Could Aliens Have Already Visited Earth And We Just Didn't Know It?

Could it be possible that aliens have already visited our planet and we just didn't know it?

NASA says it might be possible.

Dr. Silvano P. Colombano said at a recent conference that "given the fairly common presence of elements that might be involved in the origin of life throughout the universe, it is a reasonable assumption that life “as we know it” was at least a common starting point, but our form of life and intelligence, may just be a tiny first step in a continuing evolution that may well produce forms of intelligence that are far superior to ours and no longer based on carbon “machinery”.

So essentially we should be looking beyond the carbon-based "standard" and consider that other forms of life might be out there.

What do you think? Do you think extraterrestrial life has visited the earth?