The
Quaternary period saw the
extinctions of numerous predominantly larger, especially
megafaunal, species, many of which occurred during the transition from the
Pleistocene to the
Holocene epoch. However, this extinction wave did not stop at the end of the Pleistocene, but continued, especially on isolated islands, in
human-caused extinctions, although there is debate as to whether these should be considered separate events or as the same event. Among the main causes hypothesized by
paleontologists are natural
climate change and overkill by
humans, who appeared during the
Middle Pleistocene and migrated to many regions of the world during the
Late Pleistocene and Holocene. A variant of the latter possibility is the second-order predation hypothesis, which focuses more on the indirect damage caused by overcompetition with nonhuman predators. The spread of
disease is also discussed as a possible reason.