Also around this time, large mammals including mammoths, mastodons, horses, camels, and saber-toothed cats went extinct in North America.
Previous hypotheses have suggested that early humans wiped out the large animals in a prolonged act of slaughter referred to by scientists as overkill.
(Read related story: "Humans to Blame for Ice Age Extinctions, Study Says" [August 10, 2005].)
Also around this time, the prehistoric Clovis culture disappeared in North America, while other ancient cultures such as the Folsom began to flourish.
James Kennett, a geologist at the University of California, Santa Barbara, is one of the main proponents of the comet-impact hypothesis.
He said the theory is consistent in explaining and linking these various phenomena.
"We suggest it's a series of aerial bursts, more of a multiple Tunguska event like a shotgun," he said, referring to the explosion of an extraterrestrial object over Siberia in 1908.
(Read related story: "Small Asteroid Caused Mysterious 1908 Blast, Study Says" [December 21, 2007].)
This would also explain evidence of fires across swaths of North America, Kennett said.
He and his colleagues have also found widespread and abundant minuscule diamonds and magnetic particles in the layer of Earth that dates to this time.
These features were formed in the extremely hot and high-pressure environment created by the series of explosions, Kennett suggests.
"It's obviously an outrageous hypothesis in the sense that it wasn't predicted—it has come out of left field," Kennett said.
"But all I can say is that I don't know of any other process that can account for the wide display of data that we have and continue to generate other than some kind of an extraterrestrial impact."
South American Quandary
In Fiedel's critique of the theory, he also cited evidence from the archaeological record that he said is inconsistent with the comet hypothesis.
"There's the apparent lack of synchrony with what goes on in South America," he noted.
Radiocarbon dating and other data suggest that the megafauna of South America survived for centuries after their cousins up north were wiped out, Fiedel said.
"You have to ask what kind of blast might peter out by the time it gets to Mexico and not have much effect on South America," Fiedel said.
Kennett agreed that this could be seen as a discrepancy.
"South America is a critical testing point of [the theory]," he said.
But the northern and southern extinction dates based on radiocarbon techniques could be viewed as synchronous, he argued, given their margins of error.
More data need to be collected from the region to better understand the exact timing of the extinctions in South America, he added.
The Challenges of Dating
Gary Haynes is an anthropologist at the University of Nevada in Reno.
He said he isn't sure these questions could ever be answered based on radiocarbon data.
"[The Younger Dryas event] is occurring around the time [when] it's almost impossible to get precise dates, because of radiocarbon variations in the atmosphere," Haynes said.
An increase of atmospheric carbon dioxide during the Younger Dryas event is believed to have made radiocarbon dating from the period imprecise.
"That's always going to be very hard to decide for sure," said Haynes.
"So talking about causation based on that sort of imprecision, you can't do it."
For his part, Kennett pointed out that the theory is still in its early days.
At the moment, only one academic paper about the theory has been published. More are in the works, he said, based on the additional data and analyses he and his colleagues have done over the past year.
"This group is the first group that's ever systematically carried out any examination for the possibility of an extraterrestrial impact at about the time of the megafaunal extinction," he said.
"Now, basically, we are in the mode of testing the hypothesis. It's going to take some time."
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