About 11,000 years ago, the final Ice Age end and humans began to spread out to all corners of the globe. Shortly thereafter, the world's megafauna went extinct. So was it climate or humanity that killed the last megafauna?
To solve a question like this, the ideal test case would be one where the emergence of humans as the region's apex predator happened at a completely different time than when the climate changed. That's why New Zealand, which remained completely isolated from humans until the first Polynesian settlers arrived roughly 800 years ago, is such a potentially fascinating test case.
Because of the islands' complete isolation from the rest of the world — for its size, New Zealand is probably the most remote inhabitable landmass on the planet — New Zealand developed an ecosystem unlike any other. Before the Maori arrived bringing with animals like pigs and dogs, the only mammals native to New Zealand were three species of bat. All the ecological niches normally filled by large mammals were instead filled by equally gigantic birds, with the 10-foot tall moa emerging as New Zealand's great grazing beast.
Much like the bison in the 19th century American west, the arrival of human colonists quickly led to overhunting of the moa, with the various subspecies dying out likely at least a century before the arrival of the first European explorers. What's unique about this particular megafauna extinction is that it occured about 500 to 700 years ago, at a point when the planet's climate was more or less stable. That gives scientists a unique chance to study how the moa dealt with earlier climate change independent of human interference, as Australia-based research Dr. Nic Rawlence explains:
"Until now it has been difficult to determine how megafauna responded to environmental change over the past 50,000 years, because human arrival and climate change occurred simultaneously in many parts of the world. Using ancient DNA, radiocarbon dating and stable dietary isotope analysis, we have been able to show that before humans arrived, moa mitigated the effects of climate change by tracking their preferred habitat as it expanded, contracted and shifted during warming and cooling events. Moa were not in serious decline before humans arrived, as has been previously suggested, but had relatively stable population sizes. The overwhelming evidence suggests that the extinction of moa occurred due to overhunting and habitat destruction, at a time of relative climatic stability."
While it's hard to say how applicable the case of the moa is to that of their more ancient megafauna counterparts elsewhere, they are probably the best evidence that ancient humans most definitely could drive a species to extinction irrespective of climate change. Whether that means humans are actually responsible for, say, the extinction of the woolly mammoth is still an open question, but this data would suggest that megafauna had the capacity to weather massive climate fluctuations. Climate change in and of itself was almost certainly not a death sentence. As fellow researcher Jamie Wood puts it:
[The results] show that range shifts and minor population fluctuations observable in the fossil and genetic record are a natural response to environmental change and do not necessarily lead to extinction."
Check out the University of Adelaide website for more.