That makes for a good sci-if movie.

Well, if there ever were advanced beings from outside earth, then they probably would have left evidence of their arrival behind. Unfortunately there is no evidence that jumps out--besides the weird stories and pictures ancient people made. If there were microbes inside and on the skin of the aliens, as there are in humans, then wouldn't those microbes have been left behind on earth and then be DNA testable? If such microbes were tested, either the combinations of the nitrogenous bases would be 100% untraceable to any other kingdom of life, or the chemical structure would be different; it would be conceivable that microbes originating on other planets wouldn't even have DNA or ATP or even proteins as we know them. Also, wouldn't aliens have to defecate at some point on the surface of the earth. Would there be evidence of ancient alien garbage deposits or rare metal mines from thousands of years ago? There simply is nothing which makes it seem as though alien theorists underestimate the imagination of humans, unless the aliens were painfully careful not to leave any evidence behind, knowing full well that humans in the 19th and 20th century would develop advanced chemical and forensic capabilities.

makes sense