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High adult mortality among Hiwi hunter-gatherers: Implications for human evolution [1]

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Date: 2007-04-01  

Age-specific mortality rates are invoked in theoretical explanations of a multitude of human biological and behavioral characters. This includes comparisons between mortality in humans and other primates as an explanation of slow childhood development, late age at first reproduction, menopause, delayed senescence, and brain expansion in humans (e.g., Hawkes et al., 1998, Blurton Jones et al., 1999, Kaplan et al., 2000, Hill et al., 2001). Likewise, high mortality rates in some human groups have been implied to result in atypical childhood growth patterns (Walker et al., 2006), shortened life span due to early infections (Finch and Crimmins, 2004), changed marriage and mating patterns (Low, 1988, Low, 1990, Hill and Hurtado, 1996), and accelerated sexual maturity and associated medical and behavioral problems (Draper and Harpending, 1982, Belsky et al., 1991, Ellis, 2004). In addition, high adult mortality has extensive implications for human social structure and settlement patterns (Howell, 1982, Early and Peters, 1990). The origin of humanlike mortality profiles, the biological vs. cultural causes of increased longevity, and the implications for human evolution have been hotly debated in recent years (e.g., Caspari and Lee, 2004, Caspari and Lee, 2005a, Caspari and Lee, 2005b, Caspari and Lee, 2006, Hawkes and O'Connell, 2005).

In order to evaluate models of human biology, and in order to understand the adaptive pressures that helped shape human traits, we need to document the causes and rates of mortality during different periods of human evolution. Although the mortality profiles of our distant ancestors can only be ascertained through indirect means, studies of modern hunter-gatherers provide direct information on the likely mortality profiles of more recent ancestors. While it has long been recognized that hunter-gatherers show different health profiles from people living in modern societies (e.g., references in Dunn, 1968), systematic demographic studies began only with Howell's (1979) seminal research on the !Kung. In order to apply these studies to problems in human evolution, we must carefully consider whether their results reflect parameters typical of past forager populations or if these parameters have been significantly altered by subtle current conditions (see Blurton Jones et al., 2002).

Hunter-gatherers have very different lives than people in the modern societies. They are exposed to climatic challenges, risks from foraging activities, predation, nutrient and pathogen stress, violence from conspecifics, and other insults from a difficult lifestyle that is more typical of human ancestors and without the benefit of any modern medical treatments (Hill and Hurtado, 1996). Mobile hunter-gatherers often must recover or perish in a short time period before the residential band is forced to move on in search of food. Moreover, hunter-gatherers, because they live in small, mobile residential units, may be less exposed to infectious diseases that require large and dense groups and more exposed to zoonotic infectious agents and trauma than most sedentary human populations. This may result in a different immune profile than that found in sedentary and state-level societies (Hurtado et al., 2003). Hunter-gatherers eat a diet high in protein-lipid, low in carbohydrate (and very low glycemic load), low in salt, and high in fiber relative to modern people (Eaton and Konner, 1985, Cordain et al., 2000), and they also experience high rates of activity through much of the life span. They are minimally exposed to factors such as chemical toxins, hormone and antimicrobial animal-feed additives, and electromagnetic waves. Hunter-gatherer women experience a very different lifetime hormonal profile, with many pregnancies and long periods of lactation and anovulation, that appears to affect rates of reproductive cancers (Eaton et al., 1988, Eaton et al., 1994). Finally, hunter-gatherers may experience very different psychological conditions (high social support but constant threat of violence) than people in modern societies. The suite of all these different factors suggests that hunter-gatherer mortality profiles might be very different from those of people in modern or even ancient horticultural, agricultural, and state-level societies. Because of this, it is critical that we document mortality rates in as many hunter-gatherer societies as possible before these groups cease to exist. This will allow us to seek out patterns common to foraging societies and assess the range of variation in mortality among these groups in order to determine the impact of factors specific to certain world regions, ecozones, or cultural practices.

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[1] Url: https://www.sciencedirect.com/science/article/pii/S0047248406002193

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