Carriers of Extinction

By Carl Zimmer|Saturday, July 01, 1995
When Ross MacPhee walks in the woods around his house in New Jersey, he is sometimes beset by ghosts. He sees deer and groundhogs, and as a paleontologist, he can’t help thinking about what lived in North America 12,000 years ago. Sloths the size of dump trucks, teratorn birds with 25-foot wingspans, shaggy mastodons, herds of horses pursued by cheetahs, lions, or saber-toothed tigers--all in all, a bestiary that would have put the Serengeti Plain of East Africa to shame. Yet in just a few centuries, between 11,000 and 10,500 years ago, all this life disappeared. No one is sure why.

North America is not alone: between 40,000 and 1,000 years ago, Earth was visited by the biggest wave of large-animal extinctions since mammals took over from the dinosaurs 65 million years ago. North and South America lost 135 species, including almost all their animals over 100 pounds, but extinctions happened elsewhere as well--in Australia and New Zealand, for instance. Two explanations for the global extinction have been advanced. One theory says the climate changed too radically for many species. The other says we humans wiped out the large animals--either by wiping out their habitats with fires, logging, and farming, or simply by hunting them down.

MacPhee, chairman of the mammalogy department at the American Museum of Natural History in New York, thinks it’s time to explore a third scenario. He believes human beings were responsible for the mass extinction, but unwittingly: as we spread to new parts of the globe, MacPhee suggests, we exterminated dozens of animal species by infecting them with a deadly pathogen.

MacPhee has never liked the other two hypotheses much. The climate-change hypothesis is the easier to dispose of. While it’s true that the New World extinctions occurred at about the end of the last ice age, European megafauna suffered through much the same climate warming with many fewer extinctions, and the American animals themselves had survived many previous climate swings. Furthermore, Australia lost its giant kangaroos and rhino-size marsupials between 30,000 and 15,000 years ago, before the end of the Ice Age, while large islands like New Zealand and Madagascar lost most of their megafauna long after the Ice Age ended--as recently as the Middle Ages in the case of Madagascar.

What the timing of all the extinctions shows pretty clearly is a connection with the spread of humans. Some of the extinctions, researchers have suggested, happened because humans changed the landscapes of their new homes. MacPhee is skeptical of that idea too. Madagascar was the case everybody pointed to, the one place where our environmental impact had definitely made the difference, he says. Humans who arrived in Madagascar 2,500 years ago, the story went, gradually burned down much of the island’s species-rich forests, which were then replaced by grasslands. The story made sense--until MacPhee and his co-workers found ancient pollen that proved that the grasslands were at least 40,000 years old.

In the 1960s Paul Martin at the University of Arizona suggested a more compelling scenario: that humans had hunted the large animals to extinction. As our earliest ancestors evolved in Africa, invented tools, and spread slowly into Europe and Asia, Martin argued, large animals on those continents had enough time to evolve a fear of the bipedal ape. That’s why the diversity of Old World megafauna has declined only slowly and gradually; indeed, many species survive today--witness the teeming Serengeti. But when humans made the jump to new continents and islands beginning 40,000 years ago, they encountered naive animals. With such plentiful and easily bagged game, the hunters did not trouble to avoid waste. In a kind of blitzkrieg, they drove species after species extinct.

There is no doubt that modern hunters can wipe out an animal species. And in at least one case there’s strong evidence that an extinction was caused by premodern hunters: huge troves of butchered bones testify that New Zealand’s giant flightless birds, the moas, were exterminated in just a few centuries by Maori settlers who arrived around A.D. 1000. But for several reasons, MacPhee doesn’t think Martin’s blitzkrieg idea makes sense as an explanation of the global megafauna extinctions, and in particular of the New World ones.

Studies of surviving hunter-gatherer societies, says MacPhee, suggest that as a rule they don’t practice the kind of intense overkill required to wipe out a species. It’s extremely hard to imagine small groups of Paleo-Indians acting any differently, says MacPhee. Why would they spend all their time killing, say, ground sloths? The fossil evidence of overkill hunts is also flawed, he says. Martin interpreted piles of mammoth bones in North America as herds that humans killed en masse. Yet subsequent studies suggest that the animals simply died of natural causes, and the humans then harvested their meat.

The overkill hypothesis also has a peculiar ending: not all the large animals went extinct, it says, because humans, for reasons unknown, lost interest in that style of hunting. In North America tens of millions of bison still roamed the plains when Europeans arrived; moose and elk and caribou survive in large numbers today. If the Indians were so interested in that scale of hunting, why did everything change 10,000 years ago, and why in the ensuing ten millennia did they not force a single other megafaunal extinction? asks MacPhee.

On the other hand, MacPhee realized, if the extinctions were caused by a newly introduced virus or other pathogen, you would expect a few species to evolve disease resistance and survive. Disease had been suggested in the past, but never very seriously, MacPhee recalls. Europeans were known to have introduced smallpox and other diseases to Indians, with catastrophic results. But the notion that Indians might have transmitted a disease to ground sloths or saber-toothed tigers was another thing completely. And the idea that the disease could cause extinctions, says MacPhee, was too bizarre.

But AIDS, Ebola, and other newly emerging viruses have made the bizarre reasonable. Some of the new viruses can jump over the species barrier, and some are shockingly lethal. Researchers have begun to rethink their old notion that pathogens tend to become benign in order not to kill off their hosts; the opposite evolutionary strategy--hypervirulence-- might also work. A hypervirulent bug would knock its hosts down like duckpins, but because it was extremely contagious and able to infect new species, it could stay one step ahead of its own destruction. MacPhee began to think about a hypervirulent plague as a serious candidate for the cause of the megafaunal extinctions.

In his scenario early humans and Old World animals were exposed to the same stew of viruses, bacteria, and parasites, so they had similar immunities. But animals on other continents and islands had no exposure to the bugs and no resistance. When humans arrived, either they or their domestic animals--dogs, in the case of the North American Indians-- transmitted the pathogens to the immunologically naive native animals. One or more of the bugs then became hypervirulent. It swept ahead of the advancing humans, invading every refuge and routing out every last member of many species. Big animals were most susceptible since they had few offspring and needed a long time to reach reproductive age. They are innately more likely to disappear, says MacPhee. It’s the price you pay for being big.

MacPhee’s colleagues are skeptical of his hypothesis--not surprisingly, he acknowledges, because at the moment he has no positive evidence to back it up. No one has ever seen a hypervirulent plague that can wipe out entire species; no one has ever found a fossil pathogen. And no one is likely to believe MacPhee until he can find a fossil pathogen that can at least be associated with megafaunal extinctions.

As luck would have it, MacPhee met a man last year who could help him. Preston Marx is a virologist at the Aaron Diamond AIDS Research Center in New York; in the late 1980s he helped show that HIV was so close to simian immunodeficiency virus (SIV) that the former probably descended from the latter. When Marx approached MacPhee about getting access to some old monkey specimens at the museum--he hoped to isolate SIV DNA from them-- MacPhee asked him what he thought of the hypervirulent plague idea. At first I thought of the enormity of the question, the long-shot nature of trying to prove the idea, says Marx. I said, ‘Wow, Ross, it’s going to be smoke and mirrors.’

But he and MacPhee agreed to team up with Andrew Lackner, a veterinary pathologist at Harvard, to try to find the putative pathogen in megafaunal fossils. To prove to themselves that the whole thing is feasible, they’ll start with target practice and a relatively big target: a type of benign virus, known as an endogenous retrovirus, that permanently inserts its DNA into that of cats. It’s present in every cell of the body of a cat, up to 20 copies per cell, says Marx.

If they can find it in a fossil cat like the saber-toothed tiger, they’ll move on to DNA fragments of potential plagues--free-floating viruses and bacteria. And if we have any luck, MacPhee says, I expect my colleagues who are in this racket will want to do the same with their critters. He’ll need their help because he can prove his idea only by showing that many different fossils dating from after the arrival of humans--fossils of animal species that soon went extinct--contain DNA from the same pathogen, whereas fossils from before the arrival of humans do not.

Such proof would make MacPhee’s idea very relevant to today’s world. No one thinks about diseases as a threat to biodiversity these days, says MacPhee. But if it happened once, it should be able to happen again.
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