Human uniqueness illustrated by life history diversity among small-scale societies and chimpanzees

Background We compare life histories and selection forces among chimpanzees and human subsistence societies in order to identify the age-specific vital rates that best explain fitness variation, selection pressures and species divergence. Methods We employ Life Table Response Experiments that quantify vital rate contributions to population growth rate differences. Although widespread in ecology, these methods have not been applied to human populations or to look at species differences among humans and chimpanzees. We also estimate correlations between vital rate elasticities and life history traits to investigate differences in selection pressures and test predictions of life history theory. Results Chimpanzees’ earlier maturity and higher adult mortality drive species differences, whereas infant mortality and fertility variation drive differences among humans. Human fitness is decoupled from longevity by postreproductive survival, while chimpanzees forfeit higher potential lifetime fertility due to adult mortality attrition. Infant survival is often lower among humans, but lost fitness is recouped via short birth spacing and high peak fertility, thereby reducing selection on infant survival. Lastly, longevity and delayed maturity reduce selection on child survival, but among humans, recruitment selection is unexpectedly highest in longer-lived populations, which are also faster-growing due to high fertility. Conclusion Humans differ from chimpanzees more because of delayed maturity and adult mortality than child mortality or fertility rates. In both species, high child mortality reflects bet-hedging costs of quality/quantity tradeoffs borne by offspring, with high and variable child mortality likely regulating human population growth over evolutionary history. Among human subsistence societies, positive correlations between survival and natural fertility lead selection pressures in human subsistence societies to differ from modern populations undergoing demographic transition, due in part to positive correlations between longevity and natural fertility and negative correlations between recruitment elasticity and reproductive effort.