THE first signs of an early Siberian winter were bearing down on the mammoth hunters and they had precious little to show for their efforts, after a fortnight on the icy tundra. A fierce Arctic storm had blown through two days before, confining them to camp and flooding the tundra with rain, sleet and snow. With five days left, time was running out.
But that autumn morning in 1998 dawned brisk and clear, bringing renewed hope to Ross MacPhee and his team. They struck out early, trudging through miles of boot-sucking mud. They had decided to return to the Naskhok River, where bones often collect along the shallow banks following a storm. After hours of wading back and forth in the icy water, MacPhee struck something with his muck-covered boot. He reached down and pulled up a mammoth leg bone still greasy to the touch from the oozing bone marrow at its centre-a perfect specimen.
Wrangel Island, north-west of the Bering Strait in Russia鈥檚 Chukchi Sea, is a graveyard for extinct, ice-age mammals and the last place on Earth where mammoths are known to have lived. But MacPhee wasn鈥檛 just interested in any old bones. He was after remains containing ancient DNA, and even with a specimen as good as that leg bone there was no guarantee that he would find exactly what he was looking for. MacPhee hoped to find evidence of a pathogen lurking within the ancient remains, to bolster his theory that mammoths became extinct after being exposed to a disease introduced by humans. But as nobody has ever isolated a disease organism from an extinct mammal, the odds were stacked against him.
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The demise of the mammoths, along with other ice-age mammals such as giant ground sloths, sabre-tooth cats and America camels, is particularly curious. Sometime after the end of the last glacial maximum, between 11,000 and 20,000 years ago, dozens of large animals disappeared throughout the Americas, northern Eurasia, Australia and Madagascar. Compared with previous mass extinctions, such as that of the dinosaurs 65 million years ago, these late Pleistocene extinctions were relatively small and quite selective. Reptiles and marine organisms went largely unscathed, as did most small mammals, except for those on islands. What鈥檚 more, the losses that did occur, among large mammals, were sudden and severe in North America but moderate and staggered in southern Eurasia and Africa.
One particularly striking feature of the extinction pattern is that the mammalian losses all followed closely on the heels of human migration into new areas. According to Paul Martin, emeritus professor of geosciences at the University of Arizona in Tucson, it鈥檚 no coincidence that humans were on the scene when the mammoths died out. The people who migrated eastward from Asia over the Bering land bridge more than 11,000 years ago, were sophisticated pack hunters who found a Garden of Eden in the Americas, with naive, vulnerable animals there for the killing. 鈥淭he mammoths had never been hunted and would have been pitifully easy to dispatch,鈥� says Martin. 鈥淩emember these were very experienced hunter-gatherers.鈥�
In what he calls the overkill, or 鈥渂litzkrieg鈥� hypothesis, Martin argues that these early American hunters, the Clovis people, followed their prey southward, creating a shock wave of local extinctions as they went. As the human population rapidly expanded, they hunted more intensively until they drove entire megafaunal populations into irreversible decline. Martin admits that there aren鈥檛 many sites where hunting tools and mammoths are found together. But he points to a dozen confirmed archaeological sites with evidence of butchered mammoth remains-including some bones with embedded spear heads-as evidence that Clovis hunters were capable of dispatching the big animals.
Martin鈥檚 theory, which resonates with the zeitgeist that humans are responsible for much of the widespread loss of plant and animal species in the past century, is widely accepted and has been endorsed by such well-known scientists as Jared Diamond and Edward O. Wilson. Other experts think climate change, not human predation, was the cause of megafaunal extinctions. Climate scientists have proposed that after the glacial retreat, increased seasonality-greater temperature swings between summer and winter-combined with drought could have altered the vegetation and habitat enough to starve the large herbivores into extinction.
MacPhee doubted that either theory explained the mammoth鈥檚 demise. 鈥淭hese were incredibly successful large mammals,鈥� he says. 鈥淭here is strong circumstantial evidence that human arrival played some role, but almost no evidence of mass killing or proof that humans did more than scavenge mammoths that were presumably already dead or dying. I find it utterly unrealistic that hunters with puny stone tools could possibly be responsible for killing more than a few mammoths here and there. Energetically it doesn鈥檛 make sense. There鈥檚 no material benefit to hunting dangerous animals more often than you need to-there are only so many mammoth steaks you can eat.鈥�
As for climate change, it doesn鈥檛 explain the varying regional and temporal pattern of losses or the fact that mammoths and other mammals survived through previous and more dramatic climate swings in apparently fine shape. 鈥淭here is no correlation with species losses at the times of the most radical temperature swings,鈥� says MacPhee. 鈥淭he species that were there before that point were there after that point.鈥� He was convinced there must be a better explanation. 鈥淚 was ripe for revelation,鈥� he says.
This came from an unlikely source. MacPhee was flipping through a back copy of The New Yorker and came across an article by science writer Richard Preston. This later become the basis for Preston鈥檚 best-selling book The Hot Zone, about an outbreak of the then obscure Ebola virus in Africa. Ebola is a particularly infectious, fast-moving and deadly disease against which people seem to have no resistance. MacPhee wondered if this could be an important key to explaining the mammoths鈥� extinction.
When a pathogen like the Ebola virus jumps from an established host to a biologically naive one, the results can be extraordinarily lethal. MacPhee started to wonder if humans, or a species that travelled with them, had carried a lethal hyperdisease organism that infected mammoths and other large mammals that had no natural resistance. Could there exist a pathogen quick enough and deadly enough to cripple entire populations to the point of no return? To answer those questions, MacPhee needed the help of a disease expert. As luck would have it, one came walking through the doors of the American Museum a few months later.
A remarkable agent
Virologist Preston Marx from the Aaron Diamond AIDS Research Center in New York, had come to examine monkey skins. He was looking for molecular evidence for the origins of the simian immunodeficiency virus. Marx and MacPhee got talking and before long they had constructed a profile of the suspected mammoth killer. To match the megafaunal extinction pattern, the hyperdisease agent must be able to cross species barriers, kill its new hosts very quickly and efficiently, and have a stable carrier in which the disease was not devastating. 鈥淚t also has to spread like wildfire through a group so fast that it can鈥檛 recover,鈥� says Marx. 鈥淲e鈥檙e talking about quite a remarkable agent.鈥�
Although no such disease organism has been known to kill off an entire species, there are some cases of novel pathogens wiping out or nearly exterminating local animal populations-especially on islands. Honeycreepers and other endemic birds of Hawaii have been decimated by what some experts suspect is avian malaria accidentally introduced from North America in 1826. And in the past decade, a plague of canine distemper carried by domestic dogs jumped the species barrier and raged through populations of wildlife in east Africa including lions, hyenas, bat-eared foxes and wild dogs.
Armed with examples of the devastation that such emerging diseases can bring, as well as a profile of their suspected agent, MacPhee and Marx published their hyperdisease hypothesis in 1997 in a book called Natural Change and Human Impact in Madagascar. It was met with some interest and much scepticism. Martin describes it as a 鈥渓ong shot鈥�. 鈥淚t seems so improbable that the kinds of animals that were lost would be susceptible to the introduction of lethal disease by their first contact with humans,鈥� he says. 鈥淲e鈥檙e dealing with animals in different orders that are quite unrelated, and then only the larger ones are affected.鈥�
What MacPhee needed to bolster his hyperdisease theory was some hard evidence. Although the chances of finding an ancient pathogen were low, recent advances in palaeontological genetics suggested it wasn鈥檛 impossible. And Wrangel Island was the perfect place to begin the hunt. For a start, the best source of ancient DNA is bones found in permafrost, because the cold slows down the normal process of degradation by bacteria and enzymes. And because Wrangel Island was the last outpost for mammoths it should yield relatively fresh samples. Mammoths survived there until 4000 years ago, 6000 years after they became extinct on the Siberian mainland, and just about the same time that the first humans arrived.
MacPhee鈥檚 1998 expedition to Wrangel Island yielded 19 bone samples. Back at the American Museum he handed them over to molecular biologist Alex Greenwood, together with samples from mainland Siberia and Alaska. Working with ancient DNA is extraordinarily difficult, and only a few labs even attempt it. DNA starts to break down in tissues within hours of death, making it tough to find in fossils and old bones. Even the rare good sample yields only tiny bits and pieces of genes. Then there鈥檚 the problem of contamination. 鈥淚f we found a flu virus that resembles modern strains, how would we know it鈥檚 ancient and not from some infected researcher?鈥� says Greenwood. He went to great lengths to avoid charges of sloppy practice. Despite this, his findings are bound to create controversy.
The study, published this month in Molecular Biology and Evolution, reveals that among the fragments of mammoth genes there were indeed bits of viral DNA, of a type known as endogenous retroviruses. ERVs, which are found in many mammals, are the legacy of once free-living viruses that have inserted themselves into a host鈥檚 germline to be replicated and passed along from generation to generation. The ancient DNA turned out to contain a wide variety of these viral remnants in large numbers. What鈥檚 more, some of the ERVs were found in all the samples-dating from 26,000 to 4500 years old-as well as in modern elephants. Their persistence over thousands of years of evolution suggests that they may play a functional role in the genome, according to Greenwood.
They are not the disease organisms MacPhee had hoped to find, though. 鈥淓RVs are not usually pathogenic in the host species,鈥� says Marx, the team鈥檚 disease expert. 鈥淪ome of these viruses could be infectious, but we don鈥檛 know that with the data we have so far.鈥� All the same, the discovery marks a significant advance in the hunt, as it shows that it is possible to find viral DNA in ancient bones. 鈥淭his is the first provirus ever isolated from an extinct animal,鈥� says Greenwood. 鈥淚t鈥檚 proof of the principal that you can do this work,鈥� adds MacPhee. 鈥淲e鈥檙e working on the edge of what鈥檚 considered possible with ancient DNA, but now we鈥檙e ready to take the next step and start searching for direct evidence of a pathogen.鈥�
That means looking beyond the mammoth DNA for microbes that might once have been living and multiplying in the animals鈥� cells. The team plans to search for ancient viral cells in coprolites-fossilised droppings-as well as bones, using sophisticated microscopes. They will test hundreds of fossil samples for antibodies of known viral groups, from the flu to rabies and pox virus. And MacPhee has a whole new batch of bones awaiting genetic analysis, which he collected in Russia last summer.
It will take a massive effort to track down the potential hyperdisease agent-and there鈥檚 no guarantee of success. 鈥淚 tell Ross, he鈥檚 looking at a decade or decades to nail it,鈥� says Greenwood. But MacPhee argues that the effort is necessary because despite all the talk of biodiversity loss at the moment, so many extinctions theories lack scientific rigour. 鈥淭o really address whether we鈥檙e currently in another extinction event, we need to understand the agents in past extinctions,鈥� he says. 鈥淢y feeling is that disease has been completely undervalued as a cause of extinction.鈥�
Books such as The Hot Zone have increased our awareness of emerging diseases as a threat to humans. What we tend to forget is that when we penetrate farther into uncharted territory we expose other animal populations to our diseases. If the cost of that exposure is the possible loss of whole species, says MacPhee, we should be aware of the danger and do what we can to avoid it. Martin agrees: 鈥淭he more we know about extinction,鈥� he says, 鈥渢he more chance we have of avoiding disaster in the future.鈥�
