SA: North America is one of many places where megafaunal extinctions coincided with human arrivals. Yet this did not occur in Africa and southern Eurasia. Do you think the animals in these places were somehow resistant to these diseases?
RM: Yeah, the answer involves borrowing another page from Paul Martin¿s book [editor's note: Martin developed the overkill model]. The reason that overkill did not occur in Africa and south Asia, he would argue, is because the hominids and the local mammals evolved together. For every improvement in the hominid toolkit, the local animals came up with appropriate behavioral responses. So they were not na¿ve, in a behavioral sense, and were able to deal with human predation on an ongoing basis. In the New World you have the opposite state of affairs, obviously. But I would argue that the na¿vete was immunologic and genetic, rather than behavioral¿which I actually think is extremely plausible. Just on first principles, I¿ve never really been able to get past the argument that animals are so persistently na¿ve they just stand around and let themselves be butchered, especially in continental settings. But with disease you can imagine a situation, especially for herding animals, where the pathogen could be passed through a population in a matter of days, and they wouldn¿t be any the wiser. They would just be falling all over the place, without any clear threat on hand.
SA: A certain number of these big animals did survive. Are there any that you might have expected to perish if disease were as rampant as you think it must have been?
RM: I have wondered, as everybody has wondered, why elk, moose, musk ox, bison, among very few others, have managed to survive in a place like North America, or llamas down in South America, when all of them have close relatives that turned up their toes at the end of the Pleistocene during these big die-offs. From the disease scenario perspective, all I can imagine is something like the following: perhaps nearly every kind of mammal species was susceptible, but there would be groups that were affected but not devastated by the die-off, or individuals with the right genotypes to make it through. And then it just becomes a straight Darwinian selection problem. In some cases, enough immune individuals might be left to continue the species. In other cases, the die-off was so rapid that there was no possibility of coming back. This would be a good argument if there were a common feature that surviving megafauna shared¿that they all lived in one place or that they had a particular kind of birth spacing. I¿ve tried to check this out and there¿s nothing that I find very convincing. That¿s just the way it is. Which is not a very good argument, I know. But that is certainly one explanation¿that what we¿re seeing are survivors.
It is interesting, however, from a slightly different perspective, that at least half of the surviving megafauna of North America have Old World populations. This would apply to wolves, elk; this would originally have applied to musk oxen although they later became extinct in Asia; it would apply to moose also. My point here is that as catastrophic as these New World extinctions appear to have been, they may have been even worse. For example, the North American populations of something like moose could have completely disappeared at the end of the Pleistocene. The reason we have moose today, so this argument might go, is because North America was repopulated by Asian moose, who did not die out because they were much less susceptible to the pathogens. Possibly they¿d had earlier hits and survived, or what have you. You can go crazy with all of these permutations, but it is interesting to me that a significant number of the megafaunal survivors have basically a holarctic distribution, which means a distribution across all the high latitudes. It¿s not just open and shut that you had significant survival of megafauna here in North America. We don¿t really know that. We know that some megafauna survived, but if you cut out the guys with holarctic distributions, then what remains is very, very limited indeed¿pronghorns, llamas, tapirs, mountain goats and a couple of others, none very big.