Did a comet hit the Great Lakes region (North America) and fragment human populations 12,900 years ago? A 26-member team of researchers is proposing a startling new theory: that an extraterrestrial impact, possibly a comet, set off a 1,000-year-long cold spell and wiped out or fragmented the prehistoric Clovis culture and a variety of animal genera across North America almost 13,000 years ago. Driving the theory is a carbon-rich layer of soil that has been found, but not definitively explained, at some 50 Clovis-age sites in North America that date to the onset of a cooling period known as the Younger Dryas Event.
     The British journal Nature addressed the theory in a news-section story in its May 18 issue. The researchers propose that a known reversal in the world's ocean currents and associated rapid global cooling, which some scientists blame for the extinction of multiple species of animals and the end of the Clovis Period, was itself the result of a bigger event. While generally accepted theory says glacial melting from the North American interior caused the shift in currents, the new proposal points to a large extraterrestrial object exploding above or even into the Laurentide Ice Sheet north of the Great Lakes.
     "Highest concentrations of extraterrestrial impact materials occur in the Great Lakes area and spread out from there," Professor James Kennett, from the University of California in Santa Barbara (UCSB), said. "It would have had major effects on humans. Immediate effects would have been in the North and East, producing shockwaves, heat, flooding, wildfires, and a reduction and fragmentation of the human population." The carbon-rich layer contains metallic microspherules, iridium, carbon spherules, fullerenes, charcoal and soot.
     The blast, from a comet or asteroid, caused a major bout of climatic cooling which may also have affected human cultures emerging in Europe and Asia. Missing in the new theory is a crater marking an impact, but researchers argue that a strike above or into the Laurentide ice sheet could have absorbed it. Another possibility is that it exploded in the air. Kennett said that 35 animal genera went extinct at the end of the Pleistocene, with at least 15 clearly being wiped out close to 12,900 years ago. "All the elephants, including the mastodon and the mammoth, all the ground sloths, including the giant ground sloth - which, when standing on its hind legs, would have been as big as a mammoth," Kennett said. "All the horses went out, all the North American camels went out. There would have been major ecological shifts, driving Clovis survivors into isolated groups in search of food and warmth. There is evidence that pockets of Clovis people survived in refugia, especially in the western United States."
     Scientists say that they are currently evaluating the existing paleoindian archaeological datasets, which Kennett describes as "suggestive of significant population reduction and fragmentation, but additional work is necessary to test the data further." Jeff Severinghaus, a palaeoclimatologist at Scripps Institution of Oceanography in California, told Nature magazine: "Their impact theory shouldn't be dismissed; it deserves further investigation."

Sources: The Observer (20 May 2007), EurekAlert!, BBC News (21 May 2007)