Image: CLARE FLEMMING

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 discussed this hyperdisease hypothesis with MacPhee last year. 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.

Part I

SA: I'd 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 correlation-how 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 I'm 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 haven't lived before and sudden spikes in the extinction rate so that you get sudden disappearances--particularly of large-bodied animals--in a period within decades or centuries of first human arrival. So it's 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 shouldn't 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 America--between 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.

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