Originally Posted by BacteriumFendYoke
The rules of probability give a real probability of there being life on other Worlds other than our own. The sheer size of the Universe and the number of stars similar to our own combined with what appears to be a high incidence of exoplanets around most stars demonstrates a statistical likelihood of life. That even assumes that all life is like the life on Earth (carbon-based, etc; although carbon-based life is likely the most prevalent due to the chemistry of carbon).
Combine that with the latest findings on Mars - evidence of water flowing and organic chemistry in the rock and evidence of water on mars, as well as theories on Europa, I think there's a real chance that not only is there life on Earth but there was life in other parts of the Solar System at one point or another.
Interstellar distances are the real killer though when it comes to UFO arguments. As for intelligent life? Well, I'll quote Monty Python:
Not only does immense distance separate worlds, but immense stretches of time. Sentient life has existed on earth only for a tiny fraction of its existence and we may never attain interstellar travel. Sentient species may pass each other in the night, out of synch by millions of years. That's just a blink in the time span of the universe, but serious evolutionary time.