Our proposal is that human evolution has resulted in a specialized life history that is due to a particular constellation of the factors discussed above. This constellation derives from the hunter-gatherer way of life, which characterized the vast majority of human evolutionary history. While, as discussed in the next section, there are some universal features associated with this way of life, there is significant ecological variability across habitats. We also propose that as a result of exposure to such variation, human psychology and physiology have evolved to respond in systematic ways to variations in the four factors discussed above. Finally, the domestication of plants and animals and subsequent economic transformations produced new socioecological conditions to which people responded in radical shifts in parenting and mating practices.

HUMAN LIFE HISTORIES IN A COMPARATIVE CONTEXT

Relative to other mammalian orders, the primate order is slow growing, slow reproducing, long lived, and large brained. Humans are at the extreme of the primate continuum. Compared to other primates, there are at least four distinctive characteristics of human life histories: (1) an exceptionally long life span, (2) an extended period of juvenile dependence, resulting in families with multiple dependent children of different ages, (3) multigenerational resource flows and support of reproduction by older postreproductive individuals, and (4) male support of reproduction through the provisioning of females and their offspring. The brain and its attendant functional abilities are also extreme among humans.

Our theory (Kaplan et al., 2000; Kaplan and Robson, 2002; Robson andd Kaplan, 2003) is that these extreme values with respect to brain size and longevity are coevolved responses to an equally extreme commitment to learning-intensive foraging strategies and a dietary shift toward high-quality, nutrient-dense, difficult-to-acquire food resources. The following logic underlies our proposal. First, high levels of knowledge, skill, coordination, and strength are required to exploit the suite of high-quality, difficult-to-acquire resources that humans consume. The attainment of those abilities requires time and a significant commitment to development. This extended learning phase during which productivity is low is compensated for by higher productivity during the adult period, with an intergenerational flow of food from old to young. Since productivity increases with age, the time investment in skill acquisition and knowledge leads to selection for lowered mortality rates and greater longevity because the returns on the investments in development occur at older ages.

Second, the feeding niche specializing on large valuable food packages, and particularly hunting, promotes cooperation between men and women and high levels of male parental investment, because it favors sex-specific



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