If we prove that life once existed on Mars, it is proof that evolution may not work (or else the life would have evolved into something to survive Mars' harsh atmosphere). But this means that life existed somewhere besides earth. So you have proof of extraterrestrial life, but proof that evolution may not be right.
Talk about a monkey wrench. Anyway, its just a theory I thought was interesting enough to share.
Oh, and don't get into a flamewar about evolution vs creationism, so flame off.