According to a Dec. 27 report by Wetpaint, Kirkman told The Hollywood Reporter that he once promised that his popular comic book series would feature an alien invasion. “When I pitched 'The Walking Dead' originally, it was turned down, simply because there had never been a successful zombie book in the history of comics," he said. "I wasn't willing to accept no for an answer, and so I said, 'Oh, well, I forgot to tell you that this is actually a big setup for an alien invasion.'" This crazy plot twist is what got him the green light.
Kirkman told Image Comics publishers Eric Stephenson and Jim Valentino that his comic would be about aliens using zombies to destroy all humans before taking over the Earth. So basically he just pitched the plot for "Plan 9 from Outer Space" to Image Comics. The publishers loved the alien idea even though it's completely ridiculous. Why would aliens use the undead to clear the planet when they can just use giant domes instead? For Norman Reedus's sake, hopefully the aliens stay away—Daryl Dixon won't look like such a badass if aliens eventually show up to take care of the people who are left on the planet (his crossbow is no match for a ray gun).
Now you've probably gotten all worried that "The Walking Dead" will jump the shark after the E.T.s arrive, but it sounds like that crazy "Ancient Aliens" guy shouldn't get too excited—Robert Kirkman isn't going to blame the zombie apocalypse on visitors from another world, so ancient astronaut theorists can cross at least one thing off of their ever-growing list of events that extraterrestrials were involved in. Kirkman promised that he never planned on actually incorporating the alien invasion—he just used the plot twist to trick Image Comics into accepting "The Walking Dead."
That being said, it's still possible that he could revisit the idea in the future. The virus causing the dead to reanimate seemingly appeared out of thin air, and it does seem rather alien in nature. The AMC show also doesn't follow Robert Kirkman's comic books exactly, so the zombie outbreak could have a different cause in each incarnation.
Would you like to see the alien twist, or are you glad that Kirkman didn't decide to go all "American Horror Story" by combining horror genres?