I’m not certain why we’ve seen no evidence of alien civilizations.  One good guess is that the speed of light really can’t be broken, so to have a civilization out there means very slow transit.  But if that were true, why isn’t the sky lit up with old radio messages?  Or maybe there’s just something better than radio.  I don’t believe that technology runs away in every case and destroys intelligent life.  But perhaps there’s some kind of technological threshhold before interstellar spaceflight which constitutes a transcendence, as in Vernor Vinge’s A Fire Upon the Deep and other far-flung sf epics.

I do think that there’s at least a chance that a lifeform bent on destroying all competition is the dominant entity in the galactic neighborhood.  Cheery thought, but wouldn’t they be the most likely to survive?  In a very short time, we’ve dominated and subverted every other lifeform on our planet.  And we’re not even evil.  Heh.

Enjoy nature while you can, kids