specialization in embodied capital investments and generates a complementarity between male and female inputs. The economic and reproductive cooperation between men and women facilitates provisioning of juveniles, which both bankrolls their embodied capital investments and acts to lower mortality during the juvenile and early adult periods. Cooperation between males and females also allows women to allocate more time to child care and improves nutritional status, increasing both survival and reproductive rates. The nutritional dependence of multiple young of different ages favors sequential mating with the same individual, since it reduces conflicts between men and women over the allocation of food. Finally, large packages also appear to promote interfamilial food sharing. Food sharing assists recovery in times of illness and reduces the risk of food shortfalls due to both the vagaries of foraging luck and the variance in family size due to stochastic mortality and fertility. These buffers against mortality also favor a longer juvenile period and higher investment in other mechanisms to increase life span.

Thus, we propose that the long human life span, lengthening of the juvenile period, increased brain capacities for information processing and storage, intergenerational resource flows, and cooperative biparental investment in offspring coevolved in response to this dietary shift and the new production processes it entailed.

It is not yet possible to know many vital statistics and behavioral characteristics from paleontological and archeological remains. It must be recognized that modern hunter-gatherers are not living replicas of our Stone Age past, and global socioeconomic forces affect them all. Furthermore, many foragers today live in marginalized habitats that underreward male hunting efforts. Yet despite the variable historical, ecological, and political conditions affecting them, there is remarkable similarity among foraging peoples, and even the variation often makes adaptive sense. Comparisons between foraging peoples and other modern primates are an important source of information about the life histories of our ancestors and the selection pressures acting on them, the subject of the next sections.

Mortality and Production

The age-specific mortality profile among chimpanzees is relatively V-shaped, decreasing rapidly after infancy to its lowest point (about 3 percent per year) at about age 13, the age of first reproduction for females, and increasing sharply thereafter. In contrast, mortality among human foragers decreases to a much lower point (about 0.5 percent per year) and remains low with no increase between about 15 and 40 years of age. Mortality then increases slowly, until there is a very rapid rise in the 60s and 70s. The pattern is much more block U-shaped. The strong similarities in the mortal-



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement