I think it can be likened to the way the English language has evolved. You can separate the Old English (Shakespearean type language) from the post-modern English where everything has been cut down to the most convenient, simplistic (in some cases phonetic), "easier" to use forms. It's still the same language, like you say "one line" and a distinction can be made between the old and new but they are still one and the same language. Heck think of the way Science has evolved in the last 400 years, the same time-frame as Music and English within the context of this discussion, some things were at some stages considered to be "evil" and "blasphemes".. and now we have mind-blowing theories that incorporate God into science itself.

Pretty amazing really.. and yet so simple at the same time.