Menopause, Maternal Investment and Physical Anthropology:

Physical anthropologists attempt to explain why and how certain adaptations have
taken place in human developmental history.  For many years the phenomenon of
human menopause baffled anthropological experts.  If reproduction is in fact the prime
directive of natural selection then menopause, or genetically heritable infertility, would
seem completely anomalous from an evolutionary standpoint.  Furthermore, with the
exception of humans, periods of post reproductive infertility are extremely rare among
mammals. So what makes us different?

In the last decade a theory has been developed that explains why prehistoric, human
females developed menopause and how an inability to create offspring could have
conferred an evolutionary advantage upon the human species.  The answer seems to
be two fold and our discussion will concentrate on the risks associated with childbirth
and the relative importance of parental investment to the human species.

It is conceivable that older mothers that lost their sexual viability were able to spend
more of their time helping, protecting and teaching their children and grandchildren.  
Such an investment of time is referred to by behaviorists as parental investment, or
altruism.  Experiments and natural observation have shown that those animals that
have had time invested in them by family members, in the form of protection and
teaching, are much more likely to live to the age at which they are able to reproduce.  
This means that menopausal mothers in prehistoric times were better equipped to
ensure the survival of their offspring.  

Both pregnancy and childbirth are extremely detrimental to the health and longevity of
women.  Pregnancy increases a woman’s caloric intake requirements and childbirth
exposes women, especially older women, to deadly infections.  For these reasons
physical anthropologists have reached the consensus that primitive, older women
were less productive child bearers than younger women.  

On average those women who experienced menopause lived longer lives and were
better able to spend time supporting their children and their grandchildren.  The
progeny of these menopausal women benefited from additional parental investment
and were thereby more likely to live to procreate. These progeny also benefited from
inheriting their mother’s genes because these genes caused them to experience
menopause and have more prosperous progeny of their own. Thus menopause in
older woman today is a remnant of a protective evolutionary adaptation that allowed
females to better focus their maternal resources.

Authors that furthered the development of this theory of menopause include Kristen
Hawkes (the creator of the "Grandmother Hypothesis") and C.G. Williams who first
posited that menopause may be protective.
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