This thread and some of the comments herein thus far touch upon something that I've been thinking about, namely, that if our planet was designed for life in some precisely 'fine tuned' way, why has Earth been struck by asteroids/comets in the past, and why is there still that risk? This seems to me to be a product of a random set of circumstances, not a precisely fine-tuned system.
In its early history, sure, there would be a functionality in comet/asteroid strikes, such as depositing water and minerals here. But now that life has formed here, and the risk of such impacts still exists, it seems to me to indicate randomness rather than purposeful design - why would a Designer not eliminate all risks to his design? Why, in a finely-tuned system, can one fateful strike by a large celestial object wreak havoc and threaten all life on Earth?