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.
Williams suggested that at some point it became more advantageous for women to redirect reproductive efforts into increased support of existing offspring.
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.
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.
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.
[6] 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.
[6] Such demands constrained female IBIs (Inter Birth Intervals) thus providing an opportunity for selection to favor the grandmother hypothesis.
The mismatch between the rates of degradation of somatic cells versus gametes in human females provides an unsolved paradox.
[10] In female placentals, the number of ovarian oocytes is fixed during embryonic development, possibly as an adaptation to reduce the accumulation of mutations,[11] which then mature or degrade over the life course.
The practice of dividing parenting responsibilities among non-parents affords females a great advantage in that they can dedicate more effort and energy toward having an increased number of offspring.
[18] If the grandmother effect were true, post-menopausal women should continue to work after the cessation of fertility and use the proceeds to preferentially provision their kin.
A modern hunter-gatherer group in Tanzania, the post-menopausal Hadza women often help their grandchildren by foraging for food staples that younger children are inefficient at acquiring successfully.
However, some commentators felt that the role of Hadza men, who contribute 96% of the mean daily intake of protein, was ignored,[8] though the authors have addressed this criticism in numerous publications.
[27] Some critics have cast doubt on the hypothesis because while it addresses how grandparental care could have maintained longer female post-reproductive lifespans, it does not provide an explanation for how it would have evolved in the first place.
One theory is that the number of caregivers has a positive relationship on the likelihood of offspring reaching adulthood, suggesting that grandparents who contribute to the care of their grandchildren are more likely to have their genes passed down.
This also faces the task of explaining the paradox between the typical age for menopause onset and the life expectancy of female humans.