I think aliens may have visited Ancient human cultures. I just don't think Ancient Egyptians could have built those pyramids using the simple tools described by mainstream archaeologists. Some alien technology may have been involved.
How, you might ask, did these aliens show up and then scrub Ancient documents of detailed accounts of their presence? Well, first of all, we can't be sure of what form these aliens took to make their presence felt by Ancient people. Maybe they came in human form to make us more comfortable. Maybe some of the accounts are in ancient documents, but humans simply fell short of describing things that were beyond their imagination. Perhaps more likely, these aliens, who can presumably surpass or bypass the cosmic speed limit of light, can--as far as we're concerned--do whatever the fuck they want!
Why, you may ask, would aliens care to help us build massive pyramids without correcting, for example, the taking of slaves or any other ancient social ill? Well, if we found a primitive culture on another planet, would we care to do anything other than kick-start its technological progress and peace out? For an alien species, handing down a moral code to us, especially one that we'd find unnatural, might simply be a waste of time. After all, we don't waste our time intervening in the petty quarrels of rival lions fighting over a lioness. Heck, sometimes we don't even stop massive genocide within our own species (think Darfur and Rwanda).
The moral of the story: Aliens would certainly think strategically when interacting with other life forms. They may have simply helped us out in some simple way that was meaningful to us (and maybe to them) without incurring much cost to them. Also, maybe the aliens didn't find it practical to leave us with tangible AND unequivocal evidence of their visit, because--after all--we would have still been far from any advancements that would allow us to initiate communication with them on their home planets. In fact, we still are. Also, perhaps the aliens didn't want to prop themselves up as idols on our planet, which could actually impede the progress that they wanted to inspire in the first place. After all, if someone thinks that great works can only be the result of magical technology from a higher being, that person may well be resigned to lazy adoration rather than dedicated to industrious labor.
On this Fourth of July weekend I leave you with the thought that our independence, while utterly important within the proper context, is really only an illusion. We are connected to other matter and energy around us in ways that we can't fully understand. I don't mean this in a New Age hippie kind of way; I simply mean that our human notions of self-determination, self-reliance, individualism, etc. are trumped scientifically by our inextricable connections to the world around us. No one is "self-made"...and--forgive the cliché--WE ARE NOT ALONE!