What Did Our Ancestors Die Of?
Are we psychologically prewired to deal with epidemics?
Posted Mar 24, 2020
While I was working on a blog the other day about the question “What is actually likely to kill you?” my wife asked me about the evolutionary psychology of the current pandemic anxieties. She wondered whether we might not be pre-adapted to handle disease epidemics, since disease was such a threat to our ancestors. Turns out, the answer isn’t as clear as you might think.
What did our ancestors die of, anyway?
If we compare ourselves to our most recent ancestors, there has been a radical change in the causes of mortality. Over the last 150 years, the odds of dying from an infectious disease have dropped steeply. That has been true particularly in wealthier countries, but more recently, it’s true almost everywhere. There’s a lovely article in the New England Journal of Medicine that compares the causes of death in 1900 and 2010 (Jones, Podolsky, & Greene, 2012). Back in 1900, when my grandfather was still a young boy, the top cause of death was "pneumonia or influenza," which killed slightly more people than heart disease did in 2010. Tuberculosis killed more people per year in 1900 than cancer did in 2010. If you added in gastrointestinal infections and diptheria then the 4 top infectious diseases in 1900 killed as many people as did all the top 10 causes in 2010. In fact, in 2010, only one infectious disease category even made it into the top 10 – pneumonia or influenza, but the total mortality for that category was 12 times less than it had been in 1900. And in 2010, no other infectious disease even made the list.
But of course, 1900 was only 3 or 4 generations ago, not exactly the evolutionary past. What if we were to look at the causes of death back when our ancestors were hunter gatherers? Our predecessors were hunting and gathering for at least 90 percent of our species’ history, and agriculture came along a mere 10,000 years ago. Based on estimates from archaeological and genetic data, it is estimated that we Homo sapiens have been around, in more or less our current form, for at least 200,000 years. And 10,000 years is a generous start date for the agricultural revolution, since most of our ancestors did not start farming until the last 5000 years (making 98 percent a better estimate for the amount of time our species spent as hunter-gatherers) (Gurven & Kaplan, 2007).
Obviously, there wasn’t a Center for Disease Control gathering statistical data on the causes of mortality 10,000 years ago. But anthropologists, archaeologists, and demographers have come up with clever ways around that problem. One method is to look at evidence from ancient burial sites and archaeological digs, which provide bones that can be analyzed to determine how old their owners were at the time of death (Acsadi & Nemeskeri, 1974; Preston, 1995). Another method is to look at records from hunter-gatherers who were still around during the last century or two, and search for similar patterns in the causes of death in different groups living under very different conditions (in Australian deserts versus Africa grasslands versus South American jungles, for instance). Those groups can also be compared to groups that live a horticulturalist lifestyle (halfway between hunting and gathering and agriculture) and to groups that were recently brought into contact with Western civilization. ASU anthropologist Kim Hill and a team of other illustrious anthropologists were also able to compare mortality rates for contemporary hunter-gatherers and chimpanzees living in the wild and in captivity (Hill, Boesch, Goodall, Pusey, Williams, & Wrangham, 2001).
Kim Hill also conducted one of the other hallmark studies of comparative mortality rates, along with Magdalena Hurtado and Robert Walker. In that study, the research team reported the details of 722 deaths among the Hiwi of Venezuela, who had continued to live as hunter-gatherers until the late 20th century. Hill and his colleagues divided the deaths into those that had occurred before contact with Westerners (which were reported to them in extensive interviews with those Hiwi still living), and those that happened after contact. The Hiwi were found to be very likely to die violent deaths – from homicides (caused by competitions over women, reprisals by jealous husbands, and revenge for past killings) and from accidents associated with the hunter-gatherer lifestyle. Barry Hewlett’s report on mortality rates among Aka pygmies finds a lower rate of homicide but a similarly high rate of deaths from accidents (which included people falling from trees as they tried to get honey or palm nuts, being trampled by elephants, and being gored by a captured antelope) (Hewlett, Van De Koppel, & Van De Koppel, 1986). Although we don’t die in the same kinds of accidents, modern folks in Western societies have a similarly high mortality from automobile accidents.
The biggest difference between modern mortality statistics and those for hunter-gatherers comes from the extremely high rates of infant and childhood deaths amongst hunter-gatherers. Another big difference is that very few hunter gatherers died from either heart disease or cancer. The lack of heart disease is probably due to the fact that obesity is almost nonexistent in hunter-gatherers, who have different diets and spend a lot of their time moving around. The lack of cancer is possibly due to the much lower exposure to chemical toxins. It is not because no one ever lives long enough, incidentally. Michael Gurven and Hillard Kaplan (2007) integrated data from a number of studies of hunter-gatherers and found that a good percentage of those who survived into their teens went on to live for several decades, often well past age 50. The majority of Hiwi who survive into old age die of infectious diseases, not heart disease, and the same is true for the Aka.
The behavioral immune system and social distancing
So did our ancestral history of disease prepare us for dealing with the modern pandemic?
Mark Schaller and his former students Damien Murray and Justin Park have written extensively on what they call the “behavioral immune system” (Murray & Schaller, 2016; Schaller & Park, 2011). The idea is this: Our skin and mucus membranes keep out some of those hungry parasites trying to get into our bodies, but not all of them. Once a pathogen enters the body, there are other mechanisms (our white blood cells, in particular) designed to prevent the pathogens from gaining a foothold. But we don’t have resistance to every new germ that comes along, so an even better defense is to behave in ways that prevent us from catching the disease at all. Schaller and his colleagues, as well as several other teams of researchers, have uncovered evidence that people concerned about disease are more likely to avoid contact with strangers, and they've also found that people who live in places where there are lots of pathogens are less open to social contacts with strangers (Schaller & Murray, 2008, see also When it's healthy to be antisocial).
Many of the diseases our ancestors contracted came from the soil, the water, or from the animals they hunted, however, not from epidemics brought in by outsiders. And even in the modern world, diseases like influenza, although originating in faraway exotic places, are not typically contracted from strangers, but from someone in our intimate social network. Indeed, our ancestors had no way of avoiding contact with those in their close social networks, since they all lived together in small high density villages and shared food, tools, and living spaces. As Kim Hill and his colleagues have documented, hunter gatherers also gained immense survival advantages from sticking close and taking care of one another, in sickness and in health.
Hence, it may be no surprise that, even when we’re worried about disease, we human beings are not particularly skilled at keeping social distance from members of our own groups. Don’t get on a plane to an exotic faraway place: Easy! Stay in your own room and don’t go visit with your relatives and close friends: Difficult. That doesn’t mean it might not be a good idea during the current epidemic, but it does mean that social distancing is more like avoiding chocolate or reading in a foreign language, something that doesn’t come naturally, and that we have to muster a lot of self-discipline to pull off.
Some additional thoughts from Kim Hill and Mark Schaller
After reading through this, Kim Hill (whose work I describe above) had two very useful points to add: "respiratory infections have been a main killer (or the main killer) in almost every human society for which we have demographic data, even small scale tribal societies" and "zoonotic diseases were extremely common in our past (we were covered with blood of wild animals every day), but infections were mainly local and then quickly died out due to isolation between residential units (bands)." Zoonotic diseases, incidentally, are diseases that originated in other species. I discussed them the other day: "Viruses and pangolins and bats, Oh my!" The coronavirus we're dealing with now is a zoonotic disease.
Mark Schaller (author of the papers on the behavioral immune system) had another very important point to add: "The other challenge—which is why mere recommendations (rather than more draconian measures) to engage in physical distancing don’t work so well—is that there is a mismatch between the epidemiological reasons why physical distancing is so important and the more intuitive psychological rationale for it. The epidemiological argument for why I should be engaging in physical distancing isn’t just that it might keep me from getting infected by someone else, it’s more profoundly this: it might keep me from infecting someone else! (That’s super-duper important because, as we know, I might be infectious without yet knowing it!). But that’s not the way the mind intuitively works. We might have an evolved intuitive psychology (the “behavioral immune system”) designed to keep us away from folks who we might get infections from, but I don’t think we have much in a way of an evolved intuitive psychology designed to keep us away from folks who we might deliver infections to."
Acsadi, G. & Nemeskeri, J. (1974). History of human life span and mortality (book review and precis). Current Anthropology, 15, 495-507.
Gurven, M., & Kaplan, H. (2007). Longevity among hunter-gathers: A cross-cultural examination. Population and Development Review, 33, 321-365.
Hewlett, B. S., Van De Koppel, J. M., & Van De Koppel, M. (1986). Causes of death among Aka pygmies of the Central African Republic. In L.L. Cavalli-Sforza (Ed.) African pygmies, 45-63.
Hill, K., Boesch, C., Goodall, J., Pusey, A., Williams, J., & Wrangham, R. (2001). Mortality rates among wild chimpanzees. Journal of human evolution, 40 (5), 437-450.
Hill, K., Hurtado, A. M., & Walker, R. S. (2007). High adult mortality among Hiwi hunter-gatherers: Implications for human evolution. Journal of Human Evolution, 52(4), 443-454.
Jones, D. S., Podolsky, S. H., & Greene, J. A. (2012). The burden of disease and the changing task of medicine. New England Journal of Medicine, 366(25), 2333-2338.
Murray, D. R., & Schaller, M. (2016). The behavioral immune system: Implications for social cognition, social interaction, and social influence. In Advances in experimental social psychology (Vol. 53, pp. 75-129). Academic Press.
Preston, Samuel H. (1995). Human mortality throughout history and prehistory. In Julian L. Simon (Ed.), The state of humanity (pp. 30–36). Cambridge, Massachusetts: Blackwell.
Schaller, M., & Murray, D. R. (2008). Pathogens, personality, and culture: Disease prevalence predicts worldwide variability in sociosexuality, extraversion, and openness to experience. Journal of personality and social psychology, 95(1), 212-221.
Schaller, M., & Park, J. H. (2011). The behavioral immune system (and why it matters). Current directions in psychological science, 20(2), 99-103.