Grandmother hypothesis

The grandmother hypothesis is a hypothesis to explain the existence of menopause in human life history by identifying the adaptive value of extended kin networking. It builds on the previously postulated “mother hypothesis” which states that as mothers age, the costs of reproducing become greater, and energy devoted to those activities would be better spent helping her offspring in their reproductive efforts. It suggests that by redirecting their energy onto those of their offspring, grandmothers can better ensure the survival of their genes through younger generations. By providing sustenance and support to their kin, grandmothers not only ensure that their genetic interests are met, but they also enhance their social networks which could translate into better immediate resource acquisition. This effect could extend past kin into larger community networks and benefit wider group fitness.

One explanation to this was presented by G.C. Williams who was the first to posit that menopause might be an adaptation. Williams suggested that at some point it became more advantageous for women to redirect reproductive efforts into increased support of existing offspring. Since a female's dependent offspring would die as soon as she did, he argued, older mothers should stop producing new offspring and focus on those existing. In so doing, they would avoid the age-related risks associated with reproduction and thereby eliminate a potential threat to the continued survival of current offspring. The evolutionary reasoning behind this is driven by related theories.

Kin selection provides the framework for an adaptive strategy by which altruistic behavior is bestowed on closely related individuals because easily identifiable markers exist to indicate them as likely to reciprocate. Kin selection is implicit in theories regarding the successful propagation of genetic material through reproduction, as helping an individual more likely to share one's genetic material would better ensure the survival of at least a portion of it. Hamilton's rule suggests that individuals preferentially help those more related to them when costs to themselves are minimal. This is modeled mathematically as :

rb > c

Grandmothers would, therefore, be expected to forgo their own reproduction once the benefits of helping those individuals (b) multiplied by the relatedness to that individual (r) outweighed the costs of the grandmother not reproducing (c).

Evidence of kin selection emerged as correlated with climate-driven changes, around 1.8 –1.7 million years ago, in female foraging and food sharing practices. These adjustments increased juvenile dependency, forcing mothers to opt for a low-ranked, common food source (tubers) that required adult skill to harvest and process. Such demands constrained female IBIs thus providing an opportunity for selection to favor the grandmother hypothesis.

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