The book <i>Rare Earth</i> covers this topic throughly. The conclusion they come to is that complex, multicellular life is likely extremely rare in the universe. But simple, microbial life may be much more common than we think.<p><a href="http:&#x2F;&#x2F;www.amazon.com&#x2F;Rare-Earth-Complex-Uncommon-Universe&#x2F;dp&#x2F;0387952896" rel="nofollow">http:&#x2F;&#x2F;www.amazon.com&#x2F;Rare-Earth-Complex-Uncommon-Universe&#x2F;d...</a>