More evidence has emerged that a massive comet hit the Earth about 13,000 years ago, backing up a widely-disputed theory.
In 2007, Richard Firestone, a staff scientist at the Department of Energy's Lawrence Berkeley National Laboratory, found concentrations of metal spherules - micro-sized balls - and nano-sized diamonds in a sediment layer dated to 12,900 years ago at 10 of 12 archaeological sites that his team examined.
One of these was the Topper archaeological site, inhabited by the Clovis people.
This date corresponds with what's known as the Younger-Dryas, a period of extreme cooling that began around 12,900 years ago and lasted 1,300 years.
The Clovis people disappeared, along with thirty-six species, including the mastodon, mammoth and saber-toothed tiger.
Firestone's team suggests that a major impact event - perhaps a comet - was the catalyst for this cooling. However, a 2009 study led by University of Wyoming researcher Todd Surovell failed to replicate Firestone's findings at seven Clovis sites.
But there's a reason for this, says Malcolm LeCompte, a research associate professor at Elizabeth City State University.
"Surovell's work was in vain because he didn't replicate the protocol. We missed it too at first," he says.
"It seems easy, but unless you follow the protocol rigorously, you will fail to detect these spherules. There are so many factors that can disrupt the process. Where Surovell found no spherules, we found hundreds to thousands."
At all three sites he surveyed, LeCompte found the same microscopic spherules - with the Topper site the most exciting.
"What we had at Topper and nowhere else were pieces of manufacturing debris from stone tool making by the Clovis people. Topper was an active and ancient quarry at the time," he says.
And, the team found, there were up to 30 times more spherules at and just above the Clovis surface than beneath the artifacts at the site.
"The so-called extra-terrestrial impact hypothesis adds to the mystery of what happened at the YDB with its sudden and unexplained reversion to an ice age climate, the rapid and seemingly simultaneous loss of many Pleistocene animals, such as mammoths and mastodons, as well as the demise of what archaeologists call the Clovis culture," says USC archaeologist Albert Goodyear.
Earlier this year, a team found melt-glass material of a similar date at sites around the world, supporting the impact hypothesis.