A group of researchers has poured scorn on the idea that a comet wiped out the North American Clovis people 13,000 years ago.
The theory has been around a while, and won increased support last year with the identification of metal spherules at several Clovis sites. The idea was also used to explain the start of the Younger-Dryas, a period of extreme cooling that began around 12,900 years ago, and the disappearance of the mammoth and saber-toothed tiger at around this time.
But there's always been a question mark over the theory, with other teams unable to find the same metal spherules - and now researchers from Royal Holloway university and Sandia National Laboratories say it simply doesn't stack up.
No appropriately sized impact craters from that time period have been discovered, they say, and no shocked material or any other features of impact have been found in sediments.
The team also found that samples presented in support of the impact hypothesis were contaminated with modern material, and even that no physics model can support the theory.
"The theory has reached zombie status. Whenever we are able to show flaws and think it is dead, it reappears with new, equally unsatisfactory, arguments," says Professor Andrew Scott from the Department of Earth Sciences at Royal Holloway.
“Hopefully new versions of the theory will be more carefully examined before they are published”.