Over time, researchers have proposed several theories for the extinctions.
Guthrie points out that the idea of megadiseases has garnered little support. And the view that the mammoths were keystone animals whose loss paved the way for other extinctions doesn't bear up under his radiocarbon dating reconstruction of the age, he says.
A third and widely debated theory has it that people killed off large numbers of large mammals and caused their populations to plummet.
But that doesn't look plausible in light of the timeline, Guthrie says. Bison and elks were actually expanding before and during human colonization, and their fossilized bones pop up in prehistoric hunter camps, he says.
Mammoth bones are rarely found during excavations of such camps, and Guthrie has found no evidence of horse kills.
"It seems we need to shift our emphasis from hunters merely riding down the extinct Pleistocene faunas, to a vision of them coming in on a rising tide of the species that did not become extinct," such as bison and elks, he said. The Pleistocene epoch occurred between about 1.8 million and 10,500 years ago.
In the new study Guthrie notes that there is thousand-year gap between when people showed up and when mammoths died off.
This gulf indicates that humans "could well have had a hand in the gradual extinction of mammoths, but not as in a century-scale 'blitzkrieg' overkill, in which a newly arriving wave of superefficient human hunters broadly and abruptly devastated local megafaunas," he said.
Gary Haynes is an anthropologist at the University of Nevada, Reno, and president of the International Union for Quaternary Research's Commission on Palaeoecology and Human Evolution.
Haynes says Guthrie's work is important for understanding the transition from glacial to interglacial conditions and the ecological circumstances of human arrival to the Americas.
But he thinks the findings may actually support the idea that humans killed off mammoths and Ice Age horses.
"Current ideas about overkill allow for habitat changes as stressors on megafaunal species, which human foragers opportunistically exploited," Haynes said.
In other words, humans may have kicked the big mammals while they were down, due to climate-induced changes in their food supply.
"Some species probably would have recovered from the habitat changes," if only humans hadn't been around to hunt them, Haynes said.
The prehistoric horse would have been a good survival candidate, based on its long tenure in the fossil record, Haynes says.
Filling in the Blanks
Guthrie, the study author, says he's not actually aiming to debunk the overkill hypothesishe's happy to help fill a part of the fossil record that's been spotty for decades.
Radiocarbon dates from the transition period between the Pleistocene and the Holocenethe current epochare rare, partly because they're expensive to pursue, Guthrie says.
Guthrie is hopeful that further research will help fill in the picture of this period for other parts of the world, including the lower 48 states.
"Clovis sites are really important," he said, referring to the Clovis people, who are said to have been the first group to colonize the plains of inner North America.
"But [the sites] don't begin until about 11,000 years ago, and nobody knows what's going on exactly with the wild animals."
Guthrie says his cursory look at data from the continental United States seems to show similar trends: bison on the rise before and during the Clovis arrival, with elks and moose expanding after.
"As for camels, mastodon, and giant musk ox, we have no evidence that they even made it to this" time period, Guthrie said.
"They were extinct before and that likely is [due to] climate. It's tough to argue humans were here before that."
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