Unlike the large animals of Africa, the ones of North America didn't have humans gradually evolving so they could get used to the idea that humans were a danger to them......they were easy marks.
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however the mammoths of siberia and europe also disspeared during the same period. mammoths of these areas had long lived side by side with humans.
Not really....the cold areas of Siberia were populated pretty late; MUCH later than the rest of Eurasia.