A follow up to last summer's Death from Above entry: a group of researchers has discovered a layer of "shocked"* microscopic diamonds, dating to 12,900 years ago (10,900 BCE), on Santa Rosa Island off California. The crystals, properly known as Lonsdaleite, are only formed on Earth under extreme conditions, like those of a "cosmic collision." Based on the discovery of other "shock-synthesized minerals" and soot at other American sites dating to the same period, Douglas Kennett (of the University of Oregon) has hypothesized that a large comet or meteor exploded over North America about 13,000 years ago, leading to a 1,000-year cooling period that may have wiped out mammoths and other mega-fauna on the continent.
This hypothesis remains controversial, because the megafaunal species in question had just survived several Ice Ages without incident. It still seems likelier, at least to this educated layman, that newly-arrived human hunters bore primary responsibility for the North American extinctions. An old but serviceable computer model, created by Paul Martin and James Moisimann, demonstrated that a very small initial population of Paleo-Indian hunters could have exterminated every large (450-plus kilogram) herbivorous mammal species north of Mexico within 1,000 years of their arrival in America. (Alfred Crosby, Throwing Fire [Cambridge UP, 2002], 57, 60-66.) Indeed, the cometary explosion discovered by Kinnett et al. may have given some large mammal species additional breathing room, by temporarily reversing the warming trend that had been shrinking the mid-continent "mammoth steppes" and leaving mammoths and other large herbivores more vulnerable to human predation. Still, this is an important story, and reminds historians and anthropologists to Keep Watching the Skies.
* I was going to call this entry "Shocked, I Say," but I've already used that title.