Editor's note: The online version of this story was posted on December 14, 2009.
Before humans arrived, the Americas were home to woolly mammoths, saber-toothed cats, giant ground sloths and other behemoths, an array of megafauna more impressive than even Africa boasts today. Researchers have advanced several theories to explain what did them in and when the event occurred. A series of discoveries announced last fall, at first glance apparently contradictory, add fresh details to the mystery of this mass extinction.
One prominent theory pegs humans as the cause of the demise, often pointing to the Clovis people, who left the earliest clear signs of humans entering the New World roughly 13,500 years ago. The timing coincides with the disappearance of megafauna, suggesting the Clovis hunted the animals to extinction or infected them with deadly disease. Another hypothesis supposes that climate was the culprit: it had swung from cold to warm twice, including a 1,300-year-long chill known as the Younger Dryas; such abrupt shifts might have overwhelmed the creatures’ abilities to adapt.
To pin down when the megafauna vanished, paleoecologist Jacquelyn Gill of the University of Wisconsin–Madison and her colleagues analyzed fossil dung, pollen and charcoal from ancient lake sediments in Indiana. The dung of large herbivores harbors a fungus known as Sporomiella, and its amounts in the dung gives an estimate of how many mammoths and other megafauna were alive at different points in history. Pollen indicates vegetation levels, and charcoal signals how many fires burned; the extent of flora and wildfires is related to the presence of herbivores, the researchers say in the November 20 Science. Without megaherbivores to keep them in check, broad-leaved tree species such as black ash, elm and ironwood claimed the landscape; soon after, buildups of woody debris sparked a dramatic increase in wildfires. Putting these data together, Gill and her team conclude that the giant animals disappeared 14,800 to 13,700 years ago—up to 1,300 years before Clovis.
A different study, however, suggests that this mass extinction happened during Clovis. Zooarchaeologist J. Tyler Faith of George Washington University and archaeologist Todd Surovell of the University of Wyoming carbon-dated prehistoric North American mammal bones from 31 different genera (groups of species). They found that all of them seemed to meet their end simultaneously between 13,800 to 11,400 years ago, findings they detailed online November 23 in the Proceedings of the National Academy of Science USA.
But if ancient DNA recovered from permafrost is any sign, megafauna survived in the New World millennia after humanity arrived. As the permafrost in central Alaska cracked during springtime thaws, water that held DNA from life in the region leaked in, only to freeze again during the winter. As such, these genes can serve as markers of “ghost ranges”—remnant populations not preserved as fossil bones. Looking at mitochondrial DNA, evolutionary biologist Eske Willerslev of the University of Copenhagen and his colleagues suggest mammoths lasted until at least 10,500 years ago (as did horses, which actually originated in the Americas only to vanish there until the Europeans reintroduced them). The Proceedings of the National Academy of Science USA published those findings online December 14.
Although the three papers appear to conflict with one another, they could be snapshots from the beginning, middle and end of a mass extinction. “If they seem to disagree, it is for the same reason as in that fable about the three blind men trying to describe an elephant—or mammoth?—by touching different parts of it,” says ecologist Christopher Johnson of James Cook University in Australia, who did not take part in any of the studies.