W A S H I N G T O N, June 8, 2001 -- For more than a century, scientists havedebated what killed off the big animals in Australia and theAmericas. Two new studies place the blame on squarely on ancienthuman hunters equipped with fire, spears and an appetite for meat.
The studies, appearing today in the journal Science, concludethat after early humans migrated into Australia and the Americas,the heavyweight animals of these new continents were driven toextinction within a few thousand years. In the Americas, 73 percent of the large plant-eaters, alongwith the saber-toothed cat, were gone within 1,200 years afterhumans migrated to the continents about 13,600 years ago. Wiped outwere animals like mammoths, camels, mastodons, large ground slothsand the glyptodont, a strange armored creature the size of a smallcar and weighing more than 1,400 pounds. In Australia, researchers precisely dated bone specimens ofelephant-sized marsupials, giant snakes, huge lizards and otherextinct animals. They found that the wildlife disappeared within10,000 years after humans arrived at the down-under continent.
The research contributes powerful new evidence to a century-olddebate among scientists intrigued by the question: What killed offthe big animals in newly settled continents of the world? Some have long blamed humans, but other experts say it couldhave been climate change, disease or a gradual change in habitat. The two new studies pin the blame firmly on humans. "Human population growth and hunting almost invariably leads tomajor mass extinctions," said John Alroy of the University ofCalifornia, Santa Barbara, author of the study of the Americanextinctions. "The results show how much havoc our species can cause, withoutanyone at the time having the slightest idea of what is going on,much less any intention of causing harm," Alroy said. Linda Ayliffe of the University of Utah in Salt Lake City saidprecise dating of rocks and fossils from 27 sites in Australia andWest Papua New Guinea clearly show that large animals theredisappeared around 46,000 years ago, or about 10,000 years or soafter the arrival of humans. The rapid demise during that time of 55 species — every landanimal, reptile and bird in Australia weighing more than 220 pounds— is strong evidence for human involvement in theextinctions, said Ayliffe. "It is clear that the downward spiral of these animals wasafter the arrival of humans," she said. Ayliffe said the dating is significant because some researchershave blamed the extinctions on extended droughts that occurredlater. But she noted that the animals had withstood climate changespreviously; so it is unlikely they all would have succumbed tonatural forces. Also, disease is improbable since so many differentspecies of reptiles, birds and mammals disappeared at about thesame time. Diseases are unlikely to affect all species the sameway.
Largest Known Bird
Among the Australian victims was the largest known bird, aflightless, ostrich-like creature that is thought to have weighedabout 220 pounds. Another victim was a claw-footkangaroo that weighed more than 600 pounds, andstill another was a 20-foot-long lizard. Ayliffe said it is unlikely that hunting alone led to thedisappearance of so many large animals. She said there is evidencethat humans 55,000 years ago used fire as a hunting tool, burningvast areas of Australia. Such fires would change the habitat, which would make itdifficult for large animals that required plenty of forage tosurvive, she said. In his study, Alroy created a computer model that factored insuch elements as the number of hunters, the number of animals,distribution of prey species and competition among prey for food. He found that with man in the equation, virtually everycombination was bad news for the big animals of America. "In fact, it is hard to find a combination of … values thatpermits all species to survive," he said in the study. Alroy said since the animals evolved in the Americas beforehuman habitation, they probably had no fear after the hunters cameand were easy prey. "Their strategy for dealing with predators was to stand andfight, and that is the last thing they should do when dealing withhumans," Alroy said. Bison, elk and moose probably escaped extinction because theylived in areas, such as the central plains, with fewer humans andvast tracts of open land, he said. Paul Martin of the University of Arizona, Tucson, a leadingauthority on extinctions, said the two papers "strengthen the casefor human involvement in all these extinctions."