These two legends, entirely separate in origin, have only recently been linked together by Nancy Lieder, a self-declared psychic who claims she is channeling aliens.
"The story started with claims that Nibiru, a supposed planet discovered by the Sumerians, is headed toward Earth. This catastrophe was initially predicted for May 2003, but when nothing happened the doomsday date was moved forward to December 2012. Then these two fables were linked to the end of one of the cycles in the ancient Mayan calendar at the winter solstice in 2012 -- hence the predicted doomsday date of December 21, 2012." - http://www.nasa.gov/topics/earth/features/2012.html
The Truth about 2012 from NASA Lunar Science Institute on Vimeo.