The Cold Zone

Testing a new theory of mammoth extinction in

By Richard Stone|Thursday, February 01, 2001


The view inside the vintage Aeroflot helicopter skimming across the Taimyr Peninsula in northern Siberia is clouded by cigarette haze, but outside all is clear in the subzero air. Treeless hills cast bluish shadows, and ice-packed rivers snake through the frozen tundra, glittering in the late afternoon sun like golden blasts of dragon fire. A herd of shaggy reindeer below, startled by the insistent whop-whop-whop, sprint past larch trees of the Ary Mas reserve, the northernmost forest in the world. As the helicopter banks, a cluster of tents appears on the horizon. A half-dozen people seem frozen by the sight of the orange-and-blue flying machine.

Members of this expedition, led by veteran Arctic explorerBernard Buigues, have been wondering if they would ever see this bird again. It had dropped them off at the barren site two days earlier, with supplies for a single night's stay. Because the satellite phone had failed, they had no way of knowing the helicopter had been diverted to carry mechanics and parts to a remote village for the emergency repair of an electric generator. After 24 hours of hunger and uncertainty, buffeted by 25-mile-per-hour winds and temperatures as low as 50 degrees below zero, everyone seems eager to break camp. Everyone except Buigues.

He smiles at his rescuers, eager to show off. "Come, follow me," he insists. Without bothering to cover his balding head with a fur-lined parka hood, he strides past the tents and down a steep bank to a frozen stream where gusts whip new snow into curlicues. As Buigues slows, the source of his excitement becomes more obvious: The tops of five chestnut-colored spinal vertebrae jut out of the ground like the back of an old sea serpent. They are the ancient remains of a woolly mammoth that Buigues and his colleagues have dubbed Hook. Buigues gets down on his hands and knees and points to tufts of hair. Can there be frozen flesh too? "We suppose that about half the mammoth is in the permafrost," he says, uncertain because the cold has sucked the power from the batteries in the expedition's ground-penetrating radar. The team has marked the site with a 20-foot-high pole so that they can find it again, perhaps in a few months.

If exhumed, the Hook carcass could help solve one of the great mysteries of species extinction. Countless woolly mammoths— elephantlike creatures with downward sloping hindquarters, small ears, and tusks up to 16 feet long— once roamed the dry, frigid grasslands of Siberia. But about 11,000 years ago, the entire mammoth population began to die off. Scientists have speculated that the giants were either driven to extinction by prehistoric human hunters or that they succumbed to a warm-up at the end of the Great Ice Age that blighted their food supply. Buigues and a research team that includes Ross MacPhee, curator of mammals at the American Museum of Natural History in New York City, have come here hoping to gather DNA samples to test yet a third theory of mammoth extinction. MacPhee believes the beasts may have been the victims of an apocalyptic disease, something he calls "the killer plague of all time."

About 250 miles southwest of Taimyr Lake lies Khatanga, a town of 5,000 where houses are propped up on pilings to keep them from sinking into the permafrost. There, a former savings bank has been converted into a crude laboratory filled with animal bones. Dick Mol, an amateur paleontologist who serves as scientific coordinator for the expedition, sets a fragment of mammoth shinbone on a table. "Let's drill it," says MacPhee. He picks up an orange Black and Decker hand drill, pushes the 1/4-inch bit against the bone, and pulls the trigger. As smoke spirals up, he douses the deepening hole with water. "It's like it's on fire," says MacPhee as the room fills with the odor of hot collagen.

The shinbone is one of more than 1,000 specimens gathered recently by several two-man brigades hired by Buigues to scour the Taimyr Peninsula, in the largest mammoth-hunting expedition ever mounted. Relics crowding workshop shelves include everything from bow-shaped mammoth ribs to umber jawbones of extinct horses, and even a wolf skull with small teeth. Masha, a gray cat, prowls among the bones.

MacPhee reaches into a pocket of his outdoorsman's vest and finds a plastic sample tube. He uses the back end of a paintbrush to ram the 11/2-inch-long bone core out of the drill bit and into the container. He takes duplicate samples, one for dating the remains with an atomic mass spectrometer, the other to be used in DNA analysis to search for a hyperdisease that could have raced from one woolly mammoth to another, thus wiping out the entire species.

MacPhee has spent much of his career puzzling over mysteries largely unexplored by other scientists. In the early 1990s, he began to focus on extinctions in the Caribbean islands that have occurred since 1500, when Europeans began exploring the New World. MacPhee saw a pattern that repeated itself everywhere he looked. "Among mammals in the last 500 years, the worst thing was to be small, like a rodent or a bat," he says. When Europeans showed up, "the extinction rate just shot up." When MacPhee and mammalogist Clare Flemming of the American Museum of Natural History compiled lists of disappeared species, they found a remarkable contrast: In the New World, only one in every eight extinctions was a large mammal, or megafauna, defined as weighing more than 100 pounds. But at the end of the last ice age, three of every four mammals that went extinct were megafauna. And in the 10,000 years in between, not a single mammal species disappeared from North America. "Nobody ever talked about this hiatus, and that led me to wonder what was actually going on. Evolutionary biologists tend to think about how species arise," he says. "They don't think about the end."

One recurring theme of both prehistoric extinctions and the recent losses, he noted, was an encounter between people and the vanquished mammals. This "dreadful syncopation— humans arrive, animals disappear," he says, isn't a phenomenon restricted to the New World: It has occurred on nearly all habitable landmasses except Africa and Eurasia, where humans evolved alongside animals. What was it about humans, MacPhee wondered, that dealt a hammer blow to mammals large and small?

About seven years ago, descriptions of outbreaks of the Ebola virus, which emerges every now and then and wipes out villages in Africa, piqued MacPhee's curiosity: "The idea that a disease could literally burn through a population was mind-boggling to me." He began to wonder whether a pathogen could become powerful enough to drive not just a single species to extinction but several species, including the mammoth and other large mammals.

MacPhee raised the idea with Preston Marx of the Aaron Diamond AIDS Research Center in New York City. Marx had trekked to places like Gabon and Cameroon to sample green monkeys, kept as pets or shot for the bush-meat market, in search of the primate version of the AIDS virus. "He's the kind of guy who's willing to go out on extreme limbs to investigate things," says MacPhee. "So I just popped the question one day: 'What do you think of disease as a cause of extinction?' " Marx listened, then rattled off a host of problems that had to be overcome for the idea to take wing. He doubted, for instance, whether a single pathogen, no matter how vicious, could annihilate species as diverse as mammoths and camels, woolly rhinos and saber-toothed cats, while allowing others— like reindeer and bison— to escape relatively unscathed. "But he was interested enough to give it a shot," MacPhee says.

Together MacPhee and Marx developed the framework for a theory. They defined a hyperdisease pathogen as one that strikes all age groups, killing at least three of every four individuals and capable of taking that heavy a toll on several species simultaneously, without seriously harming humans. While no known pathogen fits this description, some bear hallmarks that lend credibility to the idea. For instance, the influenza virus jumps the species barrier, frequently moving from pigs to people. And devastating strains emerge: During the worldwide 1918 outbreak, at least 20 million people died. That's not a large enough chunk of humanity to exalt flu to hyperdisease status. But, says Alex Greenwood, a specialist in ancient DNA who collaborates with MacPhee, "if the 1918 flu was so bad, even though the human immune system is trained to fight it, what would it do to us if we were seeing it for the first time?"

A similar terror may have emerged recently in the amphibian world. In the early 1990s, a fungus apparently rampaged through populations of the Central American golden toad. No golden toads have been observed anywhere since 1995, suggesting the species is extinct. "This was a novel pathogen and essentially an overnight collapse," MacPhee says. "That's what a hyperdisease would be like."

Circumstantial evidence suggests a hyperdisease could be behind the demise of mammoths. After surviving nearly two dozen ice ages, the woolly mammoth in North America disappeared in short order after the first humans showed up. And while at least one Siberian mammoth population survived on Wrangel Island in the Arctic until some 3,800 years ago— 700 years after the Egyptians built their great pyramids at Giza— it died out at about the time that people are thought to have arrived there. Contact with humans, MacPhee says, might have exposed woolly mammoths to a new pathogen against which their immune systems could not mount an effective defense in time. If female mammoths were like their nearest relatives, the elephants, which are pregnant for nearly two years, even individuals resistant to the pathogen might not be able to breed fast enough to save the species.

In search of further proof, MacPhee digs in the tundra of the Taimyr Peninsula. "The fact these frozen Ice Age beasts are still around means we can test the hyperdisease theory in an empirical way," he says. Radiocarbon dating will show whether any of these individuals witnessed the last stand of their species, and thus were plausible hyperdisease victims. The key is finding a pathogen that appears only at the very end of the mammoth line.

When evidence emerged in the 1830s of glaciers advancing deep into Europe and North America thousands of years ago, biologists reasoned that the woolly mammoth must have been a cold-adapted mammal that could not cope with the abrupt transition to our modern climate. A more sophisticated version of this idea holds that during much of the Pleistocene Epoch, which began 1.6 million years ago and encompassed 23 ice ages, great swaths of the northern hemisphere were covered by cold, dry grasslands. When the world warmed at the end of the Pleistocene around 9000 B.C., shifting weather patterns brought more rain and snow. For grazing animals, winter was particularly taxing. Even the mosses and sedges— less-nutritious plants that had supplanted the steppe's grasses and herbs— lay buried beneath snowdrifts. Mammoths couldn't get enough to eat, and isolated populations winked out one by one.

In the late 1960s, ecologist Paul Martin mounted the first serious challenge to this scenario, pinning the blame for the mammoth's disappearance in North America on prehistoric human hunters. Martin cited archaeological evidence indicating that bands of hunters swept from Asia across Beringia— the Bering land bridge— into the New World at the end of the Pleistocene. He argued that these Clovis hunters, named after their fluted spear points, would have preferred to hunt big, slow-moving mammals like mammoths, mastodons, giant ground sloths, and camels— species that vanished from North America in what appears to be the space of a few centuries around 11,000 years ago.

Martin likened the Clovis tribes to a shock wave rippling across the continent. The highest human population densities were at the wave's bow, he says, where small groups wreaked concentrated havoc. "The hunters would not have needed to plan elaborate cliff drives or build clever traps," he says. In as little as 350 years after crossing Beringia, Clovis hunters had reached the Gulf of Mexico and, says Martin, they'd completely purged the continent of mammoths.

"That's a very compelling argument," says MacPhee. But he doesn't believe it. "The whole notion of big-game hunters madly dashing around the landscape, killing everything in sight, is completely inconsistent with the anthropological picture," he says. The ascendant view in North American archaeology is that female gatherers kept their clans well fed. To mete out as much punishment on mammoth herds as Martin and other adherents suggest, says MacPhee, "the hunters would have had to be total Rambos." He points to Africa, where much higher densities of people, armed with guns and able to slay elephants for sport or for ivory at will, failed to eliminate the species before protective laws were enacted last century.

Several months ago, MacPhee could not have imagined himself joining Buigues's mammoth venture, much less digging into the local specialty, reindeer steak, at Khatanga's only restaurant. That's because his first impression of the expedition— from news reports about the airlift of a giant block of permafrost containing a mammoth's remains to Khatanga for study— left an unappetizing taste, not unlike reindeer, in his mouth. "I saw this ice cube coming out of the ground with the tusks sticking out and thought it was a highly dubious project," he says. That opinion changed after he met with Buigues and Mol last March. He decided Buigues was no publicity hound— he's "a humble guy," MacPhee says— and that Mol, while an amateur, is more knowledgeable about mammoths than many experts. When Mol invited him to join the expedition, MacPhee was elated: He knows his best shot at nailing a killer pathogen is to find remains of the last mammoths that roamed the tundra.

If the bones collected on the Taimyr Peninsula are as fresh as MacPhee thinks they are, his group may find fragile strands of DNA in samples that have not degraded. The hunt will start with a search for DNA sequences from broad virus classes, such as elephant herpes. "We know these viruses jump between African and Asian elephants and cause serious disease in elephants with very low levels of immunity," MacPhee says.

But it won't be easy, as Alex Greenwood knows from his own success in isolating DNA from the nucleus of a mammoth cell two years ago. "Tissue decay creates a lot of compounds that wouldn't normally be there in living tissue," Greenwood says. Particularly vexing are compounds formed when blood breaks down. Often when extracting DNA from dead tissue, the sample takes on a brownish tinge— a bad sign, indicative of a spoiled sample. "The DNA might actually be there," Greenwood says, "but the compounds kill whatever we try to do afterwards."

Greenwood won't be hunting exclusively for DNA. "A lot of the really nasty candidate viruses we might want to look at have RNA, not DNA," he says. Tops on their RNA list are myxoviruses, including influenza, and morbilliviruses, including the species-jumping canine distemper virus. But RNA is even more fragile than DNA because it degrades more quickly. Greenwood says the best hope of finding an RNA pathogen in mammoth tissue rests on viral coat proteins surviving. If researchers do finger an RNA or a DNA virus, they won't make any claims unless they score hits in samples from other mammoths. The ancient DNA field is replete with stories of findings gone bust, often because samples were contaminated inadvertently with modern DNA. "One's reputation hangs on a thin thread," says MacPhee.

The team will use antibodies to sniff out viral proteins lurking in mammoth tissue, and they might put promising bone slices under an electron microscope to hunt for viral particles lodged in the interstices. "With a hyperdisease pathogen, you should have jillions of particles in the body," MacPhee says. If the culprit is like rabies, which hides in neural tissue, his team won't find it searching through bones. "But if it's like flu or distemper," he says, "we stand a chance."

If the Taimyr samples are a bust, MacPhee will have other opportunities. Buigues plans to excavate the Hook mammoth near Taimyr Lake in April. This summer his group will send teams across the peninsula to gather mammoth remains from local residents who have agreed to collect them. The expedition also plans to venture in August 2001 into the New Siberian Islands, an Arctic archipelago separating the Laptev and East Siberian seas. Alexei Tikhonov, a Russian mammalogist on the expedition, says it may be the best place in the world to find frozen soft tissue. Four chunks of mammoth flesh were recovered during joint Russian and Japanese expeditions on the isolated and barren islands in the mid-1990s. "It's a real mammoth freezer," he says. And that is just the kind of talk that melts MacPhee's heart.



In Search of Flesh

The ice cave carved out of the rock-solid permafrost from a hillside in the Siberian town of Khatanga would be a perfect set for an arctic horror movie. Opaque crystals cling to ribbed arches in the 12-foot-high ceiling, and the temperature hovers around 5 degrees Fahrenheit year-round. One chamber is filled with the carcasses of slaughtered reindeer— a local food staple. Another houses a 23-ton block of frozen sediment, which measures 10 x 9 x 8 feet and holds the remains of Jarkov, a woolly mammoth named after the family that discovered its six-foot-long tusks sticking out of the frozen tundra at a site 200 miles away.

French explorer Bernard Buigues airlifted Jarkov to Khatanga in October 1999 and kept the creature on ice while awaiting the arrival of a research team that includes Ross MacPhee of the American Museum of Natural History in New York City and Alexei Tikhonov of the Institute of Zoology in St. Petersburg, Russia. The carbon dating of a bone fragment revealed that the adult male mammoth died some 20,000 years ago. The block has already yielded pollen, tiny worms, and insects that offer clues to the climate of ancient Siberia.

One day MacPhee and Tikhonov gather in the ice cave with Dick Mol, an amateur paleontologist who serves as Buigues's scientific coordinator, to find out how much of Jarkov's flesh is intact. Whatever tissue is recovered could be used for probing everything from the mammoth's position in the animal kingdom— how closely it is related to elephants— to undertaking a real long-shot project: resurrecting the beast through cloning. The scientists train hair dryers on the top surface of the block, and the frozen sediment erodes, speck by speck, until Tikhonov exposes a section of mammoth flesh.

"It looks like muscle," says MacPhee.

"This is the first soft tissue we've seen!" exclaims Mol.

They hold their find up to the light: It is a few inches long and looks like a scrawny strip of beef jerky. Clearly, the frozen mammoth is not the exquisitely preserved carcass that Buigues envisioned. "I'm a bit disappointed," he admits. "I was expecting a lot and got a little."

Nonetheless, Buigues insists that Jarkov won't be the last Ice Age specimen hauled to this chilly meat locker for scientists to defrost and study.
— R.S.







A Discovery Channel documentary on the expedition will air on March 11. For more, see www.discovery.com/exp/mammoth/dispatch.html.


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