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Unique human phenomenon
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Menopause is the time in a woman's life when her menstruation stopspermanently and she is no longer fertile. Menopause is marked whena menstrual periods are absent for 12 months. Menopause is thepermanent cessation of the menses due to ovarian failure leading tothe termination of the menstrual life. During this time the primaryfunction of the human ovaries stops. A woman's ovaries stop makingeggs and they produce less estrogen and progesterone. These changesin hormone levels cause menopause symptoms. Menopause usuallyoccurs in women during their late 40s or early 50s and singles theend of the fertile phase of a woman's life. The age of menopause isinfluenced by both environmental and inherited factors. Smoking isthe most common environmental toxin to ovarian function; it maycause earlier menopause as well as premature morality. A studyshowed that menopause might be influenced by genetics. In the studymothers and daughters found that the age of menopause in the motherwas significant predictor of the age of menopause in theirdaughters. There appears to be genes on the X-chromosomes that arerequired for maintaining ovarian function. Deletions of importantgenetic information found on this chromosome have been associatedwith various degrees of premature ovarian failure and have beenidentified in mother-daughter pairs (Perls, 2001). The menopausaltransition starts with varying menstrual cycle lengths and endswith the final menstrual period. This menopausal transition stateis called perimenopause, which means "the time around menopause."The transition usually occurs over a course of a few years and itis not preventable. It is a natural process that occurs with aging.Post menopause is the entire period of time that comes after thelast menstrual period. Menopause is complete when a woman has nothad a menstrual period for a year. Women who are post-menopausalcan no longer get pregnant. Although menopause is a natural processit is a unique human phenomenon that continues to bewilder manypeople.

Menopause tends to get experts thinking for many reasons. Onereason is that males produce sperm continually through their lives,but females produce eggs only during a short window of time. Theybegin releasing eggs during puberty and menstruation. The numberand quality of eggs gradually decreases until menopause occurs.Experts are curious as to why is there a difference in thereproductive organs. Another reason is that the females of otherspecies also stop producing eggs but it is much closer to the endof their lives than the human female. Human females are the onlyorganisms that experiences menopause. These concepts were puzzlingand raised the question of if natural selection favors increasedreproduction, how could terminating reproduction early throughmenopause be beneficial?

Different researchers have different explanations for menopause.Possible evolutionary explanations for menopause and for thesurvival beyond reproductive maturation range from the non-adaptiveto the adaptive. Several theories have been proposed to explain whymenopause might have evolved, all based on unusual aspects of lifehistory. Four hypotheses have been proposed to explain menopause,two are non-adaptive and two are adaptive. The two non-adaptivehypotheses are explained by the "blessing of modern life" theoryand the "senescence" theory. The first hypothesis states thatmenopause may simply be a cultural artefact. This "blessings ofmodern life" theory states that menopause occurs because women livelonger now than in the past. This idea comes from the fact thatmost animals reproduce as long as they live but due to theartificially lengthening of animal lifespans they often stopreproducing before they die. This theory hypothesize that menopausemay result from medically increasing the lifespan of a primate thatis born with a fixed total number of gametes (Sherman, 1998).

The second hypothesis is explained by the "senescence" theory. Thishypothesis states that senescence occurs because the product ofreproductive value and survivorship of young individuals alwaysexceeds that of old individuals. Tendencies to be vigorous,healthy, and fertile pile up during youth, whereas somatic andreproductive maintenance is neglected later on in life, therefore,menopause may a uncontrolled deterioration of a physiologicalprocess that once was well regulated like memory and eyesight(Sherman, 1998). Advocates of these hypotheses point out that ifvirtually all adults died by age 50 or 55 in the past then themodern situation, in which individuals often live to their 80s and90s, is unprecedented in our evolutionary history. Menopause cannotbe an adaptation because our hunter-gatherer ancestors never livedlong enough to experience it (Book). Although these non-adaptivehypotheses may answer many question they still leave some questionunanswered such as "Why does gamete production terminate abruptlyin women, yet taper off gradually in men? and "Why does ovulationcease so early in a woman's life?". Critics of this hypothesispoint out that if a substantial fraction of our femalehunter-gatherer ancestors lived into their 60s and 70s they mayhave experienced menopause which means menopause needs anevolutionary explanation (Book).

The other two hypotheses suggest that menopause is a life historyadaptation that human females evolved to have. The first adaptivehypothesis states that because birth defects increase with maternalage, menopause protects the human gene pool. This theory is knownas the "group selection" theory. A problem with this theory is thatif pollution in the gene pull was one of the main factors thengenes that tended to cause women to continue having babies despitethe risk involved would outreproduce those that influenced women tostop having babies (Sherman, 1998). The second hypothesis statesthat menopause may have evolved as a counter-strategy tosenescence. The "good mother" theory and the grandmother hypothesissuggests that if the mother had more babies, she is having childrenthat she may not be able to care for and she is risking the futureo her existing children. Instead if she stops having children anddevote herself to helping the children she already has, she mayhave more total offspring who grow up to reproduce themselves. Thissuggests that early reproductive cessation is an adaptive responseto prolonged infant dependency (Sherman, 1998). As a woman getsolder there are also several trends that are likely to occur. Thesetrends include the probability that she will live long enough to beable to nurture another baby from birth to independence declines,the risks associated with pregnancy and childbirth rise and her owndaughter themselves will start to have children. The grandmotherhypothesis suggest that older women may reach a point at which theycan get more additional copies of their genes into futuregenerations by ceasing to reproduce themselves and instead helpingto provision their weaned grandchildren so that their daughters canhave more babies. This is a trade-off between investment inchildren and investment in grandchildren (Book).

A highly controversial implication of the Grandmother Hypothesis isthat menopause may arise simply as a consequence of selection onprolonged lifespan. For example, if in mammals females are bornwith a ?nite number of eggs, and a threshold number is required toinduce ovulation, it may be unsurprising that, when females livelong enough, they will eventually run out. However, there are atleast four reasons to suspect that menopause has been underselection (1) Some mammals are capable of producing enough eggs tolast until 60 years or more (2) There is now evidence from rodentsthat stem cells exist in the ovary and hence that follicles can beregenerated and eggs recreated after birth (3) Menopause in humansis to a degree ‘self-induced' (4) The variation in onset ofmenopause is both variable and heritable (Lahdenpera¨, 204).Determining whether or not menopause is an epiphenomenon ofincreased lifespan will depend on an improved understanding of itsproximate, physiological causes.

Kristen Hawks performed a study on postmenopausal women in theHadza, a contemporary hunter-gather society in East Africa. It waspredicted that if the grandmother hypothesis is correct, then womenin their 50s, 60s, and 70s should continue to work hard atgathering food. If the grandmother hypothesis is wrong, then theresearchers expect older women to relax. It turned out that theolder Hadza women work harder at foraging than any other group andat certain times of the year older women were the most effectiveforagers. The older women share their food with young relatives,improving the children's nutritional status. This data along withthe grandmother hypothesis but it is not a definitive test (Hawkes,1998). Critics of the grandmother hypothesis point out that it iscrucial to figure out if the daughters of the helpful grandmothersare actually able to have more children of their own. There hasbeen some research to support this issue but more complete data onmore cultures are needed for definitive evaluation (Book).

Some believe that menopause may act evolutionarily as a guard toprotect the ageing woman from the hazards of childbirth. Doris andGeorge Williams developed the theory that as humans evolved andbecame able to achieve older and older ages, there came a pointwhen survival during childbirth began to decline as a function offurther ageing and increased frailty. Females, who by virtue ofsome genetic mutations became infertile prior to the age of markedmortality risk, had a survival advantage over those females thatdid not have this series of mutations (Perls, 2001). An equallyimportant advantage of continued survival would be that the mothercould continue to raise and assure the survival of her childrenbeyond puberty, as well as to assist in the care of her children'schildren. From an energy allocation perspective, there probablycomes a point with older age and decreasing energy when it becomesmore efficient to care for the children one already has, and toperform other work in the society rather than devote that energy topregnancy, childbirth and breastfeeding. In primitive hunter andgatherers societies, post-reproductive women perform a largeportion of the work. Therefore, menopause provides a survivaladvantage and a means of better assuring the passing down of one'sgenes to subsequent generations. Williams and Williams thus calledmenopause an `adaptive response' to the increased mortality riskassociated with childbirth (Perls, 2001).

Reproductive investment may be particularly costly in humans, foroffspring are born helpless. This high cost is exacerbated by shortinter-birth intervals and resulting large numbers of dependentoffspring typically nurtured simultaneously in humans. The benefitsof reproducing late may also be small because pregnancies in oldage have an elevated risk of miscarriage, the fetus of old mothershas a higher risk of being born dead, having a defect or being bornsmall. In addition, late reproduction may be costly, for a motherthat dies during or shortly after childbirth will not onlyjeopardize the life of her current child, but also those of earlierchildren which are still dependent on their mother for sustenanceand protection. Menopause may have evolved because the costs ofreproducing late in life out-weigh the benefits, and consequentlygenes that caused mothers to cease reproduction early were selectedcompared to those that allowed risky reproduction late in life.Unfortunately, testing the hypothesis that women with menopauseproduce more surviving offspring than those without menopause isnot feasible, since all women now experience menopause. However,examination of two other predictions is possible:(1) Reproducinglate in life does not add signi?cantly to the number of survivingchildren already produced; and (2) Menopause equals an earlytermination of reproduction. It is on examination of these twopredictions that some shortcomings of the above ideas aboutmenopause become apparent (Lahdenpera¨, 2004).

In a study done by Daryl P. Shanley it was found that a compositemodel readily explains menopause, while individual factors do not,can account for the uniqueness of the human life history. Othersocial species such as baboons and lions show intergenerationalexchange but menopause has not evolved. A key difference may be thehigh risk of mortality in human childbirth, probably due to thelarge neonatal head size and constraints on the pelvis imposed bybipedal gait. Records of American females show an exponentialincrease in risk of maternal mortality with age. This was also seenin captive rhesus macaques, which unlike other apes, share thehuman problem of a difficult birth and are reported to have anearly termination of fertility (Shanley, 2001). However, the modelshowed that increasing maternal mortality in childbirth does not,on its own, eliminate the fitness benefit that older femalesreceive through their own reproduction. Older human femalesenhanced the survival of grandchildren through protection orprovisioning and they lift some of the burden from adult daughters,enabling higher levels of fertility and an earlier age of weaning.In lions, grandmothers may assist by provisioning their daughter'sprogeny but they do this through lactation, which requires thatthey remain reproductive themselves (Shanley, 2001)

Recent endocrinological findings concerning the role of folliculardepletion in the regulation of ovarian cycles suggest a newhypothesis for the evolution of menopause. Follicular depletion,the apoptotic process that ultimately causes menopause, occursthroughout premenopausal life and is integral to the set ofhormonal feedback relationships that maintain regular cycles. Thecharacteristics of the follicular depletion system that determinesthe age at menopause, including the size of the initial folliclereserve and the rate of atresia, are important for ovarian cyclesat younger reproductive ages and appear to be highly conservedbetween humans and chimpanzees. Some suggest that menopause andpost-menopausal life do not confer evolutionary benefits inthemselves, but they evolve antagonistic pleiotropy because ofselection operating on the follicular depletion system to maintainregular ovarian cycles at young adult ages (Wood, 2001).

According to antagonistic pleiotropy, genes that have deleteriouseffects at late ages can be actively selected if they also havebeneficial effects at early ages. Hamilton demonstrated this; heshowed that the effect of a phenotypic trait on an individual'stotal fitness declines monotonically with the age at which thetrait is expressed. Charlesworth showed that under a variety ofcondition a modest benefit early in reproductive orpre-reproductive life could then offset a larger disadvantage thatdoes not appear until late in reproductive life (Wood, 2001). Ifmenopause were a distinct event that is superimposed on areproductive system that would otherwise continue to unctionnormally, it would be difficult to see how antagonistic pleiotropycan play a part in its evolution. Since menopause is actuallycaused by an underlying process of follicular depletion that isessential for the regulation of reproduction at all ages it issuggested that it is in fact the follicular depletion system as awhole that is the evolutionarily relevant phenotype, not just itsterminus at the menopause (Wood, 2001). From this it is concludedthat menopause and post-reproductive life are not adaptivethemselves, but are the pleiotropic effects of selection acting atearlier reproductive ages. Menopause is suggested to be nothingmore than the final outcome of a decades-long process of folliculardepletion and that it is the follicular depletion system that issubject to strong selection, no menopause. It is hypothesized thatthe selective advantage of particular combinations of atresia rateand initial follicle stock in maintaining cycles at earlier agesoffsets the selective cost of terminating reproduction by thefifties when no life history phenomenon is expected to have astrong effect on fitness. Menopause is then a case ofantagonisictic pleiotropy and post reproductive life in humans is asecondary consequence of higher survival during reproductive andpre-reproductive life. The reasoning for this is still uncleartherefore more studies are needed to support this hypothesis.

While the question of whether or not menopause is the product ofnatural selection remains controversial, there is little questionthat positive selection has acted on genes that increase lifespan.There is still considerable debate as to whether or not prolongedlifespan has been selected to ensure the survival of offspring(Mother Theory) or to increase the reproductive success of thoseoffspring (Grandmother Hypotheses). This debate is largely fuelledby semantics, for the difference is immaterial. In both cases themother and/or grandmother is attempting to secure her geneticdynasty by increasing the number of genes that she forwards intothe following generation. It is suggested that women that carriedgenes for longevity were selected because of their ability toincrease the survival and reproductive output of their children.The possibility that menopause itself is/was under selection, sinceshutting down one's reproductive system would allow women with lowchances of reproducing successfully themselves to channel moreresources into somatic maintenance and hence defer the onset ofaging is not completely ruled out. Medical research aimed atincreasing our understanding of the onset of menopause at thephysiological level, the effects of sex hormone reductions on therate of aging/senescence, and the relationship between age atmenopause and expected lifespan, will greatly increase our abilityto make coherent arguments about the role of evolution on menopause(Lahdenpera¨, 2004).

In conclusion, the evolution of menopause is a highly controversialtopic that is still active in research. There are a number oftheories and hypothesis to explain the evolution of menopause. Thegrandmother hypothesis is currently the most supported hypothesis.The grandmother hypothesis not only identifies selection pressurefor increased postmenopausal longevity, but also provides aparsimonious explanation for unique features of human life historyevolution in agreement with phylogeny and evolutionary theory. Thehypothesis is developed in the context of female primate lifehistories, but other comparative evidence also validates it(Alvarez, 2000). Although what caused human females to evolve to gothrough the process of menopause is still unclear it is clear thatthe development of menopause is a significant adaptation for humanfemales because menopause allows for post-reproductive life tocontinue so that the offspring will have a greater chance atsurvival. The greater chance at survival may be due to the help ofthe grandmother or due to the decrease risk of latereproduction.




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