Source: Stevens, William K. 1997. New Suspect in Ancient Extinctions of the Pleistocene Megafauna: Disease. New York Times, 4-29-97.
When the first humans migrated from Asia to North America 15,000 years ago or more, they confronted an array of big animals more dazzling than that of modern Africa. Mammoths, mastodons, camels, horses and a stately deer called the stag-moose coexisted with giant ground sloths and beavers the size of today's black bears.
These plant-eaters were prey for meat-eaters like saber-toothed cats, savage short-faced bears, cheetahs, maned lions and dire wolves, bigger versions of today's wolves. Scores of other large species roamed the continent as well.
They all vanished about 13,000 years ago in a geological eye blink of perhaps 400 years. The cause of this mass extinction and similar ones elsewhere in the world near the end of the Pleistocene era has been a much debated mystery.
One camp in the debate contends that a rapidly changing climate at the end of the last ice age did in the Pleistocene megafauna, as they are called, by transforming their habitat. An opposing camp, pointing out that the extinctions closely followed the animals' first contact with modern humans, holds that the megafauna were essentially hunted to extinction. The hunters' blitzkrieg was made easy, they contend, because the naive victims had no fear of people and were easy prey for prehistoric humans with well-honed spears.
Neither camp is about to give in, and now a third hypothesis has emerged. Scientists who find neither the climatic nor the blitzkrieg theory convincing argue that rampant disease was the main villain. In this view, the megafauna were betrayed not by the naivete of the big animals themselves but that of their immune systems. And it was not the spears carried by people that wiped them out, but the pathogens carried by dogs, rats, birds, parasites and other living baggage that accompanied the continent's first human arrivals from Siberia.
It is simply too much to believe that "a few thousand Indian men with pointed sticks could run around a continent and bring to extinction 135 species in maybe 400 years," said Dr. Ross D.E. MacPhee, a mammalogist at the American Museum of Natural History in New York, who advances the new disease hypothesis. He described it recently at a symposium sponsored by the museum's Center for Biodiversity and Conservation.
MacPhee is the first to acknowledge that he has an uphill battle in getting his view accepted, not least because he is only beginning to search for hard evidence to support it, and because most experts at the symposium appeared to favor the blitzkrieg hypothesis. But the originator and chief advocate of that view, Dr. Paul S. Martin of the University of Arizona, said he wished he could accept the disease hypothesis, even though he considered it improbable. It would add another mechanism, in addition to hunting, by which humans could have instigated the extinction of so many species.
Others welcomed the new entry in the debate, even though they remained skeptical. "It's wonderful that it's being thrown into the mix," said a blitzkrieg supporter, Dr. Tim F. Flannery, a research scientist at the Australian Museum in Sydney.
There have been a number of mass extinctions in the earth's history, and scientists believe most have been caused by changing climate or the impact of asteroids or comets. The disappearance of the dinosaurs some 65 million years ago was one of those events. But the North American extinctions at the end of the ice age differed from earlier ones: they coincided not only with the most recent retreat of the great glaciers but also with the first appearance of human hunters, they were very rapid and they were severe only in big mammals; small mammal species were generally unaffected.
Those who subscribe to the climate-change theory of the extinctions argue that the driving force behind them was the destruction of habitat brought about by oscillating periods of warmth and cold during the Pleistocene era, the era of on-and-off glaciation that began about a million years ago.
In this hypothesis, advocated by Dr. Russell W. Graham of the Denver Museum of Natural History among others, North America in the earlier Pleistocene was dotted shore to shore by different habitats, each favorable to particular species. As a result, the megafauna were widely distributed as well, since they could find forage anywhere. But the changing climate, according to this hypothesis, changed the landscape as well; instead of widely scattered patches, habitat types like forest, savanna and prairie tended to be increasingly limited to certain parts of the continent. Over hundreds of thousands of years, this gradually shrank the ranges of the large animals, whose survival required not only suitable forage but also lots of territory from which to glean it. The stag-moose, for instance, became confined to the Great Lakes region.
Critics of this view argue that there was little difference between the climatic changes that occurred at the end of the last ice age and those at the end of several earlier glaciations, when the same kinds of animals did not go extinct. But Graham replies that since the geographic ranges of animals were gradually reduced over a long time, no out-of-the-ordinary climatic event was required.
The blitzkrieg theory, on the other hand, points to the remarkable coincidence of mass extinctions with the first arrival of prehistoric but anatomically modern humans in many parts of the world at various times over the last 40,000 years. Only Africa, where humans and megafauna evolved together over millions of years, is an outright exception. But in general, there was what MacPhee calls "this dreadful syncopation -- humans arrive, animals disappear."
Martin's blitzkrieg theory goes this way: The first humans to arrive in North America from Siberia were already highly skilled, experienced hunters of big game. The continent that greeted them was rich in big animals, and the package of nourishment in a single animal was so large that "you can waste it and go out and kill another one." These optimum conditions would have enabled the new human populations to expand to their maximum. The expanded populations would sweep over the landscape, flooding a given area with people for a year or two before moving on.
"The animals totally new to this kind of encounter will vanish," Martin said. The killing front would advance perhaps 100 miles in 10 years, he said, eventually resulting in a continent-wide mortality rate far above what large mammal populations can survive given their relatively slow reproduction rates and normally low death rates.
Critics of the blitzkrieg hypothesis point to a paucity of fossil evidence showing that hunters killed big game. Such evidence has been difficult to find, Martin said, partly because one must look for it in an extremely thin geological stratum representing only a decade. Even today, according to one report at the New York symposium, it is difficult if not impossible to find evidence in the remains of thousands of modern elephants, shot in Africa as part of recent efforts to cull herds, that they were killed rather than dying naturally.
In another objection, MacPhee points out that hunters of the 19th and 20th centuries could not wipe out the great whales (blitzkriegers say the ocean is very large) or the American bison. Blitzkriegers reply that not all big mammals would necessarily have been killed by Pleistocene hunters, and indeed bison, deer, elk, moose, wolves and mountain lions are among the survivors. Moreover, they say, the bison and some others have been saved only through present-day conservation efforts.
Nevertheless, MacPhee says he still disbelieves the idea that primitive people could wipe out a whole continent full of mammals amounting to more than 11 dozen species. "You would have to do nothing morning, noon and night except kill," he said, "and kill on a scale never seen in any technologically sophisticated society."
The disease hypothesis, worked out by Dr. MacPhee and Dr. Preston A. Marx, a virologist at the Aaron Diamond AIDS Research Center in New York, holds that the animals were infected by lethal pathogens unknown to their immune systems. (Similarly, diseases introduced later by Europeans ravaged populations of immunologically defenseless Indians.) As envisioned by Marx, disease would have advanced across the continent in the wake of humans, either reducing animal populations to levels from which they could not recover or springing up again and again to infect new generations.
For this to have happened, Marx said, several conditions would have been necessary: The killer pathogen would have had to kill rapidly. It would have had to affect all age groups in a given animal population (something hunters do not do). It would have had to have an independent host, either people or creatures that arrived with people, to provide a reservoir of immune carriers from which the disease could spread. And it would have had to affect a broad array of species without causing epidemics in humans.
A number of diseases meet some of the criteria; canine distemper and rinderpest, for instance, have all caused severe reductions in populations of several animals. But they have not caused any known extinctions. MacPhee and Marx have so far identified only two existing pathogens that fulfill all the requirements: leptospirosis, a bacterium spread in rat urine, and the rabies virus. The point is not that either of these diseases were necessarily the actual agents of extinction, the two scientists say, but rather that there exist some pathogens that conceivably could have done the job.
The disease hypothesis can be tested, MacPhee said, by examining mummified remains of Pleistocene animals to see if there are genetic traces of disease organisms in their bodies. He and Marx are setting out to perform such tests. "Our assumption is that the pathogen is still with us in a more benign state and that existing species are handling it well enough to survive," Marx said. That would provide some clue as to what to look for. But if the responsible pathogen is itself extinct and has no present-day relatives, he said, "we're going to have a tough time finding it."
Could the extinctions have had multiple causes? One expert, Dr. A.J. Stuart of the Norfolk Museums Service in Norwich, England, argues that climatic change did indeed reduce some populations of Eurasian megafauna to vulnerable levels, and that hunting pressure supplied the coup de grace. One set of extinctions took place roughly coincident with the arrival of modern humans in Europe about 40,000 years ago, but others did not take place until later. While these later extinctions did not coincide with the first arrival of humans, they did coincide with the climatic disruptions that occurred at the end of the ice age. Stuart calls this "a potent reason" for rejecting the idea that human overkill alone caused the extinctions.
Another possible factor is that big carnivores shifted their attention to more kinds of animals as humans killed their preferred prey, thereby helping to press the substitutes to extinction. Once their prey became extinct for whatever reason, it is widely thought, many of the predator species vanished as well. In any case, the American maned lion, the dire wolf, the short-faced bear and the saber-toothed cat no longer exist.
Some scientists see the idea of multiple causes as a kind of intellectual cop-out. It is better, said MacPhee, for one, to "develop and test hypotheses singly and then they either succeed or fail singly."
But the day when that happens, or when it becomes clear that several causes are responsible, appears far off.
by William K. Stevens © New York Times, 4-29-97.
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