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

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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”.

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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?