I once read a book called Rare Earth; http://www.amazon.com/Rare-Earth-Complex-Uncommon-Universe/dp/0387952896 http://www.amazon.com/Rare-Earth-Complex-Uncommon

I once read a book called Rare Earth; http://www.amazon.com/Rare-Earth-Complex-Uncommon-Universe/dp/0387952896 http://www.amazon.com/Rare-Earth-Complex-Uncommon-Universe/d... and the authors argued that the Earth has many unique circumstance necessary for the development of intelligent animal life that such life is probably exceedingly rare in the universe. The Earth/Moon and its stabilizing effect on the orbit, Jupiter the gas giant that protects the inner planets from asteroids and the development of an oxygen atmosphere, etc. and much more.