Introduction
Similar to the female species itself, as males would attest, the human female menopause is an evolutionary puzzle yet to be solved and remains poorly understood. Sometimes referred to as ‘the change of life’, menopause marks the termination of a woman’s reproductive life. Throughout menopause, ovulations ceases and the production of progesterone and estrogen no longer occurs. Referring to the final menstrual period a woman experiences, menopause, naturally occurs in women between the ages of 45 and 55 years old. These hormonal changes can result in the both physical and psychological changes such as loss of libido, hot flashes, and difficulty sleeping accompanied with mood changes (McKinlay, Brambilla, Posner, 1992).
For decades, evolutionary biologists have made many efforts to understand the significance and origin of human menopause. Reproductive senescence in human females occurs long before the deterioration of other body functions. The cause of this reproductive senescence is due to the depletion of oocytes, immature egg cells, within the ovaries. Although oocyte exhaustion is a main contributor to menopause, there is not clear reason as to why the size of the initial oocyte supply has not increased along with human longevity. This presents an evolutionary dilemma concerning natural selection, since continued reproduction should be favored. An increase in maternal age correlates strongly with abnormalities within the female’s chromosomes. This aging factor offers a reason for the female body to cease reproduction, however there is no reason why the female reproduction “shelf-life” couldn’t not have evolved more efficiently through better cellular repair mechanisms. As Steven N. Austad, a researcher in the evolution of life histories, stated, “Assuming the human body has been physiologically adapted to the conditions extant during the vast majority of human history, it may be well worth pursuing how the signs and symptoms of menopause are affected by dietary, exercise, and reproductive hormone regimes mimicking those of the late Paleolithic era.” It is fundamental for evolutionists to ask themselves is if the ultimate reason for the existence of menopause lies in the evolutionary biology of the human life history?
The Age of Menopause
A reduction in the mortality that following improved sanitation and agriculture provides a possible explanation of a non-adaptive cause of menopause (Peccei, 2001). The theory behind it is that the lifespan of a reproductive women should reflect the lifespan of a female prior to the improved advancements. If this theory was correct, within the hunter-gatherer population menopause should be absent. This would result from the lack of agriculture and modern medicine because in past time periods, females rarely survived past the age of 50. However, a study of three well-known hunter-gatherer groups: The Kung of the Kalahari, Hadza of Tanzania and Ache of Paraguay. Most women of these populations live to a post-menopausal age. A recent finding of a fossil molar wear proposes that a fraction of the humans that were surviving to become grandparents increased five fold between 300,000 and 10,000 years ago, before the development of agriculture. This newly found evidence suggests that menopause has played a role in lives of women for the past 10,000 years, and perhaps even longer.
Reproductive Fitness of Human Female Menopause
Recently there has been an increasing amount of female individuals reaching a life span of 85 years and beyond, an intriguing phenomenon. Taken from a Darwinian perspective, these long lived life spans of women after menopause are not easily justified by the energetic costs of reproduction alone (Hall, 2004). As is the case with humans, natural selection would favor females who become infertile many years before death if offspring require a vast effort of parental care. Females becoming infertile prematurely is favored by natural selection. Maternal depletion came from the rising cost of the rearing of each offspring, and it was more profitable in terms of reproductive success for the mothers to invest in surviving offspring rather than attempting in pregnancies later in life (Peccei, 1995). This adaption is necessary to ensure the survival of maternal offspring during the lengthy period of dependency and the nurturing energies that accompany pregnancy and lactation.
Endocrine reproductive fitness states are abounding with estrogen, with lactation being the only low-estrogen condition associating with effective reproductive effort. The provision of adequate and mobile nutrition enables the survival of the suckling offspring, all of which lactation justifies the guidelines of evolutionary selective pressure. The physiology of lactation is of keen interest to evolutionists because selective genetic evolutionary pressure is the cause of the only low-estrogen state. During menopause, many maternal physiologic alterations and symptoms occur due to lactational hormone shifts. Many of these changes are advantageous biologically for nursing infants at a cost for mothers.
The Dependency of Human Infants and Cooperation with Kin Groups
Human infant dependency is largely due to the pelvic constraints of the birthing canal combined with the large size of the brain of the newborn. In order to becoming capable of independent survival, babies are born with brain sizes that are close to the limit compatible with safe delivery and can result with high risks of complication (Yerushalmy et al. 1940; Grimes 1994). With increasing age maternal mortality rates also increase, consequently it would make sense to cease having children when the benefits are outweighed by the risks. Does the high cost of bearing offspring in late years outweigh the risks? As age in women increases, specifically after the age of 45, so do the rates of stillbirths and chances of the offspring being born with birth defects. As a result of senescence, an older woman may switch her resources from her own breeding attempts to help raise her grandchildren.
The grandmother hypothesis, a phrase coined that aging women gain an inclusive fitness advantage from investing into their progeny (Peccei, 2001). This hypothesis is derived from the idea evolving from an explanation of long post reproductive lifespan in human females. Menopause, in the grandmother hypothesis, is an adaption of facilitating mothering. The hypothesis is that the grandmothers must stop reproducing early in order to create a post reproductive lifespan to help with their progeny. In this way of life, fitness is enhanced by producing grandmothers with long post reproductive lifespans to assist their adult offspring by sharing in the burden of providing and protecting their grandchildren (Hawkes et al. 1998; Hawkes 2004).
When it comes to caring and providing for young, humans are unique in assisting their kin. Human infants are not able to provide for themselves realistically before the age of 5, with a nutritional dependency that can last until puberty (Kaplan 1996; Bogin 1997). In comparison with the early age of weaning infants, this high dependency seems odd unless others are also participating in the help of raising a child. The maternal grandmother is the prime candidate for such provision as she is the best-fit genetic interest in her kin’s reproductive success. Grandmother’s present have often led to children being larger and in mothers having a higher rate of fertility (Bereczkei 1998; Lahdenperä et al. 2004).
Responses to Falling Sex Steroids
There are many metabolic alterations resulting from menopausal symptoms involving immune function and disease. While these alterations are beneficial during the era of reproductive, there are cell systems affected and there can even be maladaptive consequences during the phase of menopause. While maintenance of capillary blood flow is critical during child bearing years, this could lead to cardiovascular disease and hypertension in the post-reproductive life of women. Suckling newborns need the radiant warming of their mothers, this is achieved by the body’s vasomotor control and overall changes in the normal temperature set-point. Although these temperature controls are necessary for the offspring, women oftentimes experience dreadful night sweats and hot flashes.
It is estimated that approximately 90% of women are affected with post-menopausal vasomotor symptoms. The figure below is a report from current day, sedentary populations and do not represent the hunters and gatherers. Menopausal symptoms are greatly decreased in physical women living an active lifestyle in comparison with those who are more inactive. Since hunters and gatherers are extremely vigorous and dynamic even in their late years, it was likely they would have been among those least bothered by symptoms accompanying menopause.
An increase in vigilance is mandatory to ensure the safety of offspring involving alterations in sleep pattern, however these sleep alterations have drastic effects on sleep disorders in menopausal women. Interestingly there are even alterations to the central nervous system (CNS) decreasing the immune brain barrier and having changes cognitively. These changes in the nervous system lead to dystrophies and memory deficiency. To ensure health in the offspring, the body increase its immune response to foreign antigens, these have detrimental effects in the long-term such as failure of immune response and vulnerability to the CNS and even cancer.
Genetic Factors of Menopause
There is scarce amount of evidence to validate that menopause has had a evolutionary genetic contribution for females over time. Since, women living beyond their reproductive fitness are able to care for their offspring by offering hunter-gatherer labor and resources for their children, there is an outcome of lower maternal and infant mortality rates. Other than limiting the life span of her reproduction and lengthening the time of protection and provision, there seems to be no genomic advantage due to menopause.
Natural selection is intended to favor earlier onset of alleles that are beneficial and harmful alleles later in life. However, there is an exception of lethal alleles that prematurely act throughout pubescent years. Such natural selection would create senescing, or deteriorating, organisms. It is predicted that the systems of the organism will simultaneously break down because selection most strongly acts to suspend the rate of the destructive effects that occur the earliest (Williams, 1957). To survive past the age of normal reproducing years demands a rationalization, although there is little information on the influence of genetics and the timing of menopause. A reasonable explanation for the failure to detect important predictors of age at menopause is due to the variation of menopausal age and is independently determined by the influence of genes. However, there have been recent studies showing that family history can be a predictor of early menopause and a strong correlation between the menopausal ages of mothers and their daughters. A classic theory of genetic factors of menopause suggests that genes revealing deleterious will accumulate and have an effect on post-reproductive survival. A “death barrier” should extend the maximum age for maternal reproduction resulting from males preferring younger females over older females leading to the accumulation of mutations deleterious fertility of females (Morton, Stone, Singh, 2017).
Conclusion
If they live long enough, all women will experience menopause no matter the society or culture in which she lives. From an evolutionary perspective of menopause, many researchers are still on a quest to solve the puzzle of the post-reproductive phase of a woman’s life. There are two main patterns that have evolved from evolution theorists (1) promoting there is an evolved pattern that actively promotes a non-reproductive life cycle phase of the female and (2) whether menopause is an unintentional by-product of other processes that chose good health during the human female’s reproductive years. Jocelyn Peccei, a well-known anthropologist expresses her opinion:
If premature reproductive senescence is the result of certain conditions in the environment of evolutionary adaptedness, the universality of menopause implies that those conditions must still be present today. Menopause, or the postreproductive life span, must be the response to some unvarying constellation of pressures; my vote is for prolonged offspring dependence and overlapping child care (2001).
The birth of a newborn human requires time for the brain to develop and a considerable amount of time before the offspring is able to declare independence. The infant therefore is highly dependable on a parent’s, usually maternal, attention for survival. The maternal mortality increases with age, since there are many risks that can arise from the birth of a child, it appears to make sense to terminate having more children when the threats are too high. With a new focus, those females who have experienced menopause are able to enhance their fitness by being an assist to their adult offspring and can help in the rearing of their grandchildren.
Arisen early in human evolution, many biological evolutionists suggest that menopause was a result of the high cost of childcare (lactation) arising from dependence of their offspring. Females are favored by natural selection, by those who were prematurely infertile, but allowed from the investment of children and grandchildren. Even with the physiological symptoms, genetic influences and the reproductive fitness of the human female, menopause still remains an evolutionary puzzle.