It sounds crazy... but the chances are very high that he is right.
There's a line of thought that earth was seeded for life via comets and asteroids in our distant past.. there's no law or rule stating that each time we get hit with a meteor shower or comet trail we aren't taking in more random microbes...
That's assuming you believe how life was initially created on this planet.
What do you think the whole debate about the Allan Hills 84001 meteor is about?
Because if you believe that those are fossilised microbes
http://en.wikipedia.org/wiki/Allan_Hills_84001Then you are going to get into a debate within seconds on how life showed up here, considering those fossilised microbes pre-date life on earth ....
It sounds cooky... but the point is that there is a decent chance that life found it's way here... and not started here.. and ALH 84001 is the starting point of an entire shift in public thinking on how life appeared on earth.