December 19, 2001
A debate has recently developed over the evolutionary reason for menstruation and whether current menstruation rates are natural. However, explaining the physiological processes behind menstruation has been a topic for centuries. In the third and fourth centuries B.C., early physicians hypothesized that menstrual blood was the substance from which the embryo formed. It was not until the late 19th to early 20th centuries that biologists started suggesting that menstruation is a secondary process resulting from a failure of fertilization (Finn 1996). It is now known that menstruation is the shedding of the uterine lining in the absence of an implanted embryo, which results in vaginal bleeding. An adequate explanation of menstruation for some biologists and physicians is that it marks a woman’s failure to become pregnant (Travis 1997). Recently, however, evolutionary biologists have begun questioning why menstruation occurs at all. Increased rates of women’s reproductive cancers have also lead some to question whether monthly menstruation for years at a time is a natural and healthy state. During the past 10,000 years, natural selection has made relatively small changes to the human gene pool. This suggests that modern humans are actually adapted for life during the Stone Age. However, American women experience three times as many menstrual cycles as women in these hunter-gatherer societies. Recent lifestyle changes have resulted in consideration of monthly menstruation as the "natural" health state (Eaton et al 1994). Increased rates of women’s reproductive cancers have called into question this belief. Studies suggest that women’s exposure to the hormones estrogen and progesterone have dramatically increased due to life-style changes, resulting in increased rates of reproductive cancers (Strassman 1999).
The menstrual cycle is a set of events, divided into two phases, that results in the production of a mature ovum ready for fertilization and an endometrium that is prepared for implantation (Berga 1999). The follicle phase occurs before ovulation. During this phase, estrogen levels increase, which causes the endometrium to develop and marks maturation of the follicle (Berga 1999, Seagal and Schiff 1999). Ovulation, the release of an oocyte from the mature follicle, follows (Raven and Johnson 1999). During the subsequent luteal phase, the corpus luteum secretes estrogen and progesterone, which causes further development of the endometrium. Absence of fertilization results in corpus luteum regression, which initiates an abrupt decline in estrogen and progesterone levels and causes the endometrium to recede. The menstrual cycle then starts again (Raven and Johnson 1999). In all mammals, the endometrium proliferates during the menstrual cycle and, if pregnancy and implantation do not occur, then the endometrium regresses through reabsorption. Only in humans, apes, Old World monkeys, and shrews does vaginal bleeding also occur (Strassman 1999).
In evolutionary terms, menstruation is a relatively recent development (Finn 1994). Multiple hypotheses have surfaced as to why a few mammal species menstruate while the vast majority does not. The opinion of many scientists is that since natural selection has not eliminated it, menstruation must have some benefit (Travis 1997). However, since menstruation only occurs when pregnancy does not take place, natural selection may not have had an opportunity to select against any negative effects of menstruation. Until recently, most females became pregnant within a few years of first menses, and would therefore pass on their genes before the deleterious effects of menstruation occurred (Finn 1996)
Margy Profet developed the first hypothesis based on an evolutionary explanation for menstruation to gain widespread recognition (Travis 1997). Profet argues that vaginal bleeding flushes out dangerous microorganisms introduced into the uterus by sperm capable of causing infertility, illness, or death. The protection provided by this mechanism outweighs the loss of iron and nutrients accompanying menstrual bleeding (Travis 1997). Profet’s hypothesis received criticism from many scientists. This hypothesis predicted that the promiscuity of a species would correlate to its degree of menstrual bleeding. However, published data showed that several non-promiscuous primate species have copious menstrual bleeding. Menstrual blood is also considered by many as an inappropriate expulsion medium because it is an excellent growth material for microorganisms (Travis 1997). In addition, menstruating every few weeks is an inefficient means of infection control. Any bacteria entering the uterus soon after menstruation ends would then wait three weeks before expulsion, which is long enough for the bacteria to proliferate and cause serious infections (Finn 1994). This method of defense against pathogens also leaves post-menopausal, pregnant, and lactating women unable to defend themselves as they are not menstruating. Since menstruation only occurs in the absence of implantation, and therefore in the absence of sperm, it is counterintuitive that menstruation would have evolved as a defense against bacteria brought in with sperm. The presence of sperm inhibits this "defense" mechanism while the absence of sperm, and hence bacteria, allows the "defense" (menstruation) to occur regularly (Finn 1994). Profet’s hypothesis not only initiated a barrage of criticism, it also sparked the formation of other theories explaining the evolution of menstruation.
Beverly Strassman hypothesizes that vaginal bleeding occurs as a side effect arising when there is too much blood for reabsorption into the body during endometrium regression (Strassman 1999 and Travis 1997). Shedding the endometrium, as opposed to continuously maintaining it, is hypothesized to have evolved as an energy-saving adaptation (Travis 1997). The embryo is available for implantation in only a fraction of the menstrual cycle, corresponding to the time at which the endometrium can sustain implantation. By studying endometrium oxygen consumption during buildup and regression, Strassman concludes that it requires less energy to rebuild the endometrium each cycle then to maintain it continually. Therefore, natural selection has resulted in synchronization of embryo availability and endometrium development (Strassman 1999).
Colin Finn and Peter Ellison each hypothesize that menstruation results from the invasiveness of the human blastocyst. Finn argues that the increased invasiveness of the human blastocyst through evolution led to natural selection for endometrial preparation that is independent of embryo stimulus (Finn 1994). The mother protects her uterus by undergoing changes in endometrial stroma in anticipation of pregnancy. If pregnancy does not occur, then the endometrium breaks down and bleeding occurs (Finn 1996). This differs from many mammals, where stromal changes only occur when the blastocyst is present in the uterus (Finn 1994). Ellison argues that the endometrium must terminally differentiate its cells before the invasive embryo implantation, which is required to meet oxygen and glucose demands. The endometrial cells are therefore committed to a fate before the embryo arrives and cannot be retained if implantation does not occur. This results in the shedding of the endometrium (Travis 1997). Although numerous hypotheses have been put forth on the evolution of menstruation, as of yet, none has been widely acknowledged as an acceptable theory for why humans menstruate.
While the reasons behind why women menstruate remain unclear, research shows that the number of menstrual cycles modern women experience differs greatly from the number experienced by pre-agricultural women. It is impossible to know with certainty the reproductive patterns that prevailed 10,000 years ago. However, it is likely that the reproductive patterns of Stone Age women are more closely related to those of current hunter-gatherer societies than to those of western women (Eaton and Eaton III 1999). The best opportunity to study the natural pattern of human reproduction occurs with women in current foraging societies. American women currently experience three times as many menstrual periods as women who have continued living in the ways of earlier ancestors. Foraging women are 16 years old at menarche, 19.5 year old at first birth, nurse for three to four years, have a completed family size of 5.9 live births, and an average age at menopause of 47 years. They experience a total of 160 ovulations in their lifetime. Contemporary American women are 12.5 years old at menarche, 24 years old at age of first birth, nurse for 3 months (if at all), have a completed family size of 1.8, and are 50.5 years old at menopause. American women experience approximately 450 ovulations within their lifetime (Eaton et al 1994). A study done with the Dogon women of Mali shows a similar relationship. The Dogon are a foraging society that practices natural fertility by not using modern contraceptive methods. The Dogon have a fertility rate of 8.6 ± 0.3 live births per woman. Median number of lifetime menses experienced by the Dogon was 109, with a U-shaped relationship between menstruation and age showing that, from menarche to menopause, women in primary child-bearing years (20 —34 years old) rarely menstruated (Strassman 1999). Overall, this data indicates that monthly menstruation for decades on end is not the historical norm. Today, women have earlier menarche, later first birth, and fewer pregnancies. There is also a decreased suppression of menstruation through lactation as _ of children are never breast-fed and the rest only breast-feed for 3 months. Early menarche is an especially recent development. In the 19th century, the age of first menarche was the same as in the hunter-gatherer women observed today. The earlier age of first menarche can be linked to an increase in caloric intake, while at the same time occupational, educational, and social forces have led to a later first birth (Eaton and Eaton III 1999). The consequences for these changes in menstrual cycling may be seen in cancer rates among women in industrialized nations.
The increased number of menstrual cycles experienced by western women may increase the risks of developing cancer in the reproductive organs. Breast cancer afflicts one out of eight women (Strassman 1999). Unfortunately, it is impossible to determine the breast cancer rates in ancestral hunter-gatherer societies. However, medical anthropologists estimate that it was rare. One model suggests a 100-fold increase in breast cancer rates from those in ancestral women (Eaton and Eaton III 1999). Less than 2% of current breast cancer cases are caused by heritable mutations. The other 98% of breast cancer cases are probably due to the longer time span between menses and first live birth, along with increased menstruation rates (Strassman 1999). The chances of developing breast cancer decrease with later menarche, earlier first birth, high parity, and earlier menopause. This is because the susceptibility of the breast to carcinogens is directly related to its epithelial cell proliferation rate and inversely related to its degree of tissue differentiation. After menarche, but before the first birth, epithelial cells in the intralobular terminal ducts are especially susceptible to carcinogens. Pregnancy and lactation induce differentiation of these cells into well-developed secretory lobules that have a slower proliferation rate and are more resistant to carcinogens. Increased exposure to estrogen from an increased number of menstrual cycles also elevates the risk of breast cancer as estrogen accelerates breast epithelial cell proliferation (Eaton et al 1994). By increasing the cell proliferation rate in breast epithelium, there is an increased risk of random genetic errors causing malignancy (Strassman 1999). Asian women have lower breast cancer incidence rates than western women, and have lower serum and urinary estrogen levels. Studies also suggest that there is a positive relationship between estrogen replacement therapy and breast cancer (Eaton et al 1994).
Early menarche, late menopause, and low parity all increase the number of menstrual cycles experienced and therefore increase estrogen levels, leading to increased endometrial carcinoma risks. The "estrogen excess hypothesis" was developed based on the evidence that women with endometrial cancer typically exhibit signs of high estrogen effect (ovarian stromal hyperplasia and high vaginal cornification index). Their plasma estrogen levels also tend to be higher than controls (Eaton et al 1994).
The "incessant ovulation hypothesis" may explain increased rates of ovarian cancer (Eaton et al 1994). Ovarian cancer is the fourth leading cause of cancer in women and is fatal in 80% of cases (Coutinho and Segal 1999). The hypothesis states that each ovulation mechanically injures the ovarian epithelium, while the escaping follicle exposes the tissue to locally high hormone levels. This means that the more a woman ovulates, the greater her chances are for developing ovarian cancer. Pregnancies, lactation, and oral contraceptives all reduce the total number of ovulations, and therefore decrease the chances of developing ovarian cancer. The high number of ovulations experienced by women in western societies is a departure from the number experienced by pre-agricultural women and may explain the high rates of ovarian cancer (Eaton et al 1994).
Decreasing the rates of women’s reproductive cancers may depend on using modern pharmacology along with changing eating and breast feeding habits in order to recreate the hormonal balance of hunter-gatherer women (Eaton and Eaton III 1999). Oral contraceptives could be used to decrease rates of ovarian and endometrium cancers. Millions of women currently use oral contraceptive pills, which have been shown to reduce the risk of developing ovarian and endometrium cancers. However, there is no evidence that oral contraceptives decrease breast cancer rates. Women using oral contraceptives also have a "pseudo period" each month caused by hormone withdrawal. Oral contraceptives were developed under the assumption that regular menstruation is normal. They therefore contain a weeks worth of sugar pills, which induce the "pseudo period". Given that menstruation every month is likely an unnatural state, this "pseudo period" could be eliminated (Strassman 1999). The use of a gonadotropin releasing hormone agonist (GnRHA) could be used to reduce the risks of developing breast cancer as well as endometrial and ovarian cancers (Strassman 1999). GnRHA totally, but reversibly, inhibits ovarian production of estrogen and progesterone. It would be necessary to supplement those using GnRHA with low doses of estrogen to protect against the negative effects of a lack of estrogen, such as osteoporosis. Use of GnRHA during 5 years of the reproductive period could reduce breast cancer risk by 31% and for 15 years by 70% (Eaton and Eaton III 1999). Delaying the onset of puberty could also reduce the rates of all reproductive cancers. Each year of puberty delay causes a 20% reduction in the risk of developing breast cancer (Eaton and Eaton III 1999). Administration of progesterone through existing contraceptive methods could be used to delay the onset of puberty. Another way of reducing breast cancer rates is to induce maturation of the breast epithelium through the administration of high estrogen and progesterone doses after first menstruation, which would mimic the serum levels found during pregnancy. In animal experiments, this technique has induced maturation of the breast epithelium, thereby decreasing cell proliferation and susceptibility to carcinogens (Eaton et al 1994). This does not inhibit later reproductive success or lactation ability (Eaton and Eaton III 1999)
Changing dietary habits could also decrease the rates of reproductive cancers. There is a strong correlation between fat intake and breast cancer development. Studies on lab animals strongly suggest that increased fat intake promotes breast tumor development. Women in the United States consume 40% of daily calories from fat and have five times the risk of developing breast cancer as women in Japan, who consume 19% of daily calories from fat. Paleolithic humans probably received 20 to 25% of their daily total caloric intake from fat (Eaton et al 1994). Studies suggest that reproductive cancer rates may be linked to dietary fat intake as a child. There is a clear-cut association between overall dietary fat intake and breast cancer rates within a country. However, no correlation has been found between individual fat intake and breast cancer rates in the adult women studied. This suggests that breast cancer rates may be influenced by childhood fat intake (Eaton and Eaton III 1999). Dietary fiber levels may also affect reproductive cancer rates because a high fiber diet increases intestinal reabsorption of estrogen. Non-human primates, which have almost non-existent rates of reproductive cancers, consume large amounts of fiber containing foods (Eaton et al 1994). Therefore, changing dietary habits to a low fat/high fiber diet could result in lower reproductive cancer rates. Encouraging longer periods of breast-feeding could also serve to inhibit menstruation, thereby decreasing the total number of lifetime menses.
Monthly menstruation has been accepted as a natural and normal state for women based on observations that the week of menstrual bleeding is accompanied by the relief of premenstrual discomforts (Thomas and Ellertson 2000). Most women experience low-level discomfort during their menstrual cycle, including symptoms such as bloating, fatigue, cramping, and backaches. In 30 to 40% of women, these symptoms cause significant discomfort. One in ten women suffers from endometriosis, the extremely painful condition where the uterine lining cells lost during menstruation are discharged through the fallopian tubes into the pelvic cavity where they attach to pelvic organs. The cells grow each month in response to hormonal changes in the cycle (Coutinho and Segal 1999). Viewing continuous menstrual cycling as the natural state, because it brings relief of these symptoms, is an inaccurate assessment given that throughout history women probably spent most of their time pregnant or lactating, both of which suppress menstruation. Methods that decrease the occurrence of reproductive cancers may also help relieve or eliminate the monthly discomfort and pain felt by women during menstruation.
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