What is the evidence for the Overkill Hypothesis?
Martin's argument works by a process of exclusion. For example, he doubts climate change had anything substantial to do with the megafaunal extinctions because there is no climate change indicated in the paleoclimatological record that could have driven so many taxa to extinction 11,000 radiocarbon years before present (rcyrbp). The same is true, he says, for phyletic replacement, faunal turnover, and other alternative mechanisms as the primary cause of Late Pleistocene loses.
According to Martin, there is a constellation of features of the Late Pleistocene extinction record (he lists eight) that only make sense when direct impacts by humans are viewed as the fundamental cause of the massive losses of large mammal species. The critical features are as follows:
1. The extinctions were sudden.
2. Extinctions followed the spread of humans.
3. Large mammals were decimated, reflecting human hunters' selection.
4. Large mammals survived in Africa, where humans and megafaunal had co-evolved. Therefore, African mammals knew humans as dangerous, and had developed antipredator behaviors against them.
5. Extinctions occurred without the appearance of new species immigrating from elsewhere. Thus the end-Pleistocene extinction of North American mammals was not the result of competition by exotic species coming in over the Bering land bridge.
6. Kill sites testify to the slaughter of large mammals by human hunters. Although few of these sites have been identified, Martin argues that "few (sites) would be expected if man's impact were truly swift and devastating." His reasoning is as follows: (1) fossilization is an unlikely event at the best of times; (2) even though carcasses of hunted mammals would have been present in huge quantities, if the extinctions occurred very rapidly no more than a small fraction would be buried swiftly enough to be preserved; (3) thus the 15 sites that have produced evidence for mammoth hunting in North America, for example, should be seen not as evidence of rare hunting, but rare preservation.