I thought of this because of an article from Stephen Hawking in the Times recently in which he stated that we shouldn't talk to aliens for risk of their coming to get us.
He stated that it was pretty likely that intelligent life exists out there someplace.
My problem with this is not so much that we shouldn't be attempting to contact them, but rather that the likelihood of intelligent life coexisting in the same time and approximate space seems incredibly unlikely.
Here's a serious question. If intelligent life had visited earth in the past, would we know? The answer seems obvious until you do the math. In the 4.55 billion years the earth has been in existence, life has been on it about 3.4 billion. Of that 3.4 billion, the genus homo has only been in existence for 2.5 million (with an M). Of that 2.5 million years, we have only had the ability to pass generational knowledge for about the last 10,000. The universe itself is over 13 billion years old.
Considering the VAST stretch of time in which intelligent life is possible, the likelihood of it occurring coincidentally inside a time slice of 10,000 years and within proximity to contact one another seems rather slim.
If we wanted to leave a lasting monument to human existence for an alien visitor some 2 million years from now, how would we go about it? Nothing we have ever constructed will be around then. What could we possibly leave that they would find and identify as evidence of intelligence and existence? If we were to visit a barren rock that would, in 2 billion years time develop intelligent life of an unpredictable form, how would we leave evidence of our visit? Why would we bother?