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The SDMB’s favorite nonfiction book, Guns, Germs, and Steel touches on this.
As they co-evolved with the ancestors of modern humans (and associated relations), the successful large mammals in Africa adapted behavior that included a healthy fear of the brainy bidepal apes with the sharp sticks and projectiles.
Most of the large mammals in North America did not have time to develop this behavior before fully modern humans arrived on the scene 12-14,000 years ago and rapidly hunted them to extinction.
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It’s doubtful and highly disputed that humans “hunted them to extinction”. After all, humans have been around in asia for millions of years too, and as the OP pointed out- few large mammals. Next, N. America had billions of Bison, and the natives had hardly put a dent in the population. Youd think that if humans had hunted the giant sloth and the mastodon to extinction, they’d have gone for the easier to kill and more useful bison first.
Next- hunting rarely drives any species to extinction (hunting drives species to *near * extinction, instead). Go ahead- name a few large, successful mammalian species that humans have driven to extinction by hunting in the last few hundred years- after modern firearms made killing so much easier.
Here’s an abstract of an article about Australian megafauna extinction:
http://www.blackwell-synergy.com/doi/abs/10.1046/j.1466-822X.1998.00285.x?journalCode=geb
"The worldwide extinction of large terrestrial mammals in the late Pleistocene has been attributed to climate change, overkill by human populations colonizing new continents, or some combination of these two processes. Lack of precision in chronologies for human colonization and mega- faunal extinction suggests that the role of overkill will be difficult or impossible to resolve from the archaeological and fossil record. In this study, we used a simple two species predator-prey model to examine the population density and hunting efficiency of a colonizing Aboriginal population necessary for overkill to have led to extinction (equilibrium density=0), of a range of different sized megafauna (250–1000 kg bodyweight) inhabiting an hypothetical tract of north Australian Eucalyptus savanna. We referred our modelling to this region because estimates of contemporary Aboriginal population density arguably set an upper limit for those occurring during the Pleistocene, and data are available on harvesting rates of large introduced mammals such as buffalo. Modelling results indicated a compensatory trade-off between the effect of Aboriginal population density and hunting efficiency (measured as the rate of effective search for prey) on equilibrium megafauna densities. The modelling also demonstrated that although intrinsic rate of population increase and density at carrying-capacity for mammals increase with declining body size, smaller megafauna would have been more readily exterminated than larger megafauna because they would have been harvested at a higher rate to satisfy the demand for meat. However, while at contemporary Aboriginal densities it appears plausible that over-harvesting could have affected the fate of smaller megafauna (<250 kg), hunting efficiencies required to drive medium (500 kg) and larger (1000 kg) sized megafauna to extinction appear unrealistically high. This conclusion would be reinforced if Aborigines were able to switch prey at low megafauna density, a contingency not considered in our model. Collectively, these results suggest that for overkill to have driven medium to large marsupial megafauna extinct in the northern Eucalyptus savannas, Aboriginal densities must have been considerably higher than contemporary levels, or Aboriginal hunters astonishingly efficient. Alternatively, climate change in the late Pleistocene may have played an important role in the extinction of Australian megafauna."
Here’s more:http://www.pnas.org/cgi/content/abstract/99/23/14624
"*Understanding of the Pleistocene megafaunal extinctions has been advanced recently by the application of simulation models and new developments in geochronological dating. Together these have been used to posit a rapid demise of megafauna due to over-hunting by invading humans. However, we demonstrate that the results of these extinction models are highly sensitive to implicit assumptions concerning the degree of prey naivety to human hunters. In addition, we show that in Greater Australia, where the extinctions occurred well before the end of the last Ice Age (unlike the North American situation), estimates of the duration of coexistence between humans and megafauna remain imprecise. Contrary to recent claims, the existing data do not prove the “blitzkrieg” model of overkill. * "
In other words, actual scientific results show that the “overkill hypothesis” is wrong.
http://www.sciencemag.org/cgi/content/abstract/306/5693/70
*One of the great debates about extinction is whether humans or climatic change caused the demise of the Pleistocene megafauna. Evidence from paleontology, climatology, archaeology, and ecology now supports the idea that humans contributed to extinction on some continents, but human hunting was not solely responsible for the pattern of extinction everywhere. Instead, evidence suggests that the intersection of human impacts with pronounced climatic change drove the precise timing and geography of extinction in the Northern Hemisphere. The story from the Southern Hemisphere is still unfolding. New evidence from Australia supports the view that humans helped cause extinctions there, but the correlation with climate is weak or contested. Firmer chronologies, more realistic ecological models, and regional paleoecological insights still are needed to understand details of the worldwide extinction pattern and the population dynamics of the species involved. *
Three things led to the megafauna extinction: Climate change (the same climate change that allowed humans to spread), new species (of which humans were one, sure, but many other species came along- including our friend the rat), and new diseases spread by those new species (which I guess you can consider another form of “new species”).
This article puts the nail in the myth of the “Overkill position”
http://www.unl.edu/rhames/courses/current/requiem-overkill.pdf
"The arguements that human hunters were responsible for the extintion of a wide variety of large Pleistocene mammals emerged in Western Europe during the 1860’s… Today the overkill position is rejected for wetsern Europe but lives on in Austrailia and North America. The survival of this hypothesis is due almost entirely to Paul Martin… In North America, archaeologists and paleotologist whose work focuses on the late Pleistocene routinely reject Martin’s postiion for two prime reasons: there is virtually no evidence that supports it, and there is a remarkably broad set of evidence that strongly suggests that it is wrong. In response, Martin asserts that the overkil model predicts a lack of supporting evidence, thus turning the absences of empirical support into support for his beliefs. We suggest that this feature of the overkill position removes the hypothesis from the realm of science and places it squarely in the realm of faith. One may or may not believe in the overkill position, but one should not confuse it with a scientific hypothesis about the nature of the North American past."
In other words, you can believe in Overkill/Blitzkrieg just like you can believe in God or the Invisible Pink Unicorn. There is no scientific evidence for any of them.