http://www.sciam.com/article.cfm?id=did-a-comet-hit-earth-12900-years-ago
Roughly 12,900 years ago, massive global cooling kicked in abruptly, along with the end of the line for some 35 different mammal species, including the mammoth, as well as the so-called Clovis culture of prehistoric North Americans.
The classic causes for this are usually taken to be overhunting, combined with climatic change. (The strange, abrupt and anomalous cooling period known as the Younger Dryas started at this point). It's also often suggested that this cooling period helped institute the invention of agriculture in the Levant.
Now, though, the discovery of nanodiamonds found in sediments from this time period in North America point to a new possibility--a comet exploding in the atmosphere, larger than the Tunguska Event of 1908. The pieces might have hit the ice sheet, or offshore, which is why impact craters haven't been found.
A lot more work is going to be needed in order to develop this theory. I'd start, myself, by looking at those Greenland ice cores that were taken some time ago. A cometary impact or explosion in the atmosphere should definitely show a telltale at this point in the timeline.