> if they could happen, they already would have An asteroid can impact with the earth... Oh wait, that's happened already. Luckily, we haven't had such a colli

> if they could happen, they already would have An asteroid can impact with the earth... Oh wait, that's happened already. Luckily, we haven't had such a collision large enough to kill us all, yet. A plague could kill many people... Oh wait, that's happened already. Luckily, we haven't had a plague with enough virality and lethality to kill us all, yet.