"Mass Extinctions Pinned on Ice Age Hunters" by Leigh Dayton, "A Multispecies Overkill Simulation of the End-Pleistocene Megafaunal Mass Extinction" by John Alroy, and "New Ages for the Last Australian Megafauna: Continent-Wide Extinction about 46,000 Years Ago" by Richard G. Roberts, Timothy F. Flannery, et al., in Science (June 8, 2001), American Assn. for the Advancement of Science, 1200 New York Ave., N.W., Washington, D.C. 20005.
It’s an Ice Age mystery: What caused the sudden mass extinction of huge, exotic mammals and flightless birds in the late Pleistocene era, 11,000 to 50,000 years ago? Climate change has been suggested. But the evidence is mounting against the prime suspect in the case, Homo sapiens, reports Dayton, a science writer in Australia.
Dating megafauna-bearing sediments from 28 sites across Australia, scientists led by Roberts, a geochronologist at the University of Melbourne, and Flannery, a mammalogist at the South Australian Museum in Adelaide, found that a continent-wide extinction of large animals took place about 46,000 years ago—not many millenniums after humans appeared on the Australian scene. Though the evidence is circumstantial, Roberts thinks it "definitely" implicates humans. But the lethal blow that humans delivered to frightful 660-pound, claw-footed kangaroos, flightless 220-pound Genyornis birds, and other huge beasts was indirect, he believes. Aborigines habitually set fire to the landscape, perhaps to make hunting and traveling easier, and so reduced the megafauna’s food supply. Hunting and climate change may have pushed the big animals the rest of the way to extinction.
"In North America, by contrast," writes Dayton, "hunters may have been in the thick of the faunicidal fray." Ice Age America had saber-toothed tigers, giant antelopes, woolly bison, and woolly mammoths. But by the end of the Pleistocene era, 11,000 years ago, more than two-thirds of the large mammals had died out—once again, after humans had arrived on the scene. According to the "blitzkrieg" hypothesis put forth in 1967 by geoscientist Paul Martin of the University of Arizona, Tucson, early huntergatherers followed their prey across the top of Asia to North America, then southward. Wiping out animals locally, the hunters ultimately drove populations to extinction.
To test Martin’s theory, Alroy, an evolutionary biologist at the University of California, Santa Barbara, recently ran computer simulations of such an invasion of human hunters in North America, starting 14,000 years ago, and the impact it would have had on 41 species of large, plant-eating animals. "Alroy found that no matter how he adjusted the variables, mass extinctions ensued," Dayton writes. "Even the slowest, clumsiest hunters unleashed ecological devastation," and the largest animals were hardest hit. Hunting and human population growth could have done in the megafauna even without climate change.
But "not everyone is convinced," notes Dayton. Biologists Ross MacPhee and Alex Greenwood, of the American Museum of Natural History in New York City, say that Alroy’s hunter argument fails to explain why extinctions ceased 10,000 years ago, instead of continuing into the current era, the Holocene. But MacPhee and Greenwood don’t let Homo sapiens completely off the hook. They suspect that the human newcomers brought with them a lethal, highly contagious virus, and that it did in the woolly mammoth and the other behemoths of the Ice Age.