There are also some interesting counter theories. This one (not to be taken seriously) is that all the Beatles EXCEPT Paul died between 1963 and 1966. Ringo actually had to be replaced twice during this time period. Part of the evidence is the line from "Hello Goodbye" (sung by Paul) saying "I don't know why you say goodbye, I say hello", where the dead bandmates are saying goodbye and he's saying hello to their replacement.
http://udel.edu/~mm/beatles/paulLives/paulLives.htmlThere's also a theory that the Beatles never really existed as a band at all and it was just a revolving door of lookalikes. The site says it's serious but I can't take it seriously.
http://thebeatlesneverexisted.com/