From Infidel753:
North America, too, once teemed with elephants, camels, lions, giant sloths, mammoths, and other huge creatures. After thriving for millions of years, they too suddenly became extinct around 12,000 years ago. It’s been suggested that this had something to do with the end of the last ice age, but these animals had come through many previous ice ages just fine. 12,000 years ago also happens to coincide with the arrival of the first humans in the Americas. In this case we actually have, for example, mammoth skeletons with spear-points embedded in them, suggesting the real reason for the mass extinction.
Why were these animals so easily killed off by primitive hunters, while their Sub-Saharan African counterparts were not? The evolution of human intelligence, and thus of highly lethal weapons and strategies for hunting, was a slow process spanning millions of years. As proto-humans gradually grew smarter and more dangerous, the animals of Sub-Saharan Africa had time to adapt. To a lesser extent the same was true of animals in southern Asia and the Mediterranean world, who coexisted with proto-humans for up to a million years.
The native animals in Australia and the Americas had never needed to adapt to this danger, which did not exist in their environment. Then, just 40,000 and 12,000 years ago respectively, they were “suddenly” (relative to evolutionary time-scales) confronted with a fully-developed threat for which evolution had not prepared them. They probably felt no fear of the puny-looking new creatures. Long before evolution had time to breed that fear into them, the little newcomers wiped them out.