Harry Christophers referred to the emergence of polyphony as a "big-bang moment", to be compared with the first performance of Le Sacre du Printemps; Simon Russell Beale's narration talked about the shock and awe caused by the emergence of this new music.
Sadly I have no access to BBC4 where I'm located. I am sure Christophers is right about the "big-bang" idea, although in fairness it did have a rather long fuse... there's polyphony traceable from the end of the C9th, although the manuscript examples of it are tantalisingly few. The "Notre Dame" school was certainly a hothouse flower... how far its influences reached geographically are hard to guess with certainty, but one thing helped it in a way that no previous music had enjoyed... accurate, usable notation on staff-lines. The preceding generation of music was largely notated in "open field" neumes which didn't have specific pitches... they were more of an aide-memoire, and reminded you where the tune went "up" or "down", and "by a lot" or "by a little". But you couldn't sight-read unknown music from them.
Oddly enough I am in the middle of a "gothic-era notation" project myself at the moment (one of my Univ special topics from years ago, but luckily I still have the textbooks
- it's music of extraordinary quality and complexity
