Recently it has been suggested that a asteroid or comet struck the Laurentide (north America) icesheet about 12,900 years ago and that this contributed to megafaunal extinctions and the Younger Dryas cooling (Firestone et al PNAS 104:41 pg 16016). While the evidence for an impact seems sound (at least at first glance) its effect on the megafauna has been questioned. Research to be published in PNAS this week reports on the identification of wooly mammoth DNA in Alaskan permafrost dated to between 7,600 to 10,500 years old. This means that mammoths existed for 2000 to 5000 years after the impact. It also shows that they survived long after the arrival of humans, thus casting doubt on the over-hunting hypothesis.
http://media.uow.edu.au/news/UOW070684.html