Failure to adapt to a drastically changing climate, and not overkill by humans or disease, most likely led to the extinction of mammoths, wild horses and other large mammals in North America after the last Ice Age, a new study suggests.
But this fresh take on an old argument might not be the final word.
R. Dale Guthrie, professor emeritus of zoology at the University of Alaska, Fairbanks, has added 600 radiocarbon-dated fossils to the established collection, and his examination reveals that mammoths and wild horses were in serious decline before humans arrived on the scene in Alaska and the Yukon Territory.
Like the end of the dinosaurs, the topic of large mammal extinctions is a hot one. While the new results might be true of the far North, some researchers still believe over-hunting contributed to the demise of the beasts across the rest of the continent.
The study, which also analyzed the fossil record of some surviving large mammals — bison, elk, moose and humans — in the far north of North America between 18,000 and 9,000 years ago is published in the May 11th issue of the journal Nature.
Pushed to the brink
It's generally accepted that humans first entered North America from Siberia around 12,000 years ago. Since mammoths and wild horses became extinct roughly 11,500 and 12,500 years ago, some scientists have figured that hungry humans might have hunted them into oblivion.
"The old idea, that I once had, was that these animals were killed off and then the modern large mammals expanded and took their place," Guthrie said.
According to Guthrie's new data, however, bison and elk populations were doing well during this period, and those species had expanded dramatically long before other species went extinct. So why weren't bison and elk over-hunted to extinction as well?
Interestingly, the fossil record shows the two beasts were hunted more vigorously, yet they endured.
"I imagine humans were hunting anything they could get," Guthrie said. "Horse meat is probably just as tasty as bison. But their campsites don't show many mammoth and horse remains — they're full of bison and elk."
Guthrie's interpretation of the fossil record is that something else pushed mammoths and horses to the brink, and if humans did play a role in the extinctions, it was limited to just killing stragglers.
The fossil record also casts doubt on the possibility of a mega-disease that wiped out animals across the board, Guthrie said.
A deadly disease would create a distinct end for each species, which isn't reflected in the fossils. Also, diseases that infect and kill multiple species are extremely rare, and unlikely in this case since bison, elk, and moose weren't affected.
So what happened up North?
The period between 13,000 and 11,000 years ago, at the end of the last Ice Age, was a great transitional time for the far North. Although scientists don't know exactly what happened during this period, they can tell certain things from the geologic and fossil record.
"We know that animals' body size changed, there was a mass extinction, temperature changes, and humans came in," Guthrie said. "A lot of animals, such as bison, didn't do real well before that time. Then they really prospered for a while and didn't do real well after that."
Before 13,000 years ago, the food available in the region was mostly short, dry grass of little nutritional value, Guthrie said.
Then, as the Alaska and the Yukon warmed up, and water returned to the land, the dry grass was replaced with tall lush grass and bush — the type of plants grazers like elk and bison prosper on.
"Long before horses and mammoths became extinct, bison and elk began to expand," Guthrie said. "The only good way to account for that expansion would be the availability of a more abundant and nutritious food source."
But as the region continued to warm and received more rain, the plants kept growing. Boreal forest — which includes inedible trees such as pine, spruce, and birch — began sprouting and limited the amount of grassy areas for grazing.
Bison and elk populations decreased with this transformation, but, Guthrie said, they adapted to the habitat change and out-competed mammoths and horses for the remaining food.
"Humans were probably hunting some of the animals that went extinct, but 1,000 years after humans came in, [bison and elk] were still doing fine," Guthrie said.
David Steadman, a researcher at the University of Florida who believes humans drove the giant sloth to extinction, agrees that encroaching boreal forest may have been the end for large mammals in the North. But what about across the rest of the continent?
"It's a great piece of evidence — I don't doubt it; I trust his data," Steadman told LiveScience. "What happened in Alaska and the Yukon is swell, but why did these things die out in Texas and Mexico and Arizona and Florida?"
Like many researchers in the field, Steadman attributes a combination of factors to the extinction of these beasts. But he believes humans, not climate, played the leading role throughout the New World.
"There are so many things going on, and to me it's illogical to think that warming up and getting rid of ice sheets at 40 degrees latitude is a bad thing for large mammals," Steadman said. "They went through 20 glacial cycles in the last million years, and got through every one except for the last one. It has a certain odor to it, and that odor is of humans."
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