Back in the studio after a summer hiatus, greedily chomping on multi-colored cows, tractors & bunnies, a (really only-slightly-satisfying) snack that ended up in the computer bag after rejection by the 6-year old grandson. Well, he at least demonstrated his own good taste, even if the primary rationale for an upturned nose was that farm-themed crackers are for ‘babies’.

My pathetic defense for consuming these completely unhealthy crunchy animals is that it resulted from doing my grandmotherly job of providing my daughter’s weaned child with food (- food which, for this particular post-menopausal primate, is usually responsibly locally-sourced, lovingly home-cooked, & thoroughly age-appropriate… but hey, even grandmas get lazy once in a while). It so happens that this ancient grandmotherly pastime of feeding the grandkids is now being held responsible for not only the greater longevity & shorter birth intervals of humans relative to other great apes, but as well for increased male competition for mates, male tendency toward ‘mate-guarding’, and human ‘pair-bonding’ (aka coupling).

Geez, that’s a lot to plop onto Grandma’s already-full plate.

The still-slightly-controversial grandmother hypothesis regarding the unique nature of human primate longevity has been reinforced in recent years by new research and data analysis, often with anthropologist Kristen Hawkes as the lead scientist. The general hypothesis goes like this:

Menopause occurs at about the same age in humans as it does in other great apes. Unlike our closest relatives, however, humans have the physiological capacity to live decades beyond the average age of female menopause.

At some point in our evolutionary past, probably in response to a changing, more stressful environment around 2-3 million years ago, some ancestral human* females beyond their child-bearing years started regularly sharing food (e.g., food that was difficult for young children to obtain on their own, such as tubers) with their daughters’ (or nieces’ et al) weaned children when their mother was otherwise occupied with caring for & nursing a new infant. Over time, this led to a proportionally higher likelihood of survival for the mother & her offspring, as well as more frequently-spaced human births, thereby ensuring greater survivability of the assisting grandmothers’ longevity genes.

Some scientists also theorize that the grandmother effect further stimulated the ‘sharing-economy’ already in affect with mothers and their children, fostering increased sociality, empathy & cooperation among human groups.

hmm, do ya think she’s his daughter??

New research is shedding light on the pair-bonding aspect of human culture as an extension of the grandmother hypothesis: over (more) time, more humans living longer led to a higher ratio of males to females in ancient populations. Unlike females however, male apes, including humans, retain their fertility as they age; ergo, with increasing longevity, more & more males were competing for the same number of fertile females. This led to “the male tendency to guard a female mate from the competition and form a ‘pair bond’ with her instead of mating with numerous partners“ – a strategy which would have increased male reproductive success. An excess of fertile males is also “a likely source of men’s preference for young women” (- in contrast with male chimpanzees’ apparent preference for older females…aren’t they the smart ones! – of course, these mature ladies were probably still fertile…).

In some quarters, this newest extension of the grandmother hypothesis stands in direct opposition to traditional theories – promulgated over a century ago by (primarily) male anthropologists – that human pair bonding resulted from males providing meat for females & their offspring. To me however, for reasons discussed in previous blogposts about digging tubers, cooking meat & early humans’ most likely sources of digestible protein, the Grandmother angle on human evolution seems a much better fit with current science than a concept which relies on idealizing Man the Hunter.

Just sayin’.

(…with that still mouthy old feminist voice of mine. I swear that being a provisioning grandma myself has absolutely nothing to do with it!)

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* “Human” refers to Homo habilis &/or Homo erectus, or another, as yet undiscovered early Homo species – or maybe even Australopithicus! We don’t know yet; the current estimated timeframe for this longevity transition is based on mathematical modeling.

& well now, we haven’t even made it yet to talking about the newly discovered hominin species, Homo naledi!  Watch for comments about this somewhat mysterious addition to our ever-more-branching family tree, and what’s so great about this latest discovery, coming soon to The Everyday Primate.