Centenarian humans are not out of the scope of primate longevity, especially given the large numbers of human observations; that is, high numbers increase the probability of sampling the extreme right tail of the distribution. Cebus monkeys exhibit relative life span potentials similar to humans and converge in traits such as a relatively large brain, generalized ability to exploit a wide range of ecological niches over a broad geographical distribution, fruit-based omnivorous diet, and polygynous mating systems (Judge and Carey 2000). Although Cebus are female philopatric (females remain in their natal groups while males disperse), it is not known whether human ancestors were male or female philopatric. If human ancestors had the potential for 72 to 90-year life spans for one to two million years, one might wonder why prolonging the life span to 100 years under modern conditions of ecological release has not been easier?
post-reproduction expected from primate patterns. Hammer and Foley (1996) use body and raw brain volume estimates from fossil crania to predict early hominid longevity using a multivariate regression of log body weight and brain volume. Estimates based on regressions ofanthropoid primate subfamilies, or limited to extant apes, indicate a major increase in longevity between Homo habilis (52 to 56 years) and H. erectus (60 to 63 years), occurring roughly 1.7 to 2 million years ago. Their predicted life span for small-bodied H. sapiens is 66 to 72 years. From a catarrhine (Old World monkeys and apes) comparison group, Judge and Carey (2000) predict 91 years when contemporary human data are excluded from the equation. For early hominids to live as long or longer than predicted was probably extremely rare; the important point is that the basic Old World primate design resulted in an organism with the potential to survive longer than a contemporary mother's ability to give birth. Notably, Hammer and Foley's predicted life span of H. habilis exceeds the age of menopause in extant women by 7 to 11 years, and that of H. erectus exceeds menopause by 15 to 18 years. This suggests that postmenopausal survival is not an artifact of modern lifestyle but may have originated between one and two million years ago coincident with the radiation of hominids out of Africa.
Williams (1957) first suggested that menopause may be the evolutionary result of a human life history that requires extended maternal care of offspring. Diamond (1992) notes that menopause probably resulted from two distinctly human characteristics: (1) the exceptional danger that childbirth poses to mothers and (2) the danger that a mother's death poses to her offspring. Perinatal mortality tends to increase with maternal age, and the death of an older mother endangers not only her current infant but earlier-born infants still dependent on her for food, protection, and other forms of care. However, more recently, Hawkes and co-workers (1998) argue that it is postreproductive longevity that has evolved rather than an early cessation of female reproduction; the reproductive spans of human and other ape females are not appreciably different. Rather, kin selection for older relatives subsidizing the reproduction of younger female kin may have been a primary mechanism extending human life span (the ''grandmother hypothesis''). This subsidization also allowed humans a later age at maturity and, as a result, a longer period of time for growth and learning.
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