Older women, older men

Posted on August 25, 2010


The article on naturenews online is headlined ‘Grandmother Hypothesis’ Takes a Hit.

The punchline? researchers at the Max Planck Institute for Evolutionary Anthropology constructed computer simulations intended to test University of Utah anthropologist Kristen Hawkes’ decades-old proposal that human longevity is an evolutionary consequence of the adaptive advantage conferred by having older women– “grandmothers”– available to help with child care. The computer simulation showed no increase in lifespan, even over 500 generations.

The “Grandmother Hypothesis” ran like this: women today live beyond their reproductive lifespan. This is anomalous among primate species, and biologically inefficient. For this longer life span to have evolved, natural selection must have operated; there must have been an adaptive advantage to longer life spans that caused humans who lived longer to survive and reproduce more than those with shorter life spans.

For me, the Grandmother Hypothesis has always been interesting, not so much to explain longer life span, but rather, because it takes seriously the idea that social life– a cooperative group of adults (in this case, women) who together foster children– might be a factor in human development, even under the most mechanistic kinds of models (those that take everything as evaluated on the basis of contribution to Darwinian fitness).

So the interesting outcome I see here is that, while not finding any measurable effect on lifespan, the computer simulation did find that having grandmothers assist in childcare had identifiable effects:

In some simulations, when females are not nursing their own children, they provide fitness benefits to their grandchildren by reducing the age at which they’re weaned.

After about 500 generations, the model demonstrated that the assistance of a grandmother during infancy shortened the interval between the times their daughters give birth, and led to shorter reproductive windows.

Hawkes is quoted as looking forward to examining the assumptions in the computer model in detail, and expecting to find some that could be varied to produce prolonged life span.

Meanwhile, it rested with others to draw attention to the implications of the weaning effect. Writing on Wired.com, Duncan Geere argues

That means that families with grandmothers are able to have more children over the course of a mother’s reproductive cycle.

This is interesting because it does provide a mechanism by which having longer lifespans might lead to larger numbers of offspring, which is the critical link to differential reproductive success.

What interests me here is that the attention to the “grandmother hypothesis” has always been directed to longevity, rather than to the implications for social life. Perhaps now we can have a real discussion of how we imagine social life in early human groups in which older adults survived and also took on a role in the care of children.

To have that conversation, we might need to question why we only have a “grandmother” in this model: what about the older men? naturenews cites Florida State University’s Frank Marlowe as continuing to support the grandmother hypothesis, but seeking to understand the role of men

“even if males are only dragged along with females who were the target of selection”.

For some evolutionary anthropologists, presumably grandmothers are different from grandfathers because women have a greater level of security in identifying their own offspring, and thus their own grandchildren. In teaching about models of early human gender roles, I connect our continuing emphasis on paternity anxiety to nineteenth century use of the model of the “primitive horde“, a fundamentally Freudian psychological model that takes patriarchy for granted.

If we don’t begin by assuming that early humans had the same psychological worries as 19th century Viennese, then we might ask the question differently: what kind of social groups provided greater survival value for the very dependent human young, born with an immense capacity to learn (to adapt culturally to changing conditions) but with little capacity to defend or fend for themselves? The care of larger numbers of adults– males and females– might not only reduce the time to weaning, and thus increase the number of offspring; older women and men might well have improved the survival capacity, knowledge, dare I say wisdom of the young of their group.