MORTALITY AMONG PALEOLITHIC HUMANS

Violence is as important a cause of death as disease for young Hiwi adults, and for infants as well. On page 451, the paper points out that violence and accident cause as many deaths in the Hiwi young adults as occur in most other hunter-gatherers from all causes combined. Hill and colleagues discuss this issue in relation to the possible life history pressures on Paleolithic hunter-gatherers:

If high mortality, warfare, homicide, and accidental trauma are typical of our Paleolithic ancestors, the Hiwi mortality patterns may be more representative of the past than those derived from other modern hunter-gatherers. If so, several observations about the Hiwi are important. First, conspecific violence was a prominent part of the demographic profile, accounting for many deaths in all age and sex categories. Most of the adult killings were due to either competition over women, reprisals by jealous husbands (on both their wives and their wives' lovers), or reprisals for past killings. The criollo-caused killings were motivated by territorial conquest. Moreover, infanticide (especially on females) constituted the highest mortality rate component of all Hiwi conspecific violence. Second, no predation deaths were reported despite attacks by anacondas, Orinoco caimans, and piranhas, and the presence of jaguars in the area. Accidents associated with the active-forager lifestyle were common, but disease was a more important killer, accounting for nearly half of all deaths. This suggests an adaptive landscape in which success in social relations, competitive violence, and disease resistance are paramount. This may partially explain why many of the genes that appear to have been under strong selection in the past 50,000 years affect either disease resistance or cognitive function (Wang et al., 2006), presumably related to success in an atmosphere of frequent violent social competition (Hill et al. 2007:451).

The paper also includes a substantial discussion of the implications of high young adult mortality for intergenerational investments, such as grandmothering. This is an important issue, and Hill and colleagues end their discussion with a suggestion that neither the "grandmothering" nor the "embodied capital" models for the evolution of long life spans is sufficient to explain the human pattern. In their view, the key difference between humans and other primates (notably, chimpanzees) is not life span itself, but the markedly lower mortality rate among young adults. This low mortality rate directly causes the long life span (if you don't die young, you'll live long!). Hill and colleagues favor extrinsic factors such as greater protection of children, nursing the sick, and food sharing as possible causes of reduced mortality rates in humans.