DarkSyde brought up the question of extraterrestrial life, which is made problematic by the Fermi Paradox. The jist is, if there if supposed to be an abundance of intelligent life in the galaxy, as predicted by the Drake equation, and with that number increasing as we begin to understand the frequency of extrasolar planets... well... where are they?
I think the answer to his question is dismally simple--they're dead.
The key factor in the Drake equation is that nasty "L" parameter, the one specifying how long a civilization lasts. We, to cite the only example I have, have been producing radio waves for only a century (including great big bursts of radio waves that appear when we blow up nuclear weapons--probably a huge mark against us for anyone who might be out there trying to guess if we are still around). And yet, we are dangerously close already to killing ourselves off before we even have a change to get off this rock. And even as dire as things are right now, we've still exceeded the value for L predicted by Sagan and Drake. How much more time do we really have?
If you read further into the Wikipedia article on the Drake equation, you'll come to the section on how this sort of self-destruction might be typical for intelligent life. Resource hungry beings who have develop resource-extracting technology might not be able to overcome their evolutionary instinct to glut themselves at every turn, leading to an inevitable ecological and/or social collapse. If that were the case, and L is a very low number, we wouldn't see civilizations taking over every corner of the galaxy. We would see what we see right now. Nothing.
I'm willing to believe other intelligent civilizations have existed. But they're all dead. And we may be next.