Here is another argument that points to UFO-style aliens, Zetans, Annunaki and various other Whoozits-from-Space being the products of overly stimulated imaginations: They look like us.
A lot like us. Most of the aliens that PXer types describe are perfected, angelic ideals of the human form and most UFO abduction believers and whatnot describe aliens which are either the same idealized ubermen or big-headed large-eyed creatures showing features which are exaggerations of those of human infants. These beasties come from deep within the human brain, not planets orbiting distant stars. What do you suppose the odds are that beings evolved on another world would bear the slightest resemblance to humans? Sure, you might get the occasional biped with two eyes mounted on a distinct head-like extremity and possessing between two and four arms, but expecting much more congruence than that is expecting way too much. And don't give me any hogwash about the alien races "planting" humans on earth from their own stock. Genetics shows that humans are closely related to every living organism on this planet, from daisies to dormice, and in the nearly four billion years of life on this rock the only humanoid creatures ever to arise are our own ancestors and very nearly all of the creatures which are even vaguely anthropoid are our own very close primate relatives. Let's remember that evolution is not a "march of progress" but the adaptation of organisms to their environment through natural selection. Life is almost certainly plentiful in the universe but that IN NO WAY indicates that human-type intelligence is at all common. Earth's history actually argues against it. The vast majority of lifeforms have adapted and replicated and filled the seas and land with their progeny and exactly ONE genus (that'd be Homo, us and our immediate kin) has developed the kind of traits that lead to the building of spacecraft and the awareness of other worlds. There is no reason at all to think that human-style intelligence is even a survival trait in the long run. It may be in some narrow cases but intelligence of our sort is absolutely not needed for life to be completely successful. This is not to say that there can't be advanced technologically capable alien races, but there is ABSOLUTELY NO PROOF at this time of their existing at all, let alone visiting this backwater orb.

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If you can't dazzle 'em with dexterity, baffle 'em with BS.