Lets just say, i believe it is a reasonable assumption to make, that life exists elsewhere
Yep, I agree that this is a reasonable assumption.
and has been here before.
I don't think this is reasonable at all.
Even going on the distances involved it's a wild assumtion to say alien life has been here.
Unless we're getting into the intelligent-alien-life-in-this-solar-system thing, which I find somewhat far fetched to say the least, if there was a habitable moon orbiting our nearest (non solar system) planet, it would take prohibitively long to get from there to here.
The nearest place which might even concievably hold life is 10.5 light years away - unless we consider daft sci-fi notions like warp drives and assume some alien species had managed to get fissionable material safely into space (they would face the same problems with that as we do here on Earth in that repsect) and develop fission based propulsion, it would take them well over 100 years to get here.
If they set out as adults they would have to live an incredibly long time to get here, communicate with us and get back with the information.
Plus all the ancient structures etc that we could not even build today.
Such as?
I'm going to guess you mean the pyramids, we most certainly could build those today with the same available metals, tools and manpower (that last point is key).
The perfect mathematical allignments of the pyramids of Giza and the mayan and aztec temples.
Perfect allignment is not unattributable to a purely human endeavour.
For example in your opinion who do americans pay taxes to? Who owns America? These answers are not public knowledge yet they are true none the less.
Assuming you mean the federal income tax... they pay taxes to a privately owned (non-federal) bank called the Federal Reserve Bank. That same bank owns America, issues its (debased) currency and has done for many years now.
Unless you're hinting at some crazy conspiracy theory of the type championed by Icke.
While I'm completely open to the idea of intelligant extra-terrestrial life, the idea it has been to earth and interacted with us simply does not make sense. I've yet to see evidence of alien interaction with humans or alien activity that could not be attributed entirely to human activity and endeavour.