The common theme in such theories is that aliens have been among us for decades, centuries or millennia, but a government cover-up has protected the public from knowledge of ancient astronauts and an alien invasion.
Motivated by speciesism and imperialism, these aliens have been and are secretly manipulating developments and changes in human society in order to more efficiently control and exploit it.
In some theories, alien infiltrators have shapeshifted into human form and move freely throughout human society, even to the point of taking control of command positions in governmental, corporate, and religious institutions, and are now in the final stages of their plan to take over the world.
A mythical covert government agency of the United States code-named Majestic 12 is often cited by theorists as being the shadow government which collaborates with the alien occupation, in exchange for assistance in the development and testing of military "flying saucers" at Area 51, in order for U.S. armed forces to achieve full-spectrum dominance.
Life that does not originate from Earth and sometimes described as extraterrestrial biological entities (EBE). It is unknown whether any such life exists or ever existed in the past, although many scientists think that life either exists or has existed, for instance, on Mars.
Various claims have been made for evidence of extraterrestrial life, such as those listed in a 2006 New Scientist article, which the magazine describes as "hints" rather than proof. A less direct argument for the existence of extraterrestrial life relies on the vast size of the observable Universe. According to this argument, endorsed by Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth.
Theoretical physicist Stephen Hawking warned that humans should not try to contact alien life forms. He warned that aliens might pillage Earth for resources. "If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans," he said. Geographer Jared Diamond has also expressed similar concerns.