Big Chill or Big Kill?

It goes without saying (at least to those of you who have been following this blog) that human activity is the single greatest threat to Earth's biodiversity today. Species are going extinct at a rate several times greater than the projected "background" rate--the greatest rate of extinction ever recorded since that of the dinosaurs at the end of the Mesozoic. This much, experts can agree on. What experts cannot agree on, however, is how long ago this modern-day mass extinction began.
The primary point of contention for people holding these different points of view is the extinction of the so-called "Pleistocene Megafauna"--the large mammals and birds that inhabited most of the continents up until about 10,000 years ago, but today persist mainly in Africa. There are two main hypotheses for why they died out, one being that they were unable to adapt to a changing climate, and the other being that they were killed off at least indirectly by humans. Both have their advantages and drawbacks.
The hunting hypothesis suggests that humans, after arriving on a new landmass, generally wipe out the local large herbivores and top predators. It's hard to argue with this from a strictly statistical standpoint, because many of the extinctions do indeed coincide (within windows of thousands of years, of course) with the arrival of humans. However, critics of the hunting hypothesis point out that  the weapons and technology of early humans may not have been sophisticated enough, and their populations not great enough, to kill off the megafauna.
The other hypothesis is the climate hypothesis. Most of the megafauna extinctions occurred during a time when the Earth was getting warmer, a trend that continues to the present. As stated above, this fits both with the time and with the idea that humans may have been unable to kill off these animals. It should be remembered, though, that there was more than one "ice age". There were actually several, separated by warm periods like the one we live in now, called interglacials. The Earth had been through several interglacials recently before the present one began, and if the climate hypothesis were true then it is certainly odd that the megafauna were able to survive these unharmed.
Most likely, as with all things in life, the answer is not black and white. The highly specialized creatures of the ice ages certainly would have declined as the climate warmed, but that alone need not have killed them off--after all, they had survived interglacials before. But in their weakened state, they could not survive one species that had begun to flourish. They could not survive us.