In my opinion it would be downright naive to think we were the only living beings. As lots of the other guys have said, we are inside a solar system which is part of a mind boggling huge galaxy, which is part of a universe with a mind boggling number of other glaxaxies, which is part of mind numbingly huge numbers of universes (that last one is a bit iffy I guess).
I pose this question in purely scientific terms: how did life start? how did dinosaurs come about?
If you think about it, there is every chance that on countless of other inhabitable rocks life came about as well, whether it be in a different form or the same form. Evolution could be much different or evolution could possibly be the same. My money would simply be on that slightly different climates, geographic properties etc would make each inhabited rock's beings as being different (what we call extra-terrestrials/aliens). These beings could have a greater level of primitiveness about them or they could be technologically superior to us - who can definitely say we haven't been visited by other beings? who is to say they aren't watching us? But at the same time, who can say the contrary?
I could keep talking about this all day, but this is just a basic idea I have formed, and as I stated earlier, I am of the opinion that it would be naive to suggest we are the only living beings (whether they be stupid blob like things, or like us, or superior super beings)