I read the previous two comments and I agree to an extent with both of them but since this is a topic i think about a lot I wanted to add a couple of things that bounce around my head. Please indulge
Previous to recorded music, music was created only by musicians. There was a standard repertoire (it varies depending on your culture) that was passed along to every new generation of musicians. Music was only made by musicians (not cd players) which made the musicians that were talented valuable to society. If you wanted to hear music you had to go where it was being made in real time (church, juke joint, concert hall etc...). This was also true of graphic art.
in our time you can buy a recording of louis armstrong playing "On the sunny side of the street" or Led Zepplin playing "stairway to heaven". You can also decorate your walls with fairly good prints of Picasso, Van Gough, or whoever tickles your fancy. The result of the changes modern technology has brought to our lives is that people are mostly interested in "original" art and music now. Why pay the guy down the way to play you a blues song at your birthday party if you can just pop Howlin' Wolf in the CD player and listen to that? True there were changes and evolutions to music and art in the past but they were more collective and slow and mostly came about as a result of genuine changes to peoples lifestyles and new cultural influences rather than because some genius came along and switched everything up.
That all said, I think that "originality" is and has always been about reinterpretation. Some of the most creative artist of today build songs by sampling older recordings (hip hop and electronica). The early blues and rock n roll artists were reinterpreting riffs and sentiments of older music. The boon of great music that came in the 20th century was more about technological advances (recording, multitrack recording, electric instruments etc..) than actual exceptional talent. I wouldn't worry too much about originality. I try to make entertaining and imaginative music based on my cultural frame and don't care a lick about being "original". I've also never had more than marginal success If people groove to it then it works. There are only 12 notes in the western scale.
Danny