"[T]he world lost more kings of large land animals in the millennia around the end of the Pleistocene than at any similarly brief period for many millions of years, and in no areas were the losses as great as in the Americas and Australia. [...]
"... A number of scientists, most especially Paul S. Martin, have put forward a theory to explain the extinctions that has engendered great controversy among paleontologists, archeologists, and other experts, a theory that, if true, throws a strong light on the shadowy prehistory of the Neo-Europes. Martin points to a large body of evidence that there was a coincidence in timing throughout the world between the appearance of human big-game hunters and the demise of the giants, which were the most attractively large meals available. Where humans and giants had dwelled together for many millennia, as in the Old World, the latter had learned to be wary of the biped hunters, and many -- not all, but many -- of the larger animals survived well into modern times and even to our own day; elephants and lions in Africa, and elephants, tigers, wild horses, and camels in Asia. Where the large animals did not have the benefit of hundreds of thousands of years of adjusting to the human presence, as in the Americas and Australasia, the hunters were able to slaughter them in such quantities that as to eliminate most of them completely.
"The theory strikes some as outrageous. How could Stone Age hunters eliminate whole species, even genera, of such presumably dangerous animals? However, counter-theories of universal climate change (longer winters, drier summers, or what have you) seem even less satisfactory: There simply was no such change, not, at least, that affected the several parts of the world in question at the several and different times at which they lost their giants. And why would climatic change kill off large animals and not small ones? Perhaps the small ones needed less to eat, and so survived the poverty times better than the large ones. Perhaps, but the deus ex climatica theory has, at present, less evidence to shore it up than the overkill theory. Perhaps deadly parasites and pathogens, previously present only in the Old World, came into the Americas and Australasia with the hunters and the other creatures that arrived at the same time and by the same means. But why would these kill large animals but not small animals? We are back to the hunters as our best means to account for the disappearance of the giants."
* Martin 2005 p158-159
"Those who deny overkill often turn to natural causes to account for the extinctions. Such an approach has major problems. Detailed fossil pollen and macrofossil plant records are continually appearing for habitable parts of the planet and various dating methods, especially radiocarbon dating, are available for chronolgical comparisons. If large mammals throughout North and South America disappeared simultaneously around 11,000 radiocarbon years ago as a result of some extraordinary climatic shock, nothing similar is apparent outside the Americas, not even in the West Indies. Field evidence is even harder to uncover than kill sites. Where are the 'freeze sites,' if some cold shock wave is involved? Would such a shock kill off plateau- or montane-ranging species before they could descend to adjacent lower and warmer elevations, like the Mojave Desert in Arizona and California, watered by the Colorado River? In any case, to match the global 'deadly syncopation' of Ross MacPhee, the killer climatic change would have to have struck Australia long before the Americas and Madagascar, New Zealand, and the Pacific islands long after. If 'killer cold' exterminated large mammals in South America around 10,500 radiocarbon years ago, might we not expect some concurrent losses of megafauna in Australia, South Africa, and New Zealand?"
* Martin 2005 p157
"[E]ven if spears were used to kill, for example, ground sloths, some stone spear points would not necessarily be found in association with Shasta ground sloth bones. They would have been recovered and used again. This may be a partial explanation for the paucity of apparent kill sites of animals other than bison. As for the number of bison kill sites, perhaps these are attributable in part to the fact that bison were commonly killed in a herd or group. Or perhaps by Folsom time the location of quality stone for making tools was so well known that much less care was taken to recover point than in the days of the mammoth hunters."
* Stewart 2010 p9
"'In debating the role of humans in the late Pleistocene megafaunal extinctions, the problem is that they didn't really have the greatest technology for killing off these animals,' said Blaire Van Valkenburgh, professor of ecology and evolutionary biology at UCLA and co-author of the [Bioscience] study. 'But they didn't have to[;] they just tipped the balance that existed.'
"Rather than humans hunting the animals to extinction, William Ripple, the study's lead author, and Van Valkenburgh propose that by hunting various types of carnivores and herbivores, humans disrupted a delicate balance, triggering a collapse in the large herbivores and, ultimately, the carnivores that preyed on them."