Post-reproductive longevity is a robust feature of human life and not only a recent phenomenon caused by improvements in sanitation, public health, and medical advances. We argue for an adaptive life span of 68-78 years for modern Homo sapiens based on our analysis of mortality profiles obtained from small-scale hunter-gatherer and horticultural populations from around the world. We compare patterns of survivorship across the life span, rates of senescence, modal ages at adult death, and causes of death. We attempt to reconcile our results with those derived from paleodemographic studies that characterize prehistoric human lives as "nasty, brutish, and short," and with observations of recent acculturation among contemporary subsistence populations. We integrate information on age-specific dependency and resource production to help explain the adaptive utility of longevity in humans from an evolutionary perspective.
AVERAGE WORLDWIDE HUMAN life expectancy reached 66 years in the first quinquennium of the twenty-first century, with extremes at the country level ranging from 39 years in Zambia to 82 years in Japan (United Nations 2007). Average life expectancy has increased linearly at almost three months per year over the past 160 years, with improvements in sanitation, nutrition, and public health accounting for much of this change (Riley 2001; Oeppen and Vaupel 2003). As a consequence of longevity in the developed world, women currently live more than a third of their lives in a post-reproductive state following menopause.
Such high survival rates almost surely had never occurred before in human history. Agriculture and pastoralism have been practiced for only about 10,000 years, and most extensively in the past 5,000 years. The genus Homo has existed for about 2 million years, and humans have lived as hunter-gatherers for the vast majority of their evolutionary history. While some important genetic changes may have occurred in populations after the advent of agriculture, the major distinctive features of our species (Wang et al. 2006), such as large brains, long lives, marriage and male investment in offspring, long child dependency on parents, and grandparental support of grandchildren, appear to have evolved during our preagricultural history (see Kaplan 1997 for reviews; Kaplan et al. 2000, 2001). Despite recent improvements in human survivorship, it is likely that the age-specific mortality pattern and the timing and pace of development and senescence evolved during our hunter-gatherer past as well.
The purpose of this article is to assess the evolved human mortality profile and particularly the pattern of senescent mortality change with age. We address five questions:
1) Is there a characteristic shape to the human mortality profile, as it decreases first during childhood and then increases with aging?
2) How robust is the occurrence of a post-reproductive life span, and how likely is it that older adults were alive and available in human populations?
3) Is there a characteristic modal age at death for adults, and what can this mode tell us about aging and the evolution of the human life span?
4) How variable is this mortality profile among populations, and what factors shape any variation?
5) How do the...