Overkill hypothesis

The Quaternary period saw the extinctions of numerous predominantly larger, especially megafaunal, species, many of which occurred during the transition from the 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 part of the same event. Among the main causes hypothesized by paleontologists are natural climate change and overkill by early humans, who appeared during the and migrated to many regions of the world during the 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.

The Late Pleistocene extinction event saw the extinction of many mammals weighing more than 40 kg. The proportional rate of megafauna extinctions is incrementally bigger the larger the migratory distance from Africa.

The extinctions in the Americas entailed the elimination of all the larger (over 1000 kg) mammalian species of South American origin, including those that had migrated north in the Great American Interchange. Only in the continents of Australia, North America, and South America did the extinction occur at family taxonomic levels or higher.

The proportional rate of megafauna extinctions being incrementally bigger the larger the migratory distance from Africa might be related to non-African megafauna and Homo sapiens sapiens (anatomically modern humans) not having evolved as species alongside each other.

For their part specifically, Australia, North America and South America, which respectively had the highest incremental extinction rates, had no known native species of Hominoidea (apes) at all, much less species of Hominidae (greater apes), and especially not native species of the Homo subgroup (the genus Homo comprises the species Homo sapiens, which includes modern humans, as well as several extinct species classified as ancestral to or closely related to modern humans; N.B. all indigenous human groups are ultimately descendants of anatomically modern humans recently migrated out of Africa in anthropological time scale).

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