Whereas non-human primate females are fertile almost until the end of their lives, human females of about 50 years of age experience the menopause and go through the rest of their lives with no prospect of conceiving any more children. The authors of recent research into the subject explain that these differences have "prompted interest in the evolutionary factors that might explain menopause". It is known that genetic factors are at least partly responsible, and this "suggests the idea that selection may have acted to optimise the length of the fertile portion of the life cycle."
It should be noted that the link between genetic factors and selection is only possible because of the prior assumption of Darwinism. However, even allowing this, the genetic link cannot be said to "support" the idea of the menopause being a product of selection, because it may have been a spandrel effect of humans evolving from ape-like ancestors.
Explanatory concepts used to structure the research are as follows:
"The two major hypotheses to explain the evolution of menopause are based on (i) the extremely protracted dependency of human infants on protection and provisioning by adults, particularly the mother, and (ii) the opportunities for intergenerational cooperation within kin groups."
The option that the menopause points to design influences is not even considered by the authors. In fact, the two major hypotheses (above) identified by the authors to explain the evolution of menopause are equally relevant to design. This maintains that caring for dependants is a designed behaviour and that close relatives all have a part to play. Confirming the two hypotheses does not discriminate between a Darwinian explanation and a design explanation.
The authors develop a test of the two hypotheses using data from 4 villages in The Gambia and a theoretical model. They show that there is a fitness benefit of the menopause with an optimum at 50 years of age.
This paper warrants two comments:
1. Since there is a perfectly reasonable design rationale for caring grandmas having experienced the menopause, the findings can be used as a vindication of the two hypotheses but not as a vindication of the relevance of Darwinism.
2. No attempt has been made to give any account of the genetic/physiological and other changes that are needed for the menopause to occur. This is 'black box' biology, with natural selection being asked to do an amazing number of things in a short period of time to achieve the (relatively small) fitness benefits. It should be noted that genetic changes are not directly passed on to offspring, as in the normal portrayal of the way Darwinism works. We are dealing here with complex changes in females that marginally affect the survival of grandchildren. Additionally, one wonders how many caring grandmothers there actually were in the hypothetical social groups of early man where life expectancies were low.
"Testing hypotheses of menopause" would be a better title, leaving open the issue of causation and theoretical models. The Darwinian elements of this paper can be understood in terms of bias. The adoption of Darwinism to the exclusion of any other conceptual approach is an example of Availability Bias, and the way all the evidences are claimed to support the Darwinian thesis of the research is an example of Confirmation Bias. For more on bias in scholarship, go here.
Testing evolutionary theories of menopause
Daryl P. Shanley, Rebecca Sear, Ruth Mace, Thomas B.L. Kirkwood
Proceedings of the Royal Society B, 274, December 7 2007, 2943-2949 | doi: 10.1098/rspb.2007.1028
Abstract: Why do women cease fertility rather abruptly through menopause at an age well before generalized senescence renders child rearing biologically impossible? The two main evolutionary hypotheses are that menopause serves either (i) to protect mothers from rising age-specific maternal mortality risks, thereby protecting their highly dependent younger children from death if the mother dies or (ii) to provide post-reproductive grandmothers who enhance their inclusive fitness by helping to care and provide for their daughters' children. Recent theoretical work indicates that both factors together are necessary if menopause is to provide an evolutionary advantage. However, these ideas need to be tested using detailed data from actual human life histories lived under reasonably 'natural' conditions; for obvious reasons, such data are extremely scarce. We here describe a study based on a remarkably complete dataset from The Gambia. The data provided quantitative estimates for key parameters for the theoretical model, which were then used to assess the actual effects on fitness. Empirically based numerical analysis of this nature is essential if the enigma of menopause is to be explained satisfactorily in evolutionary terms. Our results point to the distinctive (and perhaps unique) role of menopause in human evolution and provide important support for the hypothesized evolutionary significance of grandmothers.
MacKenzie, D. Caring grandmas explain evolutionary role of menopause, New Scientist, 19 September 2007
Cohen, J. Menopause in Chimps? ScienceNOW Daily News, 13 December 2007
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