emotional capacities to selection pressures that arose as a consequence of ancestral grandmothering.
Ovarian aging appears to differ little between modern humans and chimpanzees, making it likely the same pattern characterized our ancestors. Before the shifts to greater longevity in our lineage, heterogeneity in ovarian and somatic aging would have been strongly culled by mortality selection across the childbearing years. If grandmother effects reduced mortality across those years, heterogeneity in ovarian aging would have expanded as more and more women outlived their fertility. Subsidies for relatives’ reproduction would have begun well before the average age at last birth, let alone the average age of menopause. By this argument, heterogeneity in ovarian aging is an ancient legacy of grandmothering in our lineage; but now such heterogeneity poses unprecedented concerns in the human populations where childbearing is delayed and nuclear families are isolated as never before. Many women find they have missed their own windows of fertility (Broekmans et al., 2009). Although aging is often seen as a process that befalls the old, evolutionary theories of aging predict that function begins to decline in early adulthood. Such declines have been documented not only in fertility, but in muscle strength and cognitive performance (Hunter et al., 2000; Salthouse, 2009); and where mortality levels have dropped to evolutionarily unprecedented lows, heterogeneity in somatic competence is increasingly well documented in those past mid-life (Mitnitski et al., 2005; Rockwood and Mitnitski, 2007). Just as grandmothering may have expanded heterogeneity in ovarian aging by lowering mortality across the childbearing years, recently dropping mortality rates at older ages expand heterogeneity well beyond them. As continuing innovations in medical and daily living technologies interact with mortality selection to produce complex dynamics in the health and welfare of elders (Manton, 2008), the heterogeneity in ovarian and somatic aging that is an aspect of our evolved life history becomes an increasing medical as well as social, economic, and political concern of our time.
I am grateful for valuable insights and advice from J. F. O’Connell, J. E. Coxworth, J. K. Blevins, C. W. Kuzawa, and S. B. Hrdy, and for research support from the National Science Foundation.