To Kill a Mammoth

ScientificAmerican.com had an interesting story last month about some fancy detective work investigating the disappearance of North America's megafauna. What we know is that 15,000 years ago, after the last great glaciation, North America was home to an incredibly impressive array of large mammals - one that would rival or exceed that seen on the plains of Africa. American lions, saber-toothed cats, mammoths, mastodons, giant sloths - this list goes on. Even giant birds like the teratorn. We also know that this tremendously diverse group of large animals disappeared rather quickly during the Pleistocene. What we don't know is why.

One possible answer is a familiar one - us. A lot of evidence points to the disappearance of North America's large mammals coinciding closely with the appearance of a moderately sized one known as Homo sapiens. In the Late Pleistocene, human hunter-gatherers were migrating from northern Asia into the Americas across the Bering Land Bridge. The guys that usually take the fall are the Clovis people, thought to have made the trek around 13,500 years ago. The southward spread of the Clovis culture, and their distinctive spear points, seems to coincide rather closely with the demise of the the mammoths and their brethren. A lot of healthy debate remains, though.

The Scientific American story details several studies that have investigated the question using dramatically different techniques. In the November 20, 2009 issue of Science, Jacquelyn Gill and coworkers describe their work in which they examine various evidence, including fossil dung. of large herbivores. The dung deposits contain a fungus known as Sporomiella, the amount of which can be used to estimate the density of mammoths and their contemporaries. In addition, pollen and charcoal deposits can be utilized to estimate plant coverage and fire frequencies, both indicators of herbivore densities. From this evidentiary potpourri, Gill and her coworkers deduce that the majority of the big guys disappeared between 14,800 and 13,700 years ago. That would put their downfall well in advance of the arrival of the Clovis culture.

A somewhat more direct line of evidence is discussed in a paper published online by in the Proceedings of the National Academy of Sciences. J. Tyler Faith and Todd Surrovel report their results from carbon-dating of fossil specimens from different genera of North American mammals. They found that, of the 35 general that went extinct during the Pleistocene, 16 could be reliably said to have have vanished between 13,800 and 11,400 years ago - well within the time interval that would allow a Clovis conviction.

But wait... Still another study, this one appearing online at PNAS on December 14, examines DNA extracted from frozen permafrost in Alaska (yes, you read that correctly). Using this pretty sexy technology, Eske Willerslev and his team show the presence of horses and mammoths in Alaska as recently as 10,500 years ago, long after the arrival of human civilization and their supposed decimation of the great North American mammal fauna.

So - three studies, three seemingly contradictory results. But, not necessarily. While it has certainly been suggested that the extermination of the megafauna could have occurred in a geological instant, that doesn't mean that it couldn't have spanned a period of a few thousand years. It's possible that the these studies are painting a portrait of a dramatic extinction event as it started, progressed, and ended.

Considered using some footage from 10,000 B.C. up top. But the cartoon was more believable.