Comets Didn't Wipe out Sabertooths, Early Americans?

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But the grains are likely from the 30,000 tons of tiny meteorites that fall to Earth each year.

He found such grains in equal or greater concentrations at many other layers dating to other time periods.

Evolution of Style

Vance Holliday, an archaeologist at the University of Arizona, added that there is no sign that the demise of the Clovis culture was caused by a comet crash.

Around the time of the cold snap, the style of spearpoints changed—which Firestone and colleagues argued was evidence that the Clovis peoples had declined due to the comet impact.

But Holliday said it reflects a normal evolution in preference. He compared spearpoint designs to the appearance—and disappearance—of tail fins on classic automobiles.

(Related: "Ancient Camels Butchered in Colorado, Stone Tools Show?")

"We really don't know what style means in the archaeological record," he said. Tastes "come and go. We don't know why."

But "an extraterrestrial impact is an unnecessary solution for an archaeological problem that doesn't exist."

Sticking to His Guns

Firestone, the original proponent of the comet theory, is sticking to his guns.

Though no one can prove the Clovis disappeared, the total number of spearpoints plummeted after the impact, he said by email.

Likewise, other research has shown a gap of more than a hundred years between the Clovis and the ensuing Folsom culture, Firestone pointed out.

As for the other evidence, the comet-impact layer is very thin, he told Australia's Cosmos Magazine, and is easy to miss if samples are diluted with soils from adjacent layers.

University of Southern Illinois' Pinter is also wrong about the charcoal and carbon fragments, he added. These substances are not widespread as soil would be, and are found only in a narrow band of sediment.

In addition, the magnetic grains are not the same composition as what's found in typical tiny meteorites, he added.

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