very like those observed in hunter-gatherers are maintained by selection against deleterious mutations (Lee, 2008).
Our grandmother hypothesis relies on Charnov’s model of life history evolution (Charnov, 1991, 1993) to explain how correlated allometries in mammalian life history features apply to humans (Hawkes et al., 1998; Alvarez, 2000). Comparisons between other great apes and humans (Robson et al., 2006) have been essential in highlighting distinctive human life history features. As noted, chimpanzees are an especially important comparative model for phylogenetic, ecological, and morphological reasons. Fig. 11.1 shows the female side of the age structure for a human hunter-gatherer population and wild chimpanzees modeled from life tables.
FIGURE 11.1 Female age structures modeled from life tables. Each bar shows the percentage of the population in the 5-year age class indicated in the vertical axis. Lightest bars, juvenile years; medium-gray bars, childbearing years; darkest bars, post-fertile years. Humans are on the right, represented by Hadza huntergatherers with Blurton Jones’s (2002) data. In this population, life expectancy at birth is 33 years. With growth rate 1.3%/year, 32% of the women (those over 15) are past the age of 45. Growing populations are younger because more are born than die. If this population was stationary, the percentage of adult women past the age of 45 would be 39% (Hawkes and Blurton Jones, 2005). The left side of the figure represents the synthetic wild chimpanzee population constructed by Hill and colleagues (2000) using data from five wild study sites. Average age at first birth is 13 in wild chimpanzees so the 10- to 14-year age class is included in the childbearing years. Fertility ends by ~45 in both species. Less than 3% of the adult chimpanzees (counted as those over 10 years) are past the age of 45. The chimpanzee model assumes a stationary population.