Ancient Aliens

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So, let the debate begin!
In recent times, History Channel has changed History for speculation, pawn shops, taxidermy, conspiracy theory (even involving the founding stone of the White House in the Freemasons' conspiracy), the end of the world (brought to you, courtesy of the Maya Civilization (TM)) and more. However, letting aside those matters, there's one that called my attention remarkably: Ancient Aliens and their advocates, especially Giorgio Tsoukalos (or however it is spelled) who attributes almost any technological knowledge in Human Civilization to Extraterrestrial Intelligence of unknown origins. Although a large part of the series' premise can be explained with simple human evolution and its consequences, it remains a small part which cannot simply be taken as a consequence of our knowledge or invention. As crazy as it sounds, Tsoukalos is right in some parts, like the extended use of 'magic' in our folklore. 'Magic' being the term used to describe an unknown technology. Imagine taking a trivial device to most of us to uncontacted tribes or people, like a cell phone or a portable television, you probably know what it is like.
That and many more things that somehow fit more adequately in the Ancient Alien theory than just pure imagination (though that would be a more reasonable explanation). Tsoukalos is also right when he says that our ancestors were not stupid but fails to mention that they might have been naive. Anyway, an interesting topic of debate.
So, ancient aliens could be real, but, if that's true, why have they not contacted us? What happened to them? Why did they come here?
Food for thought!