In Eurasia, by contrast, two waves of extinctions coincided with climate change -- the prolonged Ice Age between 45,000 and 20,000 years ago, and a final cold snap that began about 12,000 years ago and lasted for two millennia.
For years, researchers believed that the disappearance of the Irish elk, so nicknamed because the best and largest number of specimens were found in limestone-rich deposits in Ireland, was strong evidence that climate change played the dominant role in the extinctions.
These majestic creatures -- moose-size at 1,300 pounds but with enormous antlers -- co-existed across Eurasia with modern humans or their Neanderthal predecessors for hundreds of thousands of years.
They were unwieldy animals. Males grew and shed 90 pounds of antlers every year, an arduous task requiring huge amounts of calcium-rich forage. Stuart suggested that medium-growth forest and temperate climate were probably best for them. Pure grassland did not offer the right antler-friendly mineral mix, and the antlers made it impossible for the stags to navigate dense woods.
"The Last Glacial Maximum, between 20,000 and 15,000 years ago, was rough on them," Stuart said. "The animals retreated from large areas of Western Europe. Then, as the climate warmed up again, the animals migrated back."
The final cold snap about 10,500 years ago was thought to have finished off the giant deer, but Stuart's team used radiocarbon dating to find bones from animals that lived 3,000 years later. "The study weakens the climate argument," Stuart said. "Climate change had a dramatic effect on deer, but once they got past that, maybe they would have survived -- if humans hadn't been present."
The double whammy of climate change and human hunting appears to have had its greatest effect in North America, where modern humans crossed the land bridge from Asia about 11,500 years ago, as temperatures plunged for the last time. Within 750 to 1,500 years, at least 15 species of large mammals became extinct.
This epic catastrophe contrasts dramatically with prehistory in Africa, which lost almost no large species despite climate change and the presence of humans and their precursors for millions of years.
Some scientists have used the "naive animal" theory to explain this anomaly: African animals had lived with humans long enough to understand how dangerous they were, but their North American cousins did not "get it" until it was too late.
"There's very little doubt that humans had an impact in North America," Barnosky said. "This is an ecosystem that had never seen a human being, and the arrival of humans put an entirely new predator into the system."
But for how long? "It might work for one generation or even for a serious length of time, depending on how fast the humans dispersed," Haynes said. "But to be naive for 1,500 years? That seems a little excessive."
