The Grandmother Effect
G.C. Williams was the first to posit that menopause might be an adaptation. Williams suggested that at some point during evolution, it became advantageous for females to stop "dividing declining faculties between the care of extant offspring and the production of new ones" (p. 408). Since a female's dependent offspring would die as soon as she did, he argued, older mothers should stop producing new babies and focus on the offspring they already had. In so doing, they would avoid the risk of dying during childbirth and thereby eliminate a potential threat to the continued survival of current offspring.
In addition, postmenopausal women can contribute knowledge and skills to other group members to enhance group fitness. If the other group member receiving investment were kin, then this would increase the fitness of a post-menopausal woman.
This kin selection emerged 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 birth intervals and consequently their fertility; thus providing an opportunity for selection to favour the grandmother hypothesis.
Read more about this topic: Grandmother Hypothesis
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