What did I miss? Why would we not be here without the moon?
If we didn't have an unusually large natural satellite to stabilize us, the Earth's orbital inclination would wobble drastically. IOW, our axis of rotation would wander all over the place, sometimes tilting by as much as 90 degrees, and back again, unpredictably. The climate, consequently, would be a chaotic mess. In the unlikely event that humans managed to evolve under such conditions, there'd be no chance that civilization could develop. So getting hit by that Mars-sized hunk of junk four-plus billion years ago is the best thing that ever happened to us, in a certain sense.
You know, it's really a wonder that we're here at all. Just amazing.
It's good to remind ourselves of that fact every now and then.