Ever notice the massive number of shared lines stretching across blues music, part of what the liner notes to one album call "a common pool of verses"? That it's possible to (potentially) trace the development of certain songs from others (or perhaps along parallel paths). Case in point, Muddy Waters's song "I Can't Be Satisfied," in which one finds an almost identical line to the one in Blind Boy Fuller's "Pistol Slapper Blues": "I feel like slapping my pistol in your face/ Gonna let the graveyard be your resting place." Or the standard Muddy Waters tune "Rollin' and Tumblin'" which intersects a number of times with Robert Johnson's "If I Had Possession Over Judgment Day". Or a number of Robert Johnson's own lines, which crop up in tunes by various artists, even outside the blues genre (one verse in the doggerel "They're Red Hot" is nearly identical to another from Woody Guthrie's "Take a Whiff On Me").
An article I read for a class last semester discussed the blues form as a folkloric tradition. The author pointed at the "common pool" of lines, riffs, verses, etc in blues music as an illustration of the folk process: one song spawns another, which spawns another, or two similar ones, or a song referred to by the same title but with innumerable possible variations, making the result a distinct product of the artist performing it. Which, of course, no more points to there being one single "original" or "pure" version of the song "Rollin' and Tumblin'", but certainly presents variations from one artist to another, or the sharing of lines between one tune and another, in a different light than the one in which we might usually see it.
Speaking of "R&T", anyone note that Dylan has done a new version of it for his latest album?