WASHINGTON — Something dramatic happened about 12,900 years ago, and the continent of North America was never the same. A thriving culture of Americans, known as the Clovis people, vanished seemingly overnight. Gone, too, were most of the largest animals on the continent — horses, camels, lions, mammoths, mastodons, saber-toothed cats, ground sloths and giant armadillos.
Scientists have long blamed climate change for the extinctions, for it was 12,900 years ago that the planet's emergence from the Ice Age came to a halt, reverting to glacial conditions for some 1,500 years, an epoch known as the Younger Dryas.
In just the last few years there has arisen a controversial scientific hypothesis to explain this chain of events, and it involves an extraterrestrial calamity: A comet, broken into fragments, turning the sky ablaze, sending a shock wave across the landscape and scorching forests, creatures, people and anything exposed to the heavenly fire.
Now the proponents of this apocalyptic scenario say that they've found a new line of evidence: "nanodiamonds," gems measuring in the billionths of meters.
They say they've found these structures across North America in sediments from 12,900 years ago, and they argue that they had to have been formed by a high-temperature, high-pressure event, such as a cometary impact.
"This is a big idea," said Doug Kennett, an archaeologist at the University of Oregon and the lead author of a paper on the Younger Dryas Impact Hypothesis, to be published today in the journal Science.
Kennett and his colleagues say that they've found these diamonds right at the layer of sediment that marks the starts of the Younger Dryas. They're not above or below that layer.
Scientists say a comet impact could have destabilized and melted the edges of the ice sheet resting on the northern tier of North America. An impact would also have created a short-term environmental disaster. Dust from the impact and soot from continent-spanning wildfires could have risen into the atmosphere, blocked sunlight and dramatically hampered plant growth. With vast portions of the landscape burned, large animals requiring a great deal of food to survive may have died off even if they had survived the initial catastrophe.
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