The Grandmother Hypothesis and Rates of Aging
A grandmother hypothesis links the evolution of human longevity to ecological changes that left ancestral youngsters unable to get enough food on their own. Help from grandmothers allowed mothers to bear their next baby sooner while setting novel social problems for both mothers and offspring. These connections link grandmothering not only to the evolution of our long lifespans, but also to other features of human life history, physiology, and behavior. Even if only some are correct, they make human postmenopausal longevity much less of a puzzle after all.
But how do we do it? Estrogen is crucial to the maintenance of many physiological systems aside from fertility. Yet ovarian estrogen secretion depends on menstrual cycling which ends at similar ages in all great apes including humans. Like most mammals, other primates display geriatric symptoms while still cycling and rarely survive their fertile years while women remain strong and healthy beyond menopause. The contrast points to non-ovarian sources of estrogen in somatic maintenance. A likely nominee is an adrenal androgen that circulates at a higher level in humans than any other hormone and is estimated to be the precursor for most of the estrogen in women’s peripheral tissues even before menopause. Chimpanzee levels of this steroid are much lower, consistent with the hypothesis that shifts in adrenal function are an important mechanism for slowed aging in our lineage. Comparisons with other apes also reveal intriguing puzzles about somatic maintenance in our closest living relatives.