Anyone familiar?
Using physical evidence and the myths of ancient civilisations, those study and believe Ancient Astronaut Theories think that at some point in history, life has been intervened by extra terrestrial intelligence.
Their examples include:
Egyptian pyramids
Stone Henge
Easter Island
Ancient Baghad Batteries
Nazca Lines
The idea of these examples is that the technology would've been too basic for the civilisations of that time to create said items.
Now, I'm not saying I believe or don't believe in this theory, but it's one hell of an interesting read.
http://en.wikipedia.org/wiki/Ancient_astronaut