A new paper seems to assign part of the blame for the disappearance of large prehistoric North American mammals ("megafauna") to human overhunting and part to environmental factors.
(I am linking to Tech Central Station, but their link to the paper itself is not working for me.)
"The large carnivores became extinct because their large prey disappeared," said Harris. "The larger herbivores digest their food in one of two ways: ruminants have a multi-chambered stomach and chew their food several times, known as chewing the cud, whereas non-ruminants have less complex stomachs and the bulk of their digestion takes place through fermentation in the hind gut.
"When you review the herbivores that survived you'll note they comprise ruminants (bison, deer) and omnivores (peccaries). Horses, ground sloths and proboscideans are hind-gut fermenters that failed to survive."