The pure Clovis model has taken some hits, but I think, like some have said about vulgar Marxism, it's substantially a correct appraisal of the colonization of the America's. If some maritime populations pushed south along a coastal route a thousand or two thousand years before the main big game hunter gatherer culture, I don't think this really changes the picture as far as megafauna extinctions are concerned. It is, in the end, the latter group which is the prime suspect here.
As far as ground sloths are concerned, it's interesting to me that latest known mainland populations occurred in Chile, which presumably is the last part of the America's settled by the aboriginal Indians. And the most recent populations and species of ground sloth persisted in the large islands of the Caribbean, which weren't settled for another six thousand years after that. Incidentally, it may be I was off with the last occurrence of mainland ground sloths, they seem to have become extinct earlier than I thought, which simply pushes it back closer to the arrival of man:
Asynchronous extinction of late Quaternary sloths on continents and islands
David W. Steadman,*† Paul S. Martin,‡ Ross D. E. MacPhee,§ A. J. T. Jull,¶ H. Gregory McDonald,∥ Charles A. Woods,** Manuel Iturralde-Vinent,†† and Gregory W. L. Hodgins¶
Whatever the cause, it is extraordinary that dozens of genera of large mammals became extinct during the late Quaternary throughout the Western Hemisphere, including 90% of the genera of the xenarthran suborder Phyllophaga (sloths). Radiocarbon dates directly on dung, bones, or other tissue of extinct sloths place their “last appearance” datum at ≈11,000 radiocarbon years before present (yr BP) or slightly less in North America, ≈10,500 yr BP in South America, and ≈4,400 yr BP on West Indian islands. This asynchronous situation is not compatible with glacial–interglacial climate change forcing these extinctions, especially given the great elevational, latitudinal, and longitudinal variation of the sloth-bearing continental sites. Instead, the chronology of last appearance of extinct sloths, whether on continents or islands, more closely tracks the first arrival of people.
Link: <http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1187974>
Also, some intriguing computer simulation research on how low density hunter gatherer populations could have driven large, slow reproducing animals to extinction:
In 2001, John Alroy set out to test this theory using computers once again, this time using better programs and newer technology. The parameters he set included "slow human population growth rates", "random hunting and low maximum hunting effort." The results seem to show that the carrying capacity of each inhabited region of North America may have been over extended by the growth of human populations. As such the accelerated extinction of big game animals, the focus of Paleoindian hunters, would have been inevitable. The tests Alroy performed resulted in varying distributions of extinction times for the late Pleistocene, however the most likely scenario involved three waves. The first wave of extinction would have occurred within 1000 years of human arrival, the second in the next few hundred years and the third would have involved hunting the few groups remaining. Varying factors such as "human induced habitat change," "the introduction of pandemic diseases to native herbivore species by humans" and "selective human hunting of individual prey species" were missing from the model Alroy presented. He theorized that a model with these factors added in would result in faster extinction rates with slower population growth necessary. His results were discounted by Donald Grayson, however, who found that although Alroy's model closely matches the data used, he assumes that all of the extinctions fall during the period after the arrival of the Clovis and uses over hunting to explain their demise despite the lack of evidence of hunting of certain species.
Recently another computer simulation was performed. This time the model included the body masses for a range of 198 extinct and 433 surviving species from around the world, as well as parameters for "maximal replacement rate and equilibrium density of mega faunal prey populations, density of human populations, maximal rate of off-take by human hunters, and relative naivety of prey." The model showed that the larger the species became the slower their reproduction rates would be, in turn dwindling their numbers and making them less able to cope with rapidly changing circumstances and social disruptions. All of these factors would explain the swift extinction rate of the large mammals as human hunters moved across the continent. The populations would have taken more time to rebuild than the large quantity of mammoth bones at sites like Blackwater suggests it took to kill them in. With a few sites of 15, 18 and 13 kills each it's easy to picture how animals such as the mammoth could have been wiped out in 1000 years assuming they were slow to reproduce. In addition the offspring would have been easy prey to carnivores such as the saber tooth tiger, leopard and the flat faced bear, who may have become more aggressive killers in competition with man.