About 12,800 years ago, the last Ice Age was coming to an end. Mammoths and other large mammals roamed North America, and humans were beginning to settle down and cultivate wild plants. Suddenly, the planet returned to near-glacial temperatures for more than a millennium. The mammoths disappeared at about the same time, as well as the Clovis culture that hunted them.
     Scientists agree that something pretty big happened 12,800 years ago, allowing a cold weather blooming plant known as the Dryas octopetala to suddenly thrive.
     A number of causes have been suggested, including changes in ocean currents due to melting glaciers and volcanic activity. In 2007, a diverse group of 26 researchers led by nuclear chemist Richard Firestone of the Lawrence Berkeley National Laboratory in California formally proposed what is known as the Younger Dryas impact hypothesis, in which one or more extraterrestrial bodies exploded over North America, leading to widespread wildfires, and sending dust and debris across the globe.
     Firestone and his colleagues reported evidence including deposits of the element iridium (rare on Earth but abundant in meteorites), microscopic diamonds, and magnetic particles at sites dated to about 12,800 years ago. These claims were sharply contested by some specialists in the relevant fields, who either did not detect such evidence or argued that the deposits had other causes.
     Now a team of researchers from several universities in the USA say that of the 29 different sites in the Americas, Europe, and the Middle East referenced by the earlier team, only 3 could be dated to 12,800 years ago.
     Team leader David Meltzer, an archaeologist at Southern Methodist University in Dallas, Texas, instead points toward more nuanced explanations, such as Clovis points disappearing because early Americans turned to other forms of stone tool technology as the large mammals they were hunting went extinct, as a result of the changing climate or hunting pressure.
     Impact proponents appear unmoved. "We still stand fully behind the [impact hypothesis], which is based on more than a confluence of dates," Firestone says. "Radiocarbon dating is a perilous process," he contends, adding that the presence of Clovis artefacts and mammoth bones just under the iridium, nano-diamond, and magnetic sphere deposits is a more reliable indicator that an extraterrestrial event was responsible for their disappearance.

Edited from Science, Nature (12 May 2014), PhysOrg (13 May 2014)