for National Geographic News
Some 12,000 years ago, North American mammoths, ancient horses, and many other large mammals vanished within the short span of perhaps 400 years.
Scientists cannot be sure what killed them, but a new study suggests that humans aren't off the hook just yet.
The large animals' disappearance at the end of the Pleistocene era (50,000 to 10,000 years ago) happened at about the same time that many large animals, or megafauna, went extinct around the globe.
Victims included species such as the saber-toothed cat and the diprotodona rhinolike beast that was the world's largest marsupial.
Now a new study of the fossil record fuels the debate about the cause of the creatures' fate.
In North America two major events occurred at about the same time as the megafaunal extinctions: The planet cooled, and early humans arrived from Asia to populate the continent.
For decades scientists have debated which of these factors was responsible for widespread megafaunal extinctions.
Was the climate change simply too much for the animals to withstand? Or did the ancient mammals succumb to human hunting pressure?
Many experts suggest a combination of these factors and perhaps others, such as disease.
"It's hard to see this as one of those things where a single piece of evidence will make it obvious what happened," said Scott Wing, a paleobiologist at the Smithsonian Institution's National Museum of Natural History.
"The phenomenon that people are trying to explain is not something that happened in one place at one time. It happened across the globe, at different times on different continents. I think that there are clearly multiple factors involved."
Humans Not Exonerated
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