You can't have it both ways. If they happened before, who caused them. If not humans, who? If humans have both the ability to cause and stop global warming, then logic would suggest that we have the capacity to adapt to the climate, whatever it becomes. If we can't, if we aren't as smart and powerful as we think we are, then we become one of the "extinction events". You can't belong to both groups. Either we can overcome or we can't. So why worry?
Maybe the Mayans were right. Mayber next year is the end. I think I'm worried.