Megafauna extinction mystery
WASHINGTON As recently as 20,000 years ago, North America had an array of large mammals to rival the spectacular wildlife of modern Africa. Mammoths bigger than African elephants, as well as smaller, pointy-toothed mastodons, ranged from Alaska to Central America. Herds of horses and camels roamed the grasslands, while ground sloths the size of oxen lived in the forests and bear-size beavers built dams in streams.
By about 10,000 years ago, all these animals and others, such as saber-toothed cats and giant bears were gone. Some 70 North American species disappeared, three-quarters of them large mammals. Why?
The question has fascinated archaeologists, geologists, biologists and anthropologists for decades. One long-popular theory holds that the Clovis people, Stone Age immigrants from Asia who appeared in North America about 11,000 years ago, swept across the continent and hunted most of its large mammals to extinction.
But proponents of alternative theories suggest that the animals died of natural causes. According to one view, rapid climate shifts at the end of the Ice Age altered the pattern of North American vegetation, shrinking the habitats of the big mammals until they became extinct.
Another scenario casts human immigrants (or perhaps animals or insects they brought with them) as unwitting deliverers of a killer virus that devastated the continent's wildlife.
Did hunters wipe out the American megafauna? Did climate change do it? Or was it a plague?
Scientists are limited by what can be proved through examinations of fossils and stone spear points. Fossils can't tell precisely when animal populations died out, although they can suggest an approximate chronology. In rare cases, genetic or immunological tests on well-preserved soft tissues may yield evidence of infectious diseases. Paul Martin, professor emeritus of geosciences at the University of Arizona in Tucson, is the most vigorous proponent of the "overkill" theory. Because so many species of large American mammals disappeared about 11,000 years ago, he argues, over-hunting by the new arrivals is the most plausible explanation. There are abundant examples of extinction occurring soon after humans arrived on islands, apparently caused by hunting, and Martin believes the same thing could have happened continent-wide.
"People can do it really fast and it won't leave much evidence behind," he said. The Clovis people "found a favorable environment. Their numbers would increase without serious limit, at a really rapid rate. Within 1,000 years, our species had swept through the Americas."
But critics say that if Clovis hunters killed off the mammals, there should be more fossil evidence of the deed. Clovis people's stone weapon points have been found in association with mammoths, mastodons and bison but not with other mammals, noted Russell Graham, chief curator at the Denver Museum of Nature and Science.
Climate was probably paramount, according to Graham, who presented new evidence to support his position last week at the Geological Society of America's annual meeting in Boston. "I would argue that [mammoths' and mastodons'] ranges were already collapsing" because of climate change when the Clovis hunters showed up, he said. "I think they would have gone extinct without people."
Donald Grayson, an anthropology professor at the University of Washington, agrees. He points to recent archaeological evidence from Monte Verde, Chile, that humans had settled in the Americas about 13,000 years ago, well before the Clovis people arrived and the major wave of mammalian extinction occurred.
Grayson said that at the end of the Ice Age, the melting of the glaciers that had covered Canada and the northern United States caused dramatic alterations in climate and vegetation. In the continent's interior, both winters and summers became more extreme. Landscapes that had contained a patchwork of trees and pasture became more homogenous either all forest or all grassland.
"There were complex combinations of plants that you don't find after that period of time," he said. Many animals shifted their ranges in response to changing habitat.
By constructing computer maps of the distribution of mammal fossils from different time periods, Graham sees evidence that the ranges of species like the Columbia mammoth and the Shasta ground sloth were steadily shrinking for thousands of years before they became extinct.
"Large animals require larger geographic ranges, and as you reduce the geographic range, the probability of extinction goes up exponentially," he said. "With small distributions, local effects like fire, disease and competition become very important."
But if climate change was severe enough to cause a wave of extinction in the Americas, it should have caused the same phenomenon globally, argues Ross MacPhee, curator of vertebrate zoology at New York's American Museum of Natural History. Yet most other regions were spared, even the nearby West Indies. "Why weren't things falling down in droves in Africa?" he asked.
Like Martin, MacPhee is struck by the fact that extinction in the Americas and several other places seems to have closely followed the arrival of humans. But he doubts that over-hunting is the explanation, noting that no whale or seal species has been driven to extinction in the past 200 years despite extreme over-hunting.
Instead, MacPhee is betting that a virus or other microbe new to the Americas arrived with human settlers and killed off many mammal species that had no natural resistance. He points to the devastation caused later among Native Americans by smallpox, measles and other "European" infections.
"Nothing in nature is able to cause such levels of havoc except emerging diseases," MacPhee said. "It was either the humans themselves that were vectors, or parasites of humans, or it could have been parasites of animals that came in with humans."
To fulfill MacPhee's "hyperdisease hypothesis," a new infection would have had to spread quickly among individuals of all ages and sexes, and would have been able to cross species barriers. He suspects it would have spread through the air. Candidates might include influenza and rinderpest, a disease of cattle that also affects deer, antelope and related species.
MacPhee is searching for evidence of such infections in frozen tissue from mammoths, ground sloths and other beasts that died out at the end of the Ice Age. An infected animal's immune system would make antibodies against the invading virus, chemicals that might be detectable. If antibody tests are positive, MacPhee plans to search for viral genetic material.
"No extinction is a simple matter. There's always an environment in which it happens," said MacPhee. "I need to show and convince people that disease by itself could be considered a primary factor, rather than a secondary or negligible one."
MAMMOTH LOSS Animals such as mammoths, giant beavers and camels roamed the North American continent until about 10,000 years ago. What happened to them? Some scientists think human immigrants caused their extinction. WASHINGTON POST PHOTO