14.1 Grandparents

As discussed in Chapter 10, humans are cooperative breeders. The interaction of cooperation and alloparental care combine, resulting in a social network that spans generations. Who better to support new parents than grandparents.

Grandparents hold a wealth of knowledge that is passed along to mothers and fathers, and that knowledge will again be passed along if children become parents themselves. In our evolutionary history, this wisdom may come in the form of support during pregnancy, birth, and the caring of neonates. It comes in the form of teaching how to care for sick children, procuring additional resources to feed toddlers, and balancing competing needs of newborns and their older siblings. Current thought suggests that our post-reproductive lives evolved specifically so we could offer this care and teaching to a new generation of parents.

The Grandmother Hypothesis

Inspired by Hamilton’s Rule, Robert Trivers (1943 – ) argues that parents and children in the EEA are in constant conflict over resources. This would be especially so if a mother is continuing to reproduce at the same time as her own offspring. This parent-offspring conflict should result in mothers who selfishly keep resources to themselves, refusing to share with reproductive daughters and in extreme cases, infanticide during times of severe food insecurity.

Following this logic, The Grandmother Hypothesis argues that menopause evolved as a mechanism to guarantee older women support and invest in their daughters and grandchildren. It makes sense that human mothers evolved a post-reproductive life specifically so they could support their daughters during motherhood. No longer is there a need to compete. Instead, by supporting her daughters and her daughters’ children, grandma not only secures their immediate survival – she also secures her continued evolutionary and genetic legacy. Grandfathers, on the other hand, are an incredibly important spandrel of grandmothers.

What About Grandfathers?

Frank Marlowe (1954-2019) proposed an alternative to The Grandmother Hypothesis. The Patriarch Hypothesis turns from a focus on alloparenting and considers the decline of fecundity over human lifetimes. Women experience perimenopause and the related inconsistencies in ovulation by their mid-to-late 40s, with complete cessation of fertility by their early 50s. Yet men continue to experience spermatogenesis throughout their lifetime, though quality and quantity of sperm may begin to decline by their late 50s.

For this reason, Marlowe suggested that evolutionary pressures extended the male lifespan to allow for continued reproduction. While a woman’s reproduction is constrained by the size of her ovaries, a man can reproduce until death. This allowed men who were more successful in surviving and attaining resources in the EEA to father more children – and, Marlowe argues, is also a potential explanation for the “mid-life crisis” that drives so many men to seek younger women.

Does It Have to Be One or the Other?

In review, both the Grandmother Hypothesis and the Patriarch Hypothesis are incomplete. As such, it is highly likely that some combination of the two are the reason for the post-reproductive lifespan in humans. The Active Grandparent Hypothesis is a new effort to explain and understand why, and how, humans have such long, post-reproductive lives. Developed by Daniel Lieberman, Timothy Kistner, Daniel Richard, and Aaron Baggish at Harvard, the Active Grandparent Hypothesis suggests that pressures, including but not limited to provisioning offspring, selected for energy allocations that slow senescence and reduce vulnerability to disease. This may be why individual life expectancy relies upon habits of physical activity and proper diet.