A controversial theory about prehistoric North America - that an impact by an extraterrestrial object 12,800 years ago triggered a cold spell that killed off mammoths and devastated early human populations - has been debunked.
Believers in this notion point to evidence from 29 sites, from North America to Europe, that supposedly contain a thin layer of sediments that date to the start of the cosmic event.
But researchers reported today in the journal Proceedings of the National Academy of Sciences that only three of the 29 sites in-fact date to prehistoric times. The other sites either have not been dated using the typical radiometric methods, or are much older or younger than the reported impact.
"The chronology doesn't hold up," team leader David Meltzer, an archaeologist at Southern Methodist University in Dallas, Texas, told Nature News.
"The supposed impact markers are undated or significantly older or younger than 12,800 years ago," the authors reported. "Either there were many more impacts than supposed, including one as recently as 5 centuries ago, or, far more likely, these are not extraterrestrial impact markers."
Scientists do not doubt that come significant event occurred some 12,800 years ago, but they differ in what they believe caused such drastic climate changes during that time.
Temperatures in the Northern Hemisphere plummeted in a cold front known as the Younger Dryas, and the Clovis people, who were sophisticated hunters, vanished from what is now the western United States. Large mammals native to the North American region, like the famous mammoths or sabre-toothed cats, went extinct.
Opponents of the cosmic collision advocate that if there was indeed a comet blast, it would have left a noticeable fingerprint in the geological record.
But theory proponents insist that evidence like tiny diamonds formed in the high pressure of an impact, and soot and charcoal from fires possibly ignited by it, support their claim.
"Meltzer's analysis of the dates is overly simplistic and clearly biased towards his conclusions," Richard Firestone, a nuclear chemist and leader of the impact claim, said.
However, the errors in radiometric methods are undeniable to researchers.
"For now, there is no reason or evidence to accept the claim of an extraterrestrial impact," the scientists wrote.