Complex sentence: Among the main causes hypothesized by paleontologists are natural climate change and overkill by early humans, who appeared during the Middle Pleistocene and migrated to many regions of the world during the Late Pleistocene and Holocene.
Split into simpler sentences: Among the main causes hypothesized by paleontologists are overkill by the widespread appearance of humans and natural climate change. A notable human presence first appeared during the Middle Pleistocene and afterwards migrated to many regions of the world during the Late Pleistocene and Holocene.