Some 13,000 years ago, the Clovis people wandered North America, hunting ground sloths, mammoths, and other creatures—until hunters and prey both vanished. What happened? A team of scientists now think they know: A miles-wide comet, they announced in May, seems to have exploded just north of the Great Lakes, triggering a 1,000-year cold spell that helped bring on the extinction of the Clovis and the animals.

For years, the disappearance of the Clovis culture and sudden extinction of 35 genera of animals were explained by two competing theories. One blamed climate change, although similar change at other times had not resulted in mass extinction. The other fingered the humans themselves: Newly arrived from Asia, the Clovis killed off everything in a murderous spree and subsequently starved. “They would be very strange hunters, if you look at the ethnographic record, to knock out 35 genera that quickly,” says Douglas Kennett, an archaeologist at the University of Oregon who conducted the research with 25 colleagues.

The key to the new hypothesis is a thin layer of black soil found at more than 50 North American sites. In it are magnetic grains containing iridium, an element thought to indicate extraterrestrial origins. The sediments also contain metallic and carbon spherules, as well as melted charcoal, likely the result of forest fires that swept the continent after the impact. Although no crater has been found, concentrations of these indicators are highest around the Great Lakes. Perhaps the impact was absorbed and erased by the Laurentide Ice Sheet, which at the time reached from the Arctic Ocean to that point, the researchers say. Or maybe the comet exploded before it hit Earth. “Think about it—people would have seen it coming,” says Kennett. “This was a bad day.”

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