Are we alone?

That's the question posed by Iain Murray in this fascinating, if depressing, article. His discussion about the odds of there being many other potentially life-supporting planets out there seemed to me to overlook one interesting point. We have another planet in our own solar system that may have supported life at one time: Mars. I'm talking about single cell life, not Marvin the Martian, but the very fact of Mars' existence has to undermine an argument about how the odds are against there being other Earth-like planets out there. After all, the odds of there being two life-supporting planets in the same solar system can't be good. Am I missing something? It's at least something to think about.