Researchers have found shock-synthesized hexagonal diamonds on one of California's Channel Islands, which they say is the strongest evidence yet that a comet exploded in the atmosphere above North America, causing widespread extinctions there around 12,900 years ago. Skeptics, however, say the debate is far from over.

In 2007 researchers theorized that a comet set off continental fires that led to the mysterious disappearance of the Clovis people and the extermination of 35 mammal genera, including mammoths, mastodons, ground sloths and camels. The team documented a "black mat" of charcoal throughout North America that contains high levels of iridium, magnetic spheres, and nano-diamonds, which are consistent with such an airburst. The controversial theory also gibes with the 1908 Tunguska atmospheric detonation (also thought to be from a comet or meteorite) that leveled trees in Siberia, and it echoes the extraterrestrial impact widely believed to have wiped out the dinosaurs 65 million years ago.

Today, in Proceedings of the National Academy of Sciences, the same team reports on shock-synthesized hexagonal diamonds, known only from meteorite and other impact events, in a soot layer from Arlington Canyon on Santa Rosa Island in California. The canyon is famous for containing the earliest human remains in North America, dating back to 13,000 years, and the soot layer coincides with the disappearance of the pygmy mammoth from the island. In a documentary shown earlier this year on the Public Broadcasting Service's NOVA science show, the team also claimed that they discovered similar diamonds from the Greenland Ice Sheet dating to the same period.

But the evidence does not convince everyone. "I don't think much of this whole story," says geochemist Christian Koeberl of the University of Vienna in Austria, "Diamonds of any sort are not uniquely characteristic of impact events." He says that the major lines of evidence are still missing, including the presence of shocked minerals, including breccias and tektites as well as an impact crater. "At least three other groups searched for similar evidence in the same or similar samples and found none," he adds.

Briggs Buchanan, an archaeologist from Simon Fraser University in Burnaby, British Columbia, disputes the notion that humans declined following the purported impact. "We have shown that in California, specifically, that there [was] no severe decline in the resident population." He adds that other researchers have shown that the black mat varies in age across the continent and appears to have a variety of geologic origins.

What does the research team have to say about their doubters? "I'm so skeptical about the skeptics," says marine geologist James Kennett of the University of California, Santa Barbara. "We work in a different paradigm where different materials result from different kinds of impacts."