Raziel Davison: University of California, Santa Barbara

We investigate human life history evolution by comparing smallscale subsistence societies and chimpanzees to identify the roles that fertility and mortality play in driving population-level fitness differences. We discuss differences in the selection pressures facing individuals of different ages and to make inferences about the trajectory of human life history evolution. Most human populations are growing but post-reproductive survival decouples population growth from lifespan, with high fertility driving rapid growth in some societies and low fertility balancing longevity to maintain near-stationarity in others. Chimpanzee declines are decoupled from fertility because mortality attrition limits higher potential fertility contributions. Selection pressures suggest that variable child survival likely regulated human population dynamics over evolutionary history and may reflect bet-hedging costs of high fertility, with quality/quantity trade-offs constraining the evolution of slower life histories. Common stationarity conditions may represent invariant allometry of chimpanzee and human life histories. Among humans, production and knowledge transfers constituting fitness contributions of post-reproductive adults part the veil of selection to favor long post-reproductive lifespans, but in chimpanzees transfers
are limited and reproductive senescence tracks mortality closely.