The Hyperdisease hypothesis for what killed off the Pleistocene mega-fauna

Written by Ross MacPhee

Like many ideas in science, the "hyperdisease hypothesis" came about because of a casual, almost accidental juxtaposition of ideas that had, on the surface, nothing at all to do with one another. Extinctions are usually studied by paleontologists and evolutionary biologists; the origins of diseases and host/pathogen relationships are normally investigated by virologists, epidemiologists, and specialists in biomedical fields. In my work as a paleomammalogist I had undertaken a number of technical investigations of Quaternary extinctions, but found all available interpretations of their cause to be either implausible, or untestable, or both. I could not imagine that human hunting could possibly have been responsible for hundreds of megafaunal extinctions, even under the most favorable conditions of predation. Nor could I believe that extinctions could have been forced by "climate change," if only because the losses occurred at radically different times in different places.

A few years ago I happened to be leafing through an issue of New Yorker magazine and came across an article by Richard Preston that became the basis for his best-selling book The Hot Zone. In the article Preston described Ebola disease, a then-obscure killer that had broken out in central Africa from time to time during the past 30 years. The progress of the disease was appallingly rapid: people died within days of exposure, which could occur just by handling the corpses or bodily fluids of those who had already died. In some outbreaks, mortality rates of 60-70% and even higher were reported. Seemingly, Ebola had come out of nowhere and hit humans like a sledgehammer. Ebola's meteoric career as a human pathogen was typical of so-called "emerging diseases," diseases that, for one reason or another, managed to jump from their original host to one or more new ones. In all likelihood, Ebola exists as a non-lethal infection in some rodent, bat, or other mammal in central Africa. Its spread to humans was probably accidental, due to increased contact between the mammal host and humans, maybe as a result of regional deforestation. Until Ebola evolves into a less virulent form, or until humans build up genetic immunity to it, this will remain a true killer plague.

It struck me immediately that the story of Ebola disease may be a key to understanding some extinctions. Extinctions are caused by catastrophic drops in population sizes; for any given extinction, the problem is to find some effect or process that can do the necessary irreparable damage in a period short enough to make population recovery impossible. Sixty-five million years ago, the effective agent of extinction that caused the end of all non-avian dinosaurs was a huge bolide [asteroid] impact that created fire storms, huge tsunamis, and perhaps a global "nuclear winter." Although a variety of species was affected, groups like terrestrial dinosaurs were almost completely wiped out. What comparable effect could have occurred at the end of the Pleistocene (for example) to wipe out most of the New World's mammalian megafauna? In my view, climate change was too slow to be a cause of extinction in this case. Human hunters might be much faster, but there is no way that they could have simultaneously caused faunal collapses in every ecosystem from desert to rain forest. However, disease knows no similar bounds, and -- as Ebola illustrates -- "new" diseases can be extraordinarily lethal in species that are imperfectly adapted to them.

It remained for me to relate the spread of disease to its probable agent. The cause seemed obvious: as humans progressively moved out across the planet, they must have taken pathogens along with them. In the hyperdisease theory*, worked out with my colleague Dr. Preston Marx, this stock of pathogens included ones able to infect immunologically-naive populations of endemic mammals, causing rapid population crashes and, in many cases, outright extinction. Humans need not have been the actual carriers; the carriers could have varied from place to place (as could the diseases themselves), and might have included rats or fleas, or parasites of rats and fleas, and so on. In any case, the disease hypothesis explains why the crashes occurred so soon after human arrival, and why they were so pervasive. It also seems to explain why the emerging diseases eventually stopped causing extinctions. After a period of high virulency, the pathogens and their new hosts settled down into the conventional, "benign" kind of arrangement that characterizes most well-established diseases and their hosts, wherein fatalities are few but contagion remains wide. However, for seriously-affected species, there was never the chance to set up a benign relationship. High levels of mortality would simply have continued until the last surviving populations were reduced to numbers too low to sustain species viability. After that, all that could happen was extinction.

This point about last surviving populations takes us back to what our rationale was for going to Wrangel Island in the first place. If disease was responsible for the demise of Mammuthus primigenius, then we have to expect that we will be able to detect evidence of it in only one context -- in the remains of the individuals that made up the terminal populations of the species, wherever they occurred. Since we know that mammoths survived later on Wrangel Island than elsewhere, it made sense to try there first, to see if we could actually put our hands on the bones of the last of the mammoths. Unfortunately, finding members of earlier populations of Wrangel mammoths won't help us test the theory, since it is obvious that they were not the last of their kind. The trouble is that you cannot simply look at a bone and determine its age; for that you need a reliable dating technique, such as radiocarbon dating.

I realize that the journey between a good idea and a confirmed hypothesis is often a long one. The Wrangel Island expedition marks the start of the journey; I actually look forward to the hike, because new science is bound to emerge.