What is the grandmother hypothesis for menopause?

The grandmother hypothesis is a proposed explanation for why human females live long after their fertility ends. It holds that older women improve the survival of their genes by supporting younger generations. By providing food and support to grandchildren, a grandmother can ensure her genes pass on through descendants, rather than through further births of her own. Versions of the idea differ on whether menopause itself was selected for or is a byproduct of longer life. [1] [2]

Long postmenopausal lifespans distinguish humans from other primates. Other apes live no longer than about 50 years, and their bodily systems, including fertility, decline together. In humans, maximum lifespan is nearly 100 years, but female fertility universally ends at roughly half that, well before other signs of frailty. Among hunter-gatherers, about three-quarters of girls who reach adulthood live past 45, and at any one time a third or more of adult women are beyond childbearing age. [3] [4]

The version developed by Kristen Hawkes and colleagues draws on observations of Hadza foragers in Tanzania. Young children can pick soft fruits and berries but are too small to dig and process deeply buried tubers, which are Hadza staples. Postmenopausal women dig tubers at the same high rates as other women and help feed their daughters' and nieces' children, especially when a mother is occupied with a newborn. [3] [4]

Hawkes and Coxworth suggest that as Plio-Pleistocene African grasslands spread, tuber-like foods became more abundant, and ancestral mothers would have needed to subsidize weaned offspring. Grandmothers who did so let their daughters have the next baby sooner. Because stronger elders could help more, selection would have favored mechanisms that slowed aging, lengthening life. On this account the hypothesis also helps explain shorter birth intervals and later maturity in humans. [4]

Peccei distinguishes two versions. In the old grandmother hypothesis, menopause is an adaptation: stopping reproduction early creates a postreproductive lifespan that is used for grandmothering. In the new version, grandmothering is the adaptation that favored increased longevity, and menopause is a byproduct. Peccei reviewed the evidence for both and concluded that if menopause resulted from selection for greater maternal investment, this involved mainly mothers, not grandmothers. [5]

Hawkes's PNAS paper argues against the older stopping-early idea. Chimpanzee females whose late-born offspring have low survival odds still often keep reproducing, and human reproductive spans are at least as long as chimpanzees'. In this view, postmenopausal longevity, not early termination of fertility, is the derived human trait. The results provided do not say which view most specialists currently hold, and critics such as Peccei remain skeptical of grandmother-based explanations for menopause itself. [3] [5]