Say what you will about Erich van Daniken, but I believe that he is probably right when he asserted that the most likely form for intelligent life to manifest is a humanoid, so we probably wouldn't know the difference between an extra terrestrial or a human unless we actually saw them climb into their flying saucers and take off into the night sky. Even then I'd make sure you check for U.S. Air Force badges on their shoulders before jumping to any conclusions
On another note...It just seems to me that if some benevolent group of extra terrestrials were really trying to give you the low-down on the reality of visitation on your planet...they wouldn't be selling books for $10 a pop.
Just a thought
