The end of the last ice age, some 10,000 years ago, was a rough time for big beasts in North America. Three-quarters of the mammoths, mastodons, and other hulks disappeared forever. New evidence suggests it was human hunters, not wrenching climate shocks, that caused their extinction.
The key to sorting out this story turned out to be a dung-dwelling fungus. Microscopic spores of the fungus Sporormiella wash into ponds, lakes, and bogs from the dung left by watering large animals and leave an enduring record. Paleoecologists Guy Robinson, David Burney, and Lida Pigott Burney of Fordham University in New York City, looked for the spores and other markers in mud layers at four sites in southeastern New York.
Their analysis revealed a clear progression of events. First, the megafauna population collapsed, as indicated by an abrupt 10-fold decrease in Sporormiella spores to trace amounts. Soon after, the abundance of microscopic bits of charcoal, blown from large fires, jumped 10-fold, indicating the presence of humans. And only about a thousand years later did the pollen of boreal forest trees such as alder, birch, and spruce reappear, marking a sharp cooling at the beginning of the last gasp of the latest ice age. That means climate change was such a latecomer that it could not have played a major role, the researchers told the joint meeting of the Geological Society of America's Northeastern Section and the Atlantic Geoscience Society in Halifax, Nova Scotia, in late March.
Robinson speculates that the process started when human hunters culled the plant-eating megafauna. That led to an overabundance of fuel for fires, natural and human induced. Only much later, in the renewed cold, did the last few of the great beasts die out.
But new evidence in this contentious field takes time to sink in. “Their approach is a novel one that deserves some additional attention,” says paleobotanist Norton Miller of the New York State Biological Survey in Albany. “My own bias is more toward climate than anything, [but] the fungal story provides some good information that would argue against climate.”