-I had it on vinyl and most University and Public Libraries should have it somewhere.
It's interesting and kind of poignant that we're debating/talking about this evolution on a forum for an instrument that Mainstream Music and Jazz Education didn't believe was a real instrument in both Jeff's and my lifetimes.