Where does guitar improvisation come from?

[R_of_G]

I was shocked to find out BlueGrass is not an old rootsy form of music, but pretty much invented recently mid last century.

Prepare to be shocked again? That's inaccurate.

The term "bluegrass" to describe the style of music originated in the middle of the last century (likely due to the association with the great Bill Monroe). The music itself is a derivative of "old time" music and is most assuredly a very old form of American roots music.

Also, I don't mean to blow off the rest of your answer as you're clearly putting thought into this discussion. I'm just hung up on the idea of a course in "Rock n Roll" getting a piece of American roots music history so fundamentally wrong. I shudder to think what (if anything) the course says about New Orleans jazz.

All of which is to say I will process the rest of what you asked and try to answer that as well. I have to say, I do enjoy this thread. My two favorite discussions here this week have been about improv and Miles' 70s electric bands. The venn diagram on that one has a HUGE crossover section.