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Scientists have been ­arguing for years about what caused the die-off of both North American culture and many large animals near the end of the last ice age, about 13,000 years ago. Did hunters wipe out the mammoths, saber-toothed tigers, and giant sloths? Or did a huge drop in temperature freeze out both the animals and their hunters?

Neither, says Luann Becker, a geochemist at the University of California at Santa Barbara. Becker, along with two dozen–odd scientists, is studying a thin 12,900-year-old geologic layer across North America that she believes holds the legacy of a major extraterrestrial impact roughly half the size of the one that killed the dinosaurs.

After reviewing evidence of the blast—magnetic dust, trapped extraterrestrial gas, glasslike carbon full of tiny diamonds from the heat, and a layer of iridium from outer space—the geologists concluded that the North American fireball was a whopper. Specifically, they suggest that a three-mile-wide comet moving at 135,000 miles an hour blew up over Canada with the force of a million nuclear bombs.

Mammoths didn’t stand a chance, says Northern Arizona University space scientist Ted Bunch: “If the fires and the shock wave didn’t get them, there was a nuclear winter that blocked out the sun and made eating difficult.” The heat may have also melted vast stretches of retreating glaciers, kicking off a cold spell by slowing ocean currents.

Not everyone is convinced the evidence justifies the team’s conclusion, but it’s an intriguing theory. “One of the implications is that these impacts are far more frequent than scientists have thought,” says project geophysicist Allen West. “It’s one of the few things that could produce the extinction of humans. But for the first time in the history of our planet, we have the technology to do something about it.”