Hoping we are alone in the universe

The ever-wonderful Nick Bostrom, who to my knowledge was the first to argue scientifically that we are overwhelmingly more likely to be living in a computer simulation than otherwise, has a short essay on his hope that we are alone in the universe. The hinge of his argument is the existence of a "Great Filter", some step along the path from planetary formation to the emergence of intelligent, space-colonizing life requiring such an improbable amount of luck that it almost never occurs. If life existed on Mars or elsewhere in our cosmic backyard, then this Great Filter is most likely to remain ahead of us.