for National Geographic News
Rare diamonds found buried on an island near southern California strengthen the controversial idea that comet impacts wiped out huge beasts and an early human culture in North America about 12,900 years ago.
Similar "nanodiamonds" found in sediments across North America were presented earlier this year as proof that space rocks colliding with Earth led to the ancient mass extinction.
According to the theory, a barrage of comet debris rained down on North America during the last ice age and sparked massive wildfires. That initial heat and pressure formed tiny diamonds in the soil.
But the heat also abruptly melted ice sheets, causing an influx of freshwater that shut down a key ocean current and reversed the region's thaw.
The sudden recooling killed off mammals such as saber-toothed cats, dire wolves, and mammoths and wiped out some of North America's earliest known human inhabitants, the Clovis culture.
Opponents of the theory have been skeptical of the evidence, saying that the previously found microscopic diamonds lacked crystalline structures associated with the "shock" of being struck by extraterrestrial objects.
The newfound diamonds, however, have a unique hexagonal structure that's only been found on Earth in places where known impacts have occurred, the study authors say.
"Therefore [the diamonds'] discovery is important for this hypothesis, because it is hard to explain these away," said study co-author James Kennett, an emeritus geologist at the University of California, Santa Barbara.
Roughly 13,000 years ago California's Santa Rosa Island was part of a "mega island" that today is split into the Channel Islands, west of Los Angeles (see map).
The climate on this mega-island was much cooler than it is today, and the area was covered with juniper forests that sheltered pygmy mammoths and some of the first humans known in the Americas.
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