Nukes, I tell ya.
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. Various theories have been proposed for the die-off, ranging from
abrupt climate change to
overhunting once humans were let loose on the wilds of North America. But now nanodiamonds found in the sediments from this time period point to an alternative: a
massive explosion or explosions by a fragmentary comet,
similar to but even larger than the Tunguska event of 1908 in Siberia.
http://www.scientificamerican.com/article/did-a-comet-hit-earth-12900-years-ago/