Friday, May 26, 2006

The big kill

Here's an interesting graph from the May 11 issue of Nature, from a paper by R. Dale Guthrie titled "New carbon dates link climatic change with human colonization and Pleistocene extinctions."

The red-black columns left-to-right represent carbon-dated Alaskan and Yukon Territory fossils of mammoths, horses, bison, elks, and moose, respectively. The blue dots at right represent dated evidence of human presence, mainly hearth charcoal). The vertical scale is time, from 18,000 to 9000 years before the present.

Clearly something big happens between 13,000 and 12,000 B.P., just at the end of the last ice age. Mammoths and horses become extinct in North America (along with other species not shown on the graph). And human arrive, presumably by crossing the Bering Strait between Siberia and Alaska.

The coincidence of the extinction of many large animals with the arrival of humans has long been taken as evidence for human overkill, a terrible and decisive slaughter as humans armed with bladed spears and hurling devices moved down across a pristine continent, a truly catastrophic consequence of global warming.

But Guthrie thinks the evidence for human overkill is not so clear cut. He thinks the extinctions and human arrivals might be separately related to climate change. During the transitional period from ice age to post-ice-age it seems (from the graph) that bison and elks flourished, perhaps sufficiently altering the landscape to the detriment of mammoths and horses. Meanwhile, humans drifted into the Americas with perhaps less decisive impact than formerly believed.

Still, the graph looks gravely suspicious. Have we unfairly implicated our ancestors in one of the great ecological disruptions of all time? Or are they unfairly condemned by coincidence?