[TheDude]

Read about it in brief at the http://en.wikipedia.org/wiki/Rare_Earth_hypothesisWikipedia article. There's a book too, Rare Earth: Why Complex Life Is Uncommon in the Universe, by Peter Ward and Donald Brownlee.
This theory addresses matters like the Fermi paradox, concerning aliens: Fermi wondered, "Where are they?" Answer is that complex life is so unlikely that you'd be hard pressed to have two examples of sentinent species in existence at once.
The notion that we are perhaps unique at the moment as intelligent beings in the visible universe - which is absurdly huge, read about the scale of distances in space - I find very heartening. Are we truly it - perhaps the only life in the universe's history which has become aware of itself? Isin't that cause for celebration - for change - to give us a reason to exist? Why carry on, anyway? We could all prepare, powerdown, what have you, go back to living in the 18th century; could we really do that for another millenium? Won't our descendants simply repeat the mistakes we have made - cruelty, genocide, brutality? Certainly the fact that we know so much about history doesn't prevent us from doing so now.
I mentioned another book here, Marshall T. Savage's The Millennial Project: Colonizing the Galaxy in Eight Easy Steps, which invisaged civilizations building colonies in space, growing lots of greenery. Ultimately they'd surrounded their star in a swarm of these transparent bubble colonies, and all of the green would filter the star's light. His rather cocky explanation for the paucity of extraterrestial life was that we don't see any green stars! Savage has lots of very novel ideas for colonizing Earth and space, which are far removed from conventional images of space habitats as huge boxes/rocks, or getting into orbit with rockets - he suggests using ground-based lasers and ice.
All that said, humans being the maniacal grubby materialistic primates they are, we'll run the planet into the ground soon enough via GW or MAD or viruses or. We've already done a number on other species that'll show up in the fossil record like the K-T event. How do we leave our signature for the next batch of brainy animals (if it ever happens again?)? With strip mining?