Scientists have long pondered an event 12,900 years ago that caused the disappearance of the Clovis people of North America and the extinction of large mammals such as the mammoth, mastodon, saber-toothed cat, and the North American camel. One theory is that a comet broke into fragments and showered burning material over the continent. Now there's some evidence -a layer of nanodiamonds have been found at a layer of sediment buried 12,900 years ago. The diamonds could have only been formed by a high-pressure high-temperature event.

These diamonds are measured in nanometers -- mere billionths of meters -- and one of them would not suffice for an engagement ring unless the recipient had an extremely small finger. Indeed, these diamonds are visible only with the aid of the most advanced microscopes.

The wide distribution of the nanodiamonds could be a sign that the comet broke into pieces in space and that the fragments burned up explosively over a broad area of North America. The heat and pressure from the event transformed carbon on the planet's surface into the tiny diamonds, the scientists said.

"Imagine these fireballs exploding in the air. A Clovis hunter standing and looking at these things would have seen a canopy of fire as these things came in and exploded," said Allen West, a geophysicist and one of the paper's co-authors. "There would have been no sound. There would have been massive explosions. Brilliant light, brighter than the sun. There would have been radiant heat -- it would have been capable, at the very least, of giving him serious burns and, at the maximum, of incinerating him."

This theory would explain the climate change at the time, when the warming planet was plunged into another, shorter ice age. Skeptics cite lack of a crater or other surface evidence in refuting the theory. Link -via Digg