This looks even more odd because the Earth is the only astronomical object known to possess life. So at least as of today, humans are the only intelligent species in the entire (known) universe.
I queried this on Google, and interestingly, so many people have already asked this question!
Update (9-Jan-11): A related question that just struck me - what would happen if there are two or more intelligent species on our planet? Would they fight against each other? Would they be able to live in harmony? Is it possible that there was at least one other intelligent species on the Earth, but the early humans killed it? When many animals species are physically similar to each other in so many ways, why has intelligence not spread to any other species?