Around 11,000 years ago, at the end of the Pleistocene epoch, North America witnessed an extinction that claimed its mammoths, giant ground sloths, camels and numerous other large-bodied animals. Exactly what happened to these megafauna is unknown. Indeed, researchers have puzzled over their disappearance for decades. Traditional explanations hold that either dramatic climate shifts, or human hunting (overkill) extinguished these species. But in recent years a new hypothesis has emerged. According to Ross D. E. MacPhee, curator of mammalogy at the American Museum of Natural History in New York City, extremely lethal disease, brought over by humans unwittingly when they arrived in the New World, may have wiped out those Ice Age giants.

Scientific American writer Kate Wong recently caught up with MacPhee to discuss his hyperdisease hypothesis. The edited transcript that follows falls into four sections. In the first part MacPhee talks about the shortcomings of the climate and overkill models. In the second part he provides examples of recent extinctions caused by disease and describes how the first Americans might have introduced hyperdisease when they came to North America. Megafaunal extinctions followed human arrival in Australia, New Guinea, the West Indies and Madagascar, too. The same pattern does not apply to Africa and southern Eurasia, however. MacPhee explains how his model accounts for these exceptions and ponders the surprising survival of certain North American megafauna in part three. So far MacPhee does not have empirical evidence for his hypothesis, but he and his colleagues hope to find it in mammoth remains. In part four he describes their search for signs of lethal microbes in ancient tissue and DNA. Additional information on the North American end-Pleistocene megafaunal extinction will appear in the February issue of Scientific American.

SA: Id like to start by asking you to explain how, in your opinion, the climate and overkill models are flawed.

RM: The climate has changed radically at times when there was no extinction, and extinctions have occurred when the climate, at least roughly speaking, should have been benign. There is no question that there were catastrophic changes in temperature and probably in precipitation on many occasions in the past 100,000 years. We know that, for entirely natural reasons, temperature excursions of seven to 12 degrees Celsius occurred within that time period in the space of a century or less, which is basically 12 times the maximum rate of change in the last century of "global warming." If changes like that are meaningful for extinction, then you would expect to see a correlationhow could it be otherwise? If climate change of that radical a caliber has occurred in the past there should have been losses. And the point is that there are no correlations. So all of that, as far as Im concerned, puts climate, insofar as it's understood what we mean by climate change, out of the picture.

There is a strong correlation between arrivals of people in places where people havent lived before and sudden spikes in the extinction rate so that you get sudden disappearancesparticularly of large-bodied animalsin a period within decades or centuries of first human arrival. So its easy to see why people would assume that these losses had something to do with the arrival of humans, and since we think of humans as being red in tooth and claw, that they must have provoked these extinctions by doing something nasty like hunting at a rate that they shouldnt have. The trouble with that particular argument is that the archaeological record does not support it in any of the places where these extinctions occurred. Of course there are cases where projectile points have been found embedded in mammoth bones. But when you take a look at the number of instances, you can barely come up with a dozen for the relevant time period in North Americabetween 11, 000 and 12,000 years ago. In other words, although people were clearly hunting, it is not a demonstration by that evidence alone that they were hunting on a scale that would have made any difference to the survival of species.

Image: Clare Flemming
COLLECTING TUSKS. MacPhee lugs a mammoth tusk he found in a creek bed on Wrangel Island, north of Siberia.

SA: So, in your opinion, even if the first Americans were highly skilled hunters, could their population sizes and the population sizes of these animals have been such that overkill would even be plausible?

RM: The answer is no, by any scenario. I dont care how early you want people to get into the New World, theres absolutely no evidence of a positive sort that they were there in huge numbers. In fact, it has to be the opposite, whereas the animals, in some cases, had distributions that were continent wide. Some of the ground sloths, for example, are known from as far south as Mexico and from as far north as the Yukon. The notion that people in whatever numbers and with whatever intent could have come in and slaughtered enough sloths in every possible habitat where they lived in numbers sufficient to cause their extinctionthis is unbelievable to me.

Archaeologists say in looking at Clovis sites and similar ones in Monte Verde in South America that theres nothing at these sites that suggests anything other than band-level organization. What we know from modern ethnographic examples is that the individual family groups that compose a band tend to cooperate only for very specific objectives. As soon as that economic objective is met, its overthey dont keep a high level of organization once theres no need. So if were talking about Clovis people being at essentially the band level economically, how can it be that they would stay together for common purposes at the level necessary to cause these extinctions? Youd have to be killing things all the time, and youd have to be doing it for some purpose, even if the purpose was just to kill. And its just unimaginable to me that the people concerned would be interested merely in killing, especially large, dangerous animals like mammoths. You take out the one or two a year that you need, and then you go off gathering roots and tubers, which is in fact how most of these outfits keep goingits not by direct hunting. You cant look at the first Americans as being basically people like us who didnt wear suitsthat their objectives would be similar, that their worldview would be similar, and all the rest of it. In fact, most assuredly they were not, if ethnographic comparisons mean anything.

SA: Enter hyperdisease.

RM: Hyperdisease has its own extremely large explanatory defects. But the notions that you cant get cross-species infections, or that you cant get huge mortalities that might lead to extinction, are not among them. In fact, there are such examples. There is a group of birds native to Hawaii called the Hawaiian honeycreepers, several species of which have gone extinct probably within the last 100 years. Whereas when Europeans were first going to Hawaii at the beginning of the 19th century, honeycreepers were known at lower elevations. Nowadays, however, the surviving populations all live at high altitude. Why should this be? Researchers figured out in the 1960s that the distribution of the surviving birds is mediated by how far up in altitude avian malaria-carrying mosquitoes can go. The mosquitoes that were talking about, representing a species of Culex, were introduced from tropical North America probably in the 1850s or 1860s. And in all probability what happened was that some boat going from San Francisco or Mexico over to Honolulu had freshwater in its bilge and female mosquitoes were laying eggs there. Some of the larvae survived after the bilge was dumped, they started biting the native birds, and some of the larvae had the [malaria-causing] protozoan Plasmodium in their system, so they inoculated the birds, and the birds died in droves. To repeat the experiment and thereby document what happened, what the researchers did was take some of these individuals from surviving populations up on the high mountains, bring them down to the lab at sea level, and then introduce them to Culex individuals that were known to be carrying avian malaria. The exposed birds died without exception100 percent mortality.

What I make of this is obvious: the distribution of the surviving birds is disease-controlled. I also think that the populations or species that could not survive at high altitudethe birds that have gone extinctbecame extinct because there was nowhere for them to go. Everywhere they went they were greeted by buzzing mosquitoes who were carrying the disease, and the birds dropped out. I think that is a pretty convincing case. And all of the leading list-makerslike IUCN, Nature Conservancy, U.S. Fish and Wildlifewhen they attribute cause of extinction to those particular birds use the word disease, as opposed to habitat clearance, as opposed to introduced species, persecution, all of these other things that are believed to cause endangerment and extinction. So at least in some quarters it is accepted that those bird extinctions are due to disease per se.


DRILLING FOR DNA. MacPhee uses a specially adapted drill bit to remove cores of marrow from this mammoth bone.

The other great example is golden toad extinction in Panama and apparently huge declines in certain frog species in places like Queensland [Australia], all due to a fungal infection, chytridiomycosis. Up until the mid '90s in Panama the census numbers were consistently high. Then something happened to the golden toad. Within the space of a year or two the numbers of sightings and sound recordings and so on dropped from the average level established over many decades to zero, essentially overnight. For the past five years there have been no sightings of that particular frog in Panama and Costa Rica, which were the places where it lived. It looks like the populations have just sunk to nothing.

Peter Daszak, a parasitologist at the University of Georgia, and several other investigators got interested in this problem because they were given some autopsy specimens of golden toads to examine. What they found was that a particular chytrid was consistently present in autopsy specimens from areas in which the populations were known to have sunk to zero in the manner that I described. And what they found on further investigation was that the specific cause of death seems to have been that this fungus, which is an epidermal infection, caused local thickening of the skinespecially over an area known as the drink patch down in the pelvis, which is what these frogs use for osmoregulation, for balancing out their water. With any problem with that area of skin, the frog in effect suffocates or drowns. Interestingly, tadpoles had no epidermal infection, but they did have the chytrid present in their mouthparts. Presumably, as they metamorphosed, the chytrid infection became general on the epidermis and they died out as well. So what youve got, it seems to me, is the worst of all possible cases, which is a universal infectionthat every individual either has it or could potentially have it. Its something that is obviously very easily passed on or distributed within the environment. There would seem to be no escape for the populations that suffered from the chytrid infection: it wasnt like the chytrid would pass through the adult group and the next generation would be okay.

A similar chytridperhaps even the same chytridhas appeared in Queensland, Australia. It is not known that it has been responsible for any complete extinctions, but it has certainly been responsible for massive depressions in population in the frog groups that it affects. It has also turned up in the southern part of South America. Why this particular chytrid at this particular time? Who knows? Perhaps its because of people. Because you can get anywhere on the planets surface nowadays within about 48 hours, the chances for pathogen pollutionin other words, bringing pathogens from one area to another where they might be able to take offis incredibly enhanced over what it was even a few decades ago. From that point of view its perhaps not even all that remarkable that wed start to see diseases that are essentially pan-global. Not because theyre getting distributed better by winds or currents, or whatever, but by people moving around.


DRILLED SAMPLES of mammoth bone are sent back to the U.S. for labwork.