I wondered about this, so I asked a virus who was hanging out in my nasal passages for awhile.
"Well, you see," it explained. "Some theorize that life exists on different sizes and scales to the extent that one organism may occupy the same region of space and yet not be aware of the existence of the other." There was a chorus of agreement from some bacteria in my mouth. "Anyway," the virus continued. "If a completely alien species were to attempt communication with you, how would you know? How do you know they haven't already made the attempt?"
"That's ridiculous," I argued. "I'm a member of the dominant species on this planet! Of course we would know!"
"Right," drawled a mitochondrion sarcastically. "Of course you are!"