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This isn't very accurate. Agriculture was not invented (as far as we know) before the start of the Holocene (around 10k years ago) (and modulo some South East Asian horticulturists). Since modern humans have been around for ~150k-~200k years, and in Eurasia for around ~50k years, the leading explanation for why we didn't invent agriculture/civilization till the Holocene - when we invented it numerous times independently - is that the climate was too unstable for us to stay in one place long enough until then. I don't know that this can be considered proven beyond a reasonable doubt (given that some of the possible evidence is under water and lots of mud). But still, given that no agricultural civilization has ever survived anything of the magnitude of a glacial-interglacial transition, the thought of abrupt climate changes should probably inspire some fear - though it's essentially impossible to quantify the likelihood of one in the 21st century.
Also, the methane forcing in Pleistocene deglaciations was less important than the CO2 forcings - "methane bomb" is rather misleading. And since there doesn't seem much reason to suppose the last deglaciation was any worse than previous ones, I'm inclined to accept the theory that early human hunters were the main factor in the mass mammal extinctions.
Certainly the Clovis people in North America lived for only a very short time (about 1000 years I think) before becoming something else-- 13000 years ago.
There distinctive trademark was the large bladed stone spearhead.
It appears that at the same time a number of the North American super-mammals were extincted.
This would be consistent with the impact of the Polynesians on New Zealand-- again a wave of extinctions, including the Dodo Bird.
I believe Diamond raises this point in Guns, Germs, and Steel. Clovis culture appears to be associated with the loss of North American mega fauna. The same loss of mega fauna is also found with hominid entry to the Australian continent.
Another interesting text is Sea of Slaughter by Farley Mowat. He documents the decline of all species in the Gulf of St Lawrence area with the arrival of European settlers.
Current research is discounting hunting for these extinctions. Not enuf bone damage on found carcasses. It is likely that the food chain was disrupted by the turning of the mass grasslands North of 60 to tundra. Large parts of yukon and alaska known as Beringia were ice free for much of last ice age. Mammoths starved. Saber tooth had no prey. etc etc.
References?
http://www.beringia.com/02/02mainb.html
Stuart et al, click on the "research notes" with each link. Unfortunately most of these were written in 95/96 and have not been updated with current research being shared at local seminars and lectures ... mostly by univ of alaska & univ of calgary paleantologists.
Of all the beringia fauna, wolf has adapted best over time. Much larger now due to absence of predators and new status of near top of the food chain.
With GW, deer are penetrating our are deeper. And that is bringing larger numbers of cougars.
Interesting research presently is why do sabre tooth cats have sabre. Mammoths don't have pierced neck bones. Slashing abdomen?
DNA et al testing allows new diet info by analysis of compounds in ancient teeth.
If i can get a Powerpoint from lecturers, i'll post it another time.
I believe there's still a very active debate on this subject with neither side having convinced the other. The Wikipedia has a fair summary. I expect there's some of both going on, but I find the idea that the extra ecological stress of humans was the main cause of so many extinctions in such a short time fairly persuasive.