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Feature Story
Photo of Guy Robinson
Mammoth Mystery

by Sharon Levy

Did climate change wipe out North America's giant mammals, or did our Stone Age ancestors hunt them to extinction?

Guy Robinson stands neck-deep in a hole he's dug in the dark earth of a woodland in southeastern New York State. To reach back to the time of the first humans to walk here -- people who knew and hunted 16,000-pound mastodons -- he uses a long auger to drill beneath his muddy feet. Hugging the tubular handle to his chest, he hauls up a cylinder packed with wet bits of sedge and cattail. as he hands me a fistful of soggy vegetation, Robinson looks as happy as a kid excavating a cookie jar. "This," he says, "could be mastodon fodder." These plants grew 12,000 years ago, when they risked being trod upon or munched up by the last of North America's native elephants.

Robinson, a paleoecologist at Fordham University, is finding evidence that the first humans in North America killed off an array of spectacular mammals. Mastodons, giant sloths, bear-size beavers, native horses and camels, along with the dire wolves and saber-toothed cats that preyed on them, all vanished from the continent at the end of the Pleistocene epoch, the last great Ice Age. At about the same time, a wave of Stone Age big-game hunters are believed to have made their way from Siberia to the New World.

Conventional wisdom once held that rising temperatures at the end of the Ice Age caused the mass extinction of big Pleistocene beasts, known collectively as the megafauna. In the 1960s, Paul Martin, then a maverick paleoecologist just beginning his career at the University of Arizona, published his theory that North America's giant mammals had perished in a "blitzkrieg" of hunting by the continent's first human settlers. Since then, the cause of the die-off has been hotly debated in a flurry of studies that look back at the vanished giants in inventive ways. Gripped by an obsession with the distant past, Robinson, Martin, and their colleagues are inspiring new insights into very contemporary problems, including the impact of global warming on wildlife and the best ways to preserve biodiversity as human populations continue to expand.

Evidence that humans helped eradicate the megafauna includes scattered finds of fossil mammoths, long-horned bison, and other extinct giants, their bones nicked by stone weapons, some with distinctive spear points buried between their ribs. Improvements in radiocarbon dating show that the last of the big beasts in North America died out around 11,000 years ago. That's the same time at which an elegant style of stone spear point, associated with a group known as the Clovis culture, appears in the archaeological record. Clovis artifacts dating back about 11,000 years have been found from California to Maine; their makers seem to have hunted their way rapidly across the continent.

Establishing this chronology isn't easy, in part because of the nature of radiocarbon dating, which gives only a statistical probability of an object's age, not categorical proof. Still, it's the most reliable way of dating objects from the late Pleistocene relative to one another, so paleontologists often speak in terms of radiocarbon years when piecing together the sequence of events.

Proponents of climate change as the cause of megafauna extinction cite examples of big beasts in Alaska that died out before humans ever settled there. Thirty years ago, scientists in this camp routinely argued that habitat change caused by the warming trend at the end of the Pleistocene was the killer. More recent data have made it clear that the last of the megafauna expired before the heat wave that ended the Pleistocene, leading some scientists to suggest that the animals were wiped out by the Younger Dryas event, a final frigid spell that ended around 10,000 years ago. However, those who believe the extinctions had a human cause point out that neither of these climate changes was more severe than many the giant mammals had weathered during the waxing and waning of the Ice Age over hundreds of thousands of years.

Photo of ribs from an extinct stag mooseRobinson believes that by tracking the arrival of humans, the timing of habitat change, and the demise of large herbivores in a single limited geographic area of North America, he can bring the story of the Pleistocene giants into a new, clearer focus. He works with ancient pollen and flecks of charcoal that he digs up in the Black Dirt region of Orange County, New York, where farmers plant onions in deep peat that was once the bottom of the vanished Lake Fairchild, a broad, shallow swamp filled with glacial meltwater. For generations, plows in Black Dirt country have been turning up the bones of mastodons, stag moose, and other long-gone monsters. The very first mastodon remains found in North America, a set of weighty molars that seventeenth-century colonists mistook for the teeth of human giants, were discovered nearby.

Still, big fossils are rare, and early human artifacts much rarer. By relying too much on conventional archaeological finds of bones and spear points, says Robinson, researchers have been missing vital evidence. To tell the story of New York's last megafauna, he's turned instead to minuscule but widespread markers left behind in ancient soil. He's tracked the fossil spores of Sporormiella -- a fungus that grows only on the dung of large herbivores and is common in most sediments from the Pleistocene. By showing that the spores disappear from the sedimentary record just before a burst of microscopic charcoal marks what he contends were man-made fires, Robinson has made an intriguing new argument indicting people for the extinctions.

Bad Chemistry: A Special Report
Forecast: Dry and Brutal

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Sharon Levy spent a decade working as a field biologist in the woods of Northern California before taking up science writing full time. She is a regular contributor to National Wildlife and BioScience.

Photos: Masood Kamandy

OnEarth. Winter 2006
Copyright 2005 by the Natural Resources Defense Council