What will destroy us in 2012?

Tyler Cowen says that the most likely world-ending event for 2012 is destruction of the Earth by space aliens.

That is silly. First of all, there is no reason for the aliens to wait for the end of the Mayan calendar, and no reason why the Mayans would know the aliens' plans thousands of years in advance.

Second of all, we have no evidence for the existence of aliens (much less world-destroying ones), while we have plenty of evidence for the existence of other world-shattering events. In particular, we know about asteroid strikes. We know their frequency, and we know their power. A very big one would certainly be capable of snuffing out the human race.

As for other possible world-destroying events, I doubt a single supervolcano eruption would be enough to finish us off; it wasn't last time, after all. A series of many supervolcano eruptions might do the trick, but as far as we know, these are only caused by (or coincide with) asteroid strikes.

As for other theories - a black hole, the end of the Universe, blah blah blah - there's no evidence for these being possible, so I discount them. If the human race is destroyed in 2012, it'll be a space rock that does it.