Anthropology.net

As this paper is freely accessible for the next 7 days, I’m posting it here in the hope that as many readers as possible will have time to read it through. Molly Fox et al turn their thoughts to the question of why women are able to live for many years after they able to conceive offspring, a phenomenon seemingly at odds with the idea that members of a species tend to die off once their reproductive days are over.

The ‘Grandmother Hypothesis’ is described and expanded upon thus in the linked paper:

With menopause occurring around age 50, women survive several decades longer than their gametes (Dorland et al. 1998; Hawkes 2003). Why should women have such conservative life histories that they save up enough energy for 25 years of reproductive retirement? Human females represent a particular conundrum in evolutionary studies: after menopause they are unable to reproduce and have no obvious way to increase their genetic contribution to future generations. As this is the basis for natural selection, many researchers have sought to explain how post-menopausal longevity is adaptive in women.

Among evolutionary biologists, the ‘grandmother hypothesis’ is the most widely recognized explanation for human female post-reproductive longevity. It suggests that vigorous, skilled, elderly women are able to contribute to their grandchildren’s survivorship through nutritional provisioning (Hawkes et al. 1998; Hawkes 2003). According to Hamilton’s relatedness coefficients (Hamilton 1966), grandmothers share a quarter of their DNA with grandchildren, and so a woman can increase her genetic contribution to subsequent generations by keeping her grandchildren alive and healthy. Supporters of this model often refer to the Hadza, a modern foraging society in Tanzania, where grandmothers are more effective than children at extracting tubers from the dry ground. While a mother is breastfeeding an infant, and thus unable to forage enough to provide for weaned children, the grandmother provides food for the older siblings (Hawkes 2003; Hawkes & Jones 2005).

The authors further suggest that this may be a behavioural and survival trait that can trace its roots back to the early Pleistocene, 1.7m-1.9m years bp (described in the paper as the Plio-Pleistocence boundary, recently re-dated to 2.6m years bp), when global cooling caused the expansion of African tuber-friendly grasslands, prompting ancient hominids to include a great many more tubers in their diet. From these origins, where grandmothers are suggested to have been important contributors of nutrition to two generations of descendants, researchers such as Soffer and Adovasio have taken the theory further, suggesting that grandmothers contributed a much greater role in child-rearing than simply providing nutritional foodstuffs.

The interesting aspect of this paper is how the authors suggest simply having a grandmother isn’t equally beneficial to grandchildren of either gender, with clear differences suggesting that in some instances having a maternal grandmother can be better for boys than a paternal grandmother, whilst a grand-daughter with a maternal grandmother appears to benefit most of all, as indicated here:

Tests of the grandmother hypothesis have been carried out on modern and historical populations. Most of the data correlate grandchild survivorship with grandmother survival and/or proximity, and it is often reported that only maternal grandmothers (MGMs) are found to have a positive effect on grandchild survivorship (Sear et al. 2002; Voland & Beise 2002). Studies that have not distinguished between MGMs and paternal grandmothers (PGMs) have found no correlation between grandmother’s presence and grandchild survivorship (Hill & Hurtado 1991; Lahdenperä et al. 2004). To date, only two previous studies known to the authors have distinguished between boys and girls when investigating the effect of both grandmothers.

A study of a Japanese population found that the presence of a PGM had a negative effect on boys and a positive effect on girls, while the presence of an MGM had a positive effect on children of both sexes, especially on boys (Jamison et al. 2002). They point to cultural features of the society as a possible mechanism for these findings. A study of an Ethiopian population reported the sex-specific information, but it was not analysed in the paper (Gibson & Mace 2005). More tests of the grandmother hypothesis are summarized in Sear & Mace (2007).

The authors continue by proposing the following:

The grandmother hypothesis is based on the fact that women are genetically related to their grandchildren, and we should not overlook the nature of that genetic relatedness. Grandsons and granddaughters differ in the proportion of their X-chromosomes shared with MGMs and PGMs (figure 1). According to our proposed X-linked grandmother hypothesis, if grandmothers invest in grandchildren because of their genetic relatedness with them, then their adaptive incentive to invest may vary in a way that mirrors this variation in genetic relatedness. As a consequence, grandmothers’ differential investment in grandchildren could cause differential survivorship of those grandchildren…

…Although the X-chromosome contains only about 4.4 per cent of our DNA, with its estimated 1529 genes, it contains perhaps approximately 8 per cent of all human genes (Pennisi 2003; NIH 2007; Parang et al. 2008; NCBI 2009a). The dramatic differences in X-relatedness between grandmothers and grandchildren confound the Hamiltonian concept that grandchildren are 25 per cent genetically related to each grandparent. If approximately 92 per cent of our genes are autosomes, then a grandmother shares one-quarter of that, or approximately 23 per cent of her total genes with a grandchild, plus X-relatedness.¹

If a grandmother shares no X-chromosome with a grandchild, then their overall genetic relatedness is approximately 23 per cent, and if they share an entire X-chromosome, then it would be approximately 31 per cent. Therefore, MGMs and grandchildren are likely to share 25 per cent of their genomes, while PGM and granddaughter may share a total of approximately 31 per cent of their genes, with a likelihood of 27 per cent inheritance, while a PGM and grandson may share only approximately 23 per cent.

We can only wonder whether these conclusions were borne out in ancient communities and whether it was realised that there was closer bonding and greater survival amongst children with certain alloparental configurations, or whether child-care was undertaken on a more ad hoc basis, whereby other surrogate mothers who were still of reproductive age were also co-opted into rearing off-spring when the need arose. Moreover, infant mortality was probably high and seemingly arbitrary, especially where actions of predators, illness and infections were concerned, in which case the grandmother effect would have been diminished, except in those cases where an older person such as a grandmother would have had greater experience in recognising symptoms and knowing of potential remedies, something which presumably could also have applied also to grandfathers.

I’m not sure at what age humans living in the early and later stages of the Pleistocene would have become grandparents, but my guess would be quite young, possibly by age 30-35, though whether females would also have become unable to conceive by that age I don’t know; but it seems there could have been some overlap, during which time a grandmother could herself have still given birth to children, unless a menopausal clock kicked in soon after their own off-spring had given birth, which seems unlikely. If we still assume an age of 50 for menopause however, by the time she reached 50, a Pleistocene woman could easily find herself in the role of great-grandmother, though whether humans in significant numbers reached that ripe old age back then, again seems unlikely.

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