An archeaologist from my alma mater, working with his father, a paleooceanographer from the University of Oregon, have located a fairly consistent layer of nanodiamonds in soil samples from around North America. From this, they conclude that 12,900 years ago, a comet struck the continent and caused massive climate change -- initially melting a number of glaciers, and then triggering a global cooling period, resulting in significant extinctions worldwide and especially on the North American continent.
This is very interesting because I had been under the impression that a number of North American megafauna, like the woolly mammoth, went extinct because of overuse by human settlers who migrated here from eastern Asia. But if this report is right, then that might not be the case; instead, the extinctions were caused by force majeure. And upon close reading of the report, the report does not attribute mammoth extinction to the comet impact, but it does attribute the extinction of a number of other interesting mammals.
A couple of thoughts cross my mind here. First of all, these kinds of asteroid and comet impacts happen; anyone who's been to Arizona's Meteor Crater has seen powerful evidence of this and additional evidence of this accumulates every year. Yet nothing is being done by anyone about it because the chances of such a strike occurring in the very immediate future is remote.
Second, it's certainly the case that significant climate change is caused by things other than humans. It's also the case that humans can change their environment. Climate change is going on right now. Why that is happening is still mysterious, but the fact that is happening ought to be beyond reasonable dispute.
Third, some really interesting books I've read by a UCLA anthopologist named Jared Diamond may be in need of revision if this theory pans out. In both Guns, Germs, and Steel and Collapse, Prof. Diamond assumed that environmental factors caused civilizations in particular locations to succeed or fail, and points out the importance of geography as opposed to cultural variances between people in that question. He makes a number of case studies of pre-Columbian North American peoples and concludes that many of them overtaxed their environment, either through agriculture that was too intensive or through over-hunting game animals or other animals that could have been domesticated and used for labor. But Diamond's dismissal of the "noble savage in harmony with nature" myth may not be true -- it was hardly the fault of pre-Columbian North Americans if a comet hit their homeland.
Fourth, if there was an impact like this, where is the impact crater? Has it been eroded beyond sight? Obviously it's not as obvious an impact site as Meteor Crater or Lake Manicouagan in Quebec. The site of the But if there was an impact, it had to have happened somewhere. But Chicxulib Crater in the Yucatan -- the remnants of the Dinosaur Killer -- was not discovered until 1994, although of course it had been there all along, so there may still be discoveries to be made in the realm of geography despite our living in the world of high-detail satellite mapping of the world.