I've loved reading all the comments on this, and the mathematical geekitude, but I had another thought about the idea of us being alone: What if the universe is just far more malicious to life than we thought?

We are a tiny little solar system in one arm of one moderately-sized galaxy. And our little planet in our little solar system has already had at least one extinction-level event caused by an asteroid.

Recently, as our sensors have gotten better, we have seen not one, but two different extra-solar objects entering our system (RAMA I and RAMA II, according to kleinbl00), and I guarantee you that the first objects didn't just happen to show up when we build sensors strong enough to detect them... they've been whistling through our airspace (erm... space-space?) since forever.

What if there is just so much shit flying around and crashing into each other, that life only has the luxury of time to develop, out here in our distant suburbs; in areas that are protected in some way from the primary debris flying out of the chaotic center of our Universe?

Sure, the math says that there SHOULD be a gajillion planets out there that are habitable for things we would recognize as "life". But the practicalities of having a habitable environment, undisturbed and stable for long enough to develop intelligent human-equitable life, is a vanishingly small occurrence?

Think Seveneves but on a galactic scale, rather than a planetary one...

Hmmm....