Got Aliens in Your Nose?

This week the Royal Society in London is holding a two day meet-up for scientists to talk about the state of our search for extraterrestrial life.

At a lecture today, astrobiologist Paul Davies of Arizona State University told the crowd that he thinks aliens already walk among us. Well, maybe not walk—more like float, or wiggle, or however else bacteria may locomote.
According to the Associated Press, Davies thinks that life from elsewhere in the galaxy has made its way to Earth at several points in human history. It's possible, he says, that alien life is "right under our noses—or even in our noses."

And why not? So many science-fiction writers seem convinced that if aliens of any shape or size were to come to Earth, they'll be bad for humans and hence immediately noticable. Giant robots! Predatory stalkers!! Killer pathogens!!! Yes, Michael Crichton, I'm looking at you.

But that certainly doesn't have to be the case.

For starters, consider the odds of an intelligent race of beings existing elsewhere in the universe.