The giant stag was no Bambi. It stood 7 feet tall at the shoulders and carried a spread of antlers 12 feet wide. It would have been easy to spot in the cool woodlands of Ice Age Europe, but perhaps hard to kill.
That question lies at the heart of one of prehistory's most persistent debates: Why did two-thirds of Earth's large mammals -- the giant deer, woolly mammoths, mastodons, saber-toothed cats, American horses, short-faced bears, woolly rhinos and many others -- vanish from the planet starting about 50,000 years ago?
Was it the prolonged cold snap known as the Last Glacial Maximum, which turned forest into tundra and tundra into glaciers, driving stressed animals into smaller and smaller habitats until they starved?
Or was it the coming of age of the world's greatest predator -- modern humans -- who spread throughout the world, devising ever more sophisticated weapons and hunting strategies to bring down giant beasts that could feed a family for half a year?
Or was it both? Or something else altogether?
"Neither climate change nor human hunting has a smoking gun," said Gary Haynes, of the University of Nevada at Reno. Evidence of climate change is uneven, he said, and direct evidence of human hunting -- spearheads lodged in old bones -- is scarce. "It's still anybody's guess."
Also, added vertebrate zoology curator Ross McPhee of the American Museum of Natural History, incomplete fossil records may lead to fanciful conclusions, ruling out other variables. McPhee has suggested that disease caused the extinctions, a view he acknowledges is "out in left field," but he says data for the other two views are little better.
"The whole thing is far more complex than people thought," said paleobiologist Anthony J. Stuart of University College London. "You can clearly see the population of animals shifting radically as climate changed. Then, you need to ask, 'Did the presence of modern humans just tip them over the edge and keep them from recovering?' "
Stuart is lead author of one of two new studies suggesting that climate and human predation may have combined to cause the extinctions. Reporting in the journal Nature last week, Stuart's team showed that the giant deer, believed to have vanished at the end of the Ice Age 10,000 years ago, survived in the southern Ural Mountains, on the Europe-Asia border, for at least another 3,000 years.
The deer, also known as Irish elk, "are really a great example of how the different causes of humans and climate change may have interacted," said paleobiologist Anthony D. Barnosky of the University of California at Berkeley. "They lingered on, but why couldn't those populations once again expand?"
The obvious answer is that humans must have killed them, either directly or indirectly. By 7,000 years ago, prehistory's hunter-gatherers had begun to settle into communities, and "there were many more of them," Barnosky said.
Human hunters may have obliterated the remaining deer quickly, a phenomenon described by scientists as "blitzkrieg," or hunters may have slowly starved them to death by destroying habitat -- clearing vegetation, lighting fires, depleting water and bringing disease -- "sitzkrieg." Earlier this month in the journal Science, Barnosky and several co-authors reviewed the evidence for Ice Age extinctions around the world, concluding that humans and climate played varying roles on different continents.
In Australia, where modern humans arrived about 70,000 years ago, the extinctions of 21 genera of large mammals occurred between 50,000 and 40,000 years ago, despite little or no climate change. Sitzkrieg is the major suspect.
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."