just a question

I watched a show I believe last night or what date can not think. anyway It was one of these science shows and the jist of it was that comets was one of the thing that was changing the animal structure. 13000 years ago there were the Clovis people that were killing the mastodons(giant Elephants and this was at the end of the ice age. Yet the people was killing off the Elephants and that is what caaused them to disappear. Yet they are starting to say instead of people causing these giant animals it was a Comet that hit Earth. also there was an additional Ice Age 2000 years that came back. My question is how did the humans survive and supposedly humans were in North America some 20 k years ago. How did they survive? Just a question that there does not seem to be an answer for. It is strange the more one watches these shows and learns the question seem to come about. Don