There's a possibly apocryphal story in Britain that the two-fingered version of the gesture originated at the Battle of Agincourt in 1415. British archers were renowned for their deadliness, so when the French captured one they would cut off his index and middle fingers on his right hand (the ones he used to draw the bow string). Supposedly, during the battle the British archers waved these two fingers at the French as a symbol of defiance, which is where the gesture came from.

I'm afraid I don't know how it evolved into the one-fingered version.