Granny's legacy : did evolution select for grandmothers over an extended fertility window?

Abstract

Why are humans among the very few species in which females have an extended postreproductive lifespan? This question was first posed in 1957 by evolutionary biologist and original theorist of the Grandmother Hypothesis G.C. Williams, based on the widely accepted belief that menopause was uniquely human (Williams., 1957; Kim et al., 2018). Menopause occurs in human females well before the end of their anticipated life span and is classified as the permanent discontinuation of ovulation (Thouzeau & Raymond, 2017). Following the postulates of Darwin, any decrease in reproduction is counteractive to fitness, meaning that menopause essentially has no benefits to survival (Croft et al.,2015). The Grandmother Hypothesis asserts that grandmothers' benefits of caring for and aiding children and grandchildren counterbalance the price of lost reproduction (Cohen 2007). This hypothesis suggests that natural selection favours a prolonged post-reproductive lifespan if it allows individuals to enhance their fitness by aiding their offspring in successful reproduction. This research evaluates the utility of the Grandmother Hypothesis for understanding PRLS in <i>Homo sapiens</i> using historical (1790-1918) parish data from Nova Scotia. Using this data, I will identify whether fecundity and infant mortality rates follow the trends outlined by the Grandmother Hypothesis. Specifically, I will analyze whether there are shorter birth intervals in the mothers where their mother lives in close geographic proximity and if the child survives to reproductive age when their maternal grandmother is present. Although the results did not achieve statistical significance, the trends apparent in the data do follow the proposed trends of the Grandmother Hypothesis.