Scientists have come up with a controversial theory which suggests that huge comet impacts wiped out North America's large mammals 13,000 years ago.

According to a report in National Geographic News, 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.

Around this time, large mammals including mammoths, mastodons, horses, camels, and saber-toothed cats went extinct in North America.

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.

"This would also explain evidence of fires across swaths of North America," he added.

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 suggested.

"It's obviously an outrageous hypothesis; in the sense that it wasn't predicted. It has come out of left field," said Kennett. "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," he added.

But the theory has been debated widely since it was introduced.

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," he said.

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

In spite the debate, 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.