Overkill, Not Overchill

What ever happened to the saber-toothed tiger, the dire wolf, the mammoth, the giant beaver, and more than 100 other species of large mammals that once inhabited North America? All that paleontologists know for sure is that about 10,000 years ago, as glaciers retreated northward into Canada during the Late Pleistocene epoch, these animals suddenly became extinct. Their demise, many scientists believe, was caused either by sudden climatic change --which upset their breeding season and produced a lethal sterility--or simply by winter weather, which ironically may have become increasingly severe as the glaciers waned.

Not likely, says University of Arizona Geochronologist* Paul Martin. Writing in the current issue of Natural History, he suggests it was "overkill," not "overchill," that caused the disappearance of large numbers of species. In North America, as well as on other continents, he says, "the pattern and timing of large-scale extinction corresponds to only one event--the arrival of prehistoric hunters."

Primitive Version. To support this hypothesis, Martin points out that the large mammals that perished during the Late Pleistocene had survived previous advances and retreats of the ice sheets. And although scientists generally agree that the major climatic changes of the past 50,000 years occurred at approximately the same time throughout the world, the disappearance of species did not. Thus the antlered giraffe disappeared from Africa more than 40,000 years ago, and the rhino-sized Diprotodon and giant kangaroo became extinct in Australia about 14,000 years ago. In Europe and Asia, the woolly rhinoceros and the woolly mammoth ceased to exist between 11,000 and 13,000 years ago, before the species disappeared in North America. Yet on Madagascar, the extinction of giant lemurs and pigmy hippopotamuses did not occur until within the past 1,000 years.

To Geochronologist Martin, the pattern is clear. The demise of these animals closely follows the migration of man, the hunter. In Africa, for example, the disappearance of many species of big game seems to coincide with the first record of fire in archaeological sites. Fire, Martin speculates, was used by the early African hunters to encircle entire herds of animals. With this technique, they destroyed more animals than they needed for food and clothing--a primitive version of overkill. In North America, the musk ox suddenly died out in a large swath across what is now Canada and the U.S. between 10,500 and 12,000 years ago--around the time that the Stone Age hunters were migrating along this route after crossing the Bering Strait. But in northerly areas of Canada and Greenland that were untouched by glaciation yet isolated from hunters by the continental ice sheet, musk oxen managed to survive.

To discount his hypothesis, Martin believes, it would only be necessary to identify a major wave of extinction anywhere in the world during the Late Pleistocene prior to man's arrival. "To date," he says, "such evidence has not been found."

*A specialist in the chronology of the past as indicated by geological and paleontological data.

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