Something dramatic happened about 12,900 years ago, and the continent of North America was never the same. A thriving culture of Paleo-Americans, known as the Clovis people, vanished seemingly overnight. Gone, too, were most of the largest animals: 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 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 they have found a new line of evidence: nanodiamonds. They say they have found these tiny structures across North America in sediments from 12,900 years ago, and they argue that the diamonds had to have been formed by a high-temperature, high-pressure event, such as a cometary impact.
"This is a big idea," said Douglas J. Kennett, an archaeologist at the University of Oregon and the lead author of a paper on the Younger Dryas impact hypothesis published today in the journal Science.
The hypothesis has been hotly contested, as would be expected for a catastrophic tale that, so far, lacks anything as compelling as a crater. Nor are there signs of deformation in rock debris that is a signature of the massive impact that, 65 million years ago, apparently wiped out the dinosaurs.
But Kennett and his colleagues say that they have found these diamonds at the layer of sediment that marks the start of the Younger Dryas. They are not found above or below that layer.
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."
The hypothesis of a catastrophic impact at the start of the Younger Dryas has incited abundant skepticism in the scientific community. NASA space scientist David Morrison, an expert on impacts, said he doubts that a comet could have broken up in the manner proposed by the Kennett group.
"They talk rather blithely about a comet disintegrating in the atmosphere," Morrison said. Referring to the nanodiamonds, he said: "They may have discovered something absolutely marvelous and unexplained. But the impact hypothesis just doesn't make sense."
Morrison posed several questions: "What size impact does it take to produce diamonds? What size crater would that be? Where is it? If it hit in the ocean, would it have had the same effect? These are all questions one can ask."
Kennett's father and co-author, James Kennett, an earth scientist at the University of California at Santa Barbara who has devoted much of his career to studying the Younger Dryas, said: "I think it's totally reasonable that there should be skeptics. What we're arguing is that this impact hypothesis explains three major things that have been enigmatic and not particularly resolvable."
Those three things are the extinction of the megafauna, the disappearance of the Clovis culture and the climate change of the Younger Dryas. The general thought has been that climate change played a key role in wiping out the large animals and perhaps undermining the Clovis people, though some scientists have argued that the animals were hunted to extinction (the Pleistocene overkill hypothesis). But the fossil record has been puzzling, for many species of megafauna had survived multiple ice ages until the cool spell of the Younger Dryas.
For decades, scientists have believed that meltwater at the end of the ice ages formed a huge lake in central North America, known to scientists as Lake Agassiz. At some point, the water from that lake may have surged into the North Atlantic and shut down the dominant ocean current that brought warmer water toward higher latitudes. That, in turn, could have created a long-term climate change.
The impact scenario incorporates the meltwater scenario. The scientists say that the impact could have destabilized and melted the edges of the ice sheet resting on the northern tier of the continent. 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 may have died off, even if they had survived the initial catastrophe.
The younger Kennett acknowledged that work must be done to firm up the claim: "It's a hypothesis. . . . Basically, there's a suite of data that suggest that something like this occurred, but it still needs to be tested."