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.

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