sciencehabit writes: Things were looking up for Earth about 12,800 years ago. The last Ice Age was coming to an end, mammoths and other large mammals romped around North America, and humans were beginning to settle down and cultivate wild plants. Then, suddenly, the planet plunged into a deep freeze, returning to near-glacial temperatures for more than a millennium before getting warm again. The mammoths disappeared at about the same time, as did a major Native American culture that thrived on hunting them. A persistent band of researchers has blamed this apparent disaster on the impact of a comet or asteroid, but a new study concludes that the real explanation for the chill, at least, may lie strictly with Earth-bound events.