American Scientist have published a wishy-washy article debating whether humans or climate change caused the extinctions of megafauna at the end of the Pleistocene...

High-impact human hunting, referred to by archaeologists as "overkill," and climate change are the two most cited possible causes of the extinctions, but the role of each remains contested.

To me it is quite clear. The number of beasts that died in North America (millions) could not have been caused by humans, the ratio of kills per probably newly arrived human is too great. And climate change happens all the time... In my opinion a global cataclysm fits the bill - large creatures are more vulnerable when things topple over. Little creatures can hide in nooks and crannies. If scientists focussed on the size factor, and why it made a difference, they might one day arrive at the cataclysm theory as well.

Source: American Scientist