When competent human hunters encountered naive fauna, the biggest animals, things like mammoths and toxodons and diprotodons, all went extinct. It is not hard to see why this occurred. Large animals are more worth hunting than rabbits, and easier to catch, while having a far lower reproductive rate. Moreover, humans are not naturally narrow specialists on any one species, so are not limited by the abundance of that species in the way that the lynx population depends on the hare population. Being omnivores, they could manage even when the megafauna as a whole were becoming rare.
There were subtle factors at work as well: the first human colonists in a new land probably didn’t develop ethnic/language splits for some time, which meant that the no-mans-land zones between tribes that can act as natural game preserves didn’t exist in that crucial early period. Such game preserves might have allowed the megafauna to evolve better defenses against humans – but they never got the chance.
It happened in the Americas, in Australia, in New Zealand, in Madagascar, and in sundry islands. There is no reason to think that climate had much to do with it, except in the sense that climatic change may sometimes have helped open up a path to those virgin lands in which the hand of man had never set foot, via melting glaciers or low sea level.
I don’t know the numbers, but certainly a large fraction of archeologists and paleontologists, perhaps a majority, don’t believe that human hunters were responsible, or believe that hunting was only one of several factors. Donald Grayson and David Meltzer, for example. Why do they think this? In part I think it is an aversion to simple explanations, a reversal of Ockham’s razor, which is common in these fields. Of course then I have to explain why they would do such a silly thing, and I can’t. Probably some with these opinions are specialists in a particular geographic area, and do not appreciate the power of looking at multiple extinction events: it’s pretty hard to argue that the climate just happened to change whenever people showed when it happens five or six times.
It might be that belief in specialization is even more of a problem than specialization itself. Lots of time you have to gather insights and information from several fields to make progress on a puzzle. It seems to me that many researchers aren’t willing to learn much outside their field, even when it’s the only route to the answer. But then, maybe they can’t. I remember an anthropologist who could believe in humans rapidly filling up New Zealand, which is about the size of Colorado, but just couldn’t see how they could have managed to fill up a whole continent in a couple of thousand years. Evidently she didn’t understand geometric growth. She is not alone. I have see anthropologists argue [The revolution that wasn't] that increased human density in ancient Africa was driven by the continent ‘finally getting full’, rather than increased intellectual abilities and resulting greater technological sophistication. That’s truly silly. Look, back in those days, technology changed slowly: you would hardly notice significant change over 50k years. Human populations grow far faster than that, given the chance. Imagine a population with three surviving children per couple, which is nothing special: it would grow by a factor of ten million in a thousand years. The average long-term growth rate was very low, but that is because the rate of increase in human capabilities, which determine the carrying capacity, was very slow – not because rapid population growth is difficult or impossible.
I could explain this to my 11-year old twins in five minutes, but I don’t know that I could ever explain it to Brooks and McBrearty.