Did Comets Cause Ancient American Extinctions?

Anne Casselman
for National Geographic News
May 6, 2008
Debate has heated up over a controversial theory that suggests huge comet impacts wiped out North America's large mammals nearly 13,000 years ago.

The hypothesis, first presented in May 2007, proposes that an onslaught of extraterrestrial bodies caused the mass extinction known as the Younger Dryas event and triggered a period of climatic cooling.

The theory has been debated widely since it was introduced, but it drew new scrutiny in March at the annual meeting of the Society for American Archaeology annual meeting in Vancouver, Canada.

Stuart Fiedel from the Louis Berger Group, a private archaeological firm in Richmond, Virginia, argued that the theory fails to address some major questions—like how comet blasts could have wiped out woolly mammoths and saber-toothed cats in North America, while leaving humans unscathed.

"If this [impact] was powerful enough to fricassee mammoths and mastodons and short-faced bears and other big fauna that were on the landscape, you would think that it would have decimated the human population as well—not only by direct thermal shock but by wiping out much of their food source," said Fiedel, who presented his criticisms of the theory to a packed crowd.

"So you should have a marked fall-off or termination of human populations, and we don't see that.

"Ultimately the judgment is supposed to be based on whether this thing works when you throw it at the data, and vice versa," he said.

"And right now I don't think some fairly obvious things are explained by it."

Global Cooling Mystery

No matter how it happened, experts agree that Earth got a shock to its system 12,900 years ago.

The world was in the middle of thawing out from the last ice age, when the Younger Dryas event inexplicably plunged it back into near glacial temperatures. This anomalous period lasted for about 1,300 years.

One widely accepted hypothesis suggests that melting ice sheets and glacial lakes 12,000 years ago dumped so much meltwater into the oceans that it disrupted ocean circulation. This in turn cooled much of the planet, especially in the Northern Hemisphere.

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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