A hard bolide to swallow?

For those of you who may be wondering what is wrong with paleoanthropology that we can't just resolve the hobbit problem, I can only say one thing: We are not alone.

For instance, there is the idea that a "mammoth-killing" impact caused the Younger Dryas, suggested in a paper last year by R. B. Firestone and colleagues.

If you like the idea, this seems like a bad sign:

Archaeologist Vance Haynes, professor emeritus at the University of Arizona, Tucson, is finding likely looking magnetic spherules in the darnedest places. He has spent 30 years studying Clovis sites, many of which the Firestone group sampled. As a check on his own ongoing independent analysis of YD samples, he collected a modern sample. "I got 300 grams of dust off the roof [of my house], and it's full of magnetic microspherules," he says. Whether they are the melted, iridium-rich micrometeorites that continually drift down from the upper atmosphere or the product of high-temperature industrial processes such as coal burning, he doesn't yet know. Either way, they could be trouble. The cosmic dandruff of microspherules could have salted sediments forming 12,900 years ago with iridium, while the humanmade variety might have settled on modern outcrops before sampling.

That's from a long news article in Science by Richard Kerr, titled "Experts find no evidence for a mammoth-killer impact."

Like almost every other temperature fluctuation of the last 80,000 years, the Younger Dryas has been attributed with quasi-magical power: in this case, the power to kill mammoths and extinguish cultures. And hey, maybe it really did...but I have a lot of skepticism when associations are made on the basis of dating uncertainty. It would help if these climate changes would affect every species and culture, rather than showing catastrophic effects on particular ones without showing any signs of affecting others.

Of course, without the biological and cultural fallout, the story of a particular climate cycle isn't very interesting. Nobody would care. With the Younger Dryas, we have human archaeological evidence from all over the world 12,900 years ago. Surely something will match! Of course, the megafauna didn't become extinct precisely then, exactly. But no matter -- it's close enough. Surely these events would have been devastating for animals and plants, right? Most of them were lucky enough to escape the devastation with their lives, but a few unlucky victims because extinct, thousands of years afterward.

A meteor helps to spice it up, and in this case we have the whole package:

The catastrophe had taken place a geologic instant ago--closely coinciding with the disappearance of North America's mammoths and the continent's earliest human culture (Science, 1 June 2007, p. 1264). Then came the 26-author paper last October in the Proceedings of the National Academy of Sciences (PNAS), not to mention the hourlong National Geographic Channel documentary running on cable since last October, with more coverage on the way from the History Channel and PBS's prestigious program NOVA.

Uh, yep. Sounds familiar so far. Now is the time to bring out the spurned skeptics:

"The whole thing is contrived," says geochemist and impact specialist Christian Koeberl of the University of Vienna, Austria. "Their data don't agree with anything we know about impacts. It just doesn't make any sense. Occam's razor has been put safely in a drawer somewhere."

Kerr lists an impressive array of scientists who think the evidence for an impact doesn't add up. I don't have any particular opinion -- my main skepticism concerns the proposed link between climate change and Clovis, which is a separate issue from whether an impact occurred.

But the story is a good one to watch, and has many parallels to the Flores hobbit story, considering different standards of evidence, the interactions of specialists from different scientific specialties, and the use (or abuse) of the press.

P. S. A number of abstracts from the 2007 AGU meetings pick up the question.


Firestone RB and lots of others. 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proc Nat Acad Sci USA, 104:16016-16021. doi:10.1073/pnas.0706977104

Kerr RA. 2008. Experts find no evidence for a mammoth-killer impact. Science 319:1331-1332. doi:10.1126/science.319.5868.1331