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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-sized beavers built dams in the streams.
By about 10,000 years ago, all of these animals -- and others, such as American
lions, cheetahs, sabertooth 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
pattern of North American vegetation, progressively shrinking the habitats of the continent's big mammals until they became extinct.
Another recently proposed scenario casts human immigrants (or perhaps animals
or insects they brought with them) as unwitting deliverers of a killer virus
devastated the continent's wildlife.
Did hunters wipe out the American megafauna? Did climate change do it? Or was it a plague?
Scientists on all sides of the debate are hampered by the limits of what can
be proved by examining animal fossils and stone spear points. Fossils can't
researchers the size of animal populations at various times in prehistory or pinpoint precisely when they died out, although they can suggest an approximate
chronology. Sophisticated chemical analysis of bones can provide some clues about an animal's diet. In rare cases, genetic or immunological tests on well-preserved
soft tissues may be able to 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.
He argues that
because so many species of large American mammals disappeared about 11,000 years ago, overhunting by the new arrivals is the most plausible explanation. There
are abundant examples of extinctions occurring soon after humans arrived on islands, apparently caused by hunting, and Martin believes the same thing could have
happened on a continent-wide scale.
"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
without serious limit, at a really rapid rate. Within 1,000 years, our species can sweep through the Americas."
But critics respond that if Clovis hunters killed off the mammals, there should
be more fossil evidence of the deed. Clovis people's stone weapon points have
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
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. . . . People arrived and killed the last few on the landscape."
Donald Grayson, a professor of anthropology at the University of Washington,
agrees. He points to recent archaeological evidence from Monte Verde, Chile,
humans had settled in the Americas about 13,000 years ago, well before the Clovis people arrived and the major wave of mammalian extinctions 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
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 homogeneous -- 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 extinctions in the
Americas, it should have caused the same phenomenon globally, argues Ross D.E.
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 impressed with the fact that extinctions in the Americas
and several other places seem to have closely followed the arrival of humans.
he doubts that overhunting is the explanation, noting that no whale or seal species has been driven to extinction in the past 200 years despite extreme overhunting.
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
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."
File Date: 011.22.01
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