The X-Files states that an extraterrestrial life-form, known simply as the Colonists, were originally present on Earth in the early stages of human evolution. They highly resemble the well-known "grey aliens" in their mature form. In their immature stage, they are more yellowish colored, tall, and very aggressive, possessing fangs, claws and scale-like texture of their skin. This immature form is a protective stage, able to viciously defend itself from birth. This outer skin is eventually shed when the alien develops into its mature form.The aliens were forced to abandon the planet during the last ice age as their viral life-force is deactivated by extreme cold.
Upon their departure, they left behind underground deposits of the black oil virus, in preparation for their return. The virus apparently contains the aliens' genetic blueprints, awaiting reconstitution until the master species returns to Earth. Whilst away from Earth, the Colonists evidently sought out life throughout the universe in an effort to subdue other species and take over the universe.
Secretly, the Mayans KNEW that the Colonists planned on returning to Earth in the year 2012.
The Smoking Man reveals to Fox Mulder that the ancient Mayans were so terrified that they stopped their calendar on the exact date of colonization:
December 22, 2012.