See this article on Fermi's Paradox. This is the reply I (John Schmidt) posted here on 4/9/05: If 1) intelligent life evolved on the order of 1,000,000,000 years ago and 2) that intelligent life and its offspring continued to exist for the past 1,000,000,000 years and 3) some fragment of those offspring reached Earth long ago and 4) we have not found evidence of 1-3 (above) and 5) those offspring are still on Earth then what are the possible explanations of 1-5?