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= Human health (Menstruation): the evolutionary explanations and evolution of menstruation =

Menstruation and its origins
Menstruation is a physiological process that involves intricate interactions between vascular and inflammatory processes to stabilize the endometrium and enable a controlled loss of endometrial blood and tissues. Although there is some disagreement in definitions between sources, menstruation is generally considered to be limited to primates. Overt menstruation (where there is bleeding from the uterus through the vagina) is found primarily in humans and close relatives such as chimpanzees. It is common in simians (Old World monkeys, New World monkeys, and apes), but completely lacking in strepsirrhine primates, and possibly weakly present in tarsiers. Beyond primates, it is known only in bats, the elephant shrew, and the spiny mouse species Acomys cahirinus.

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The evolutionary origins of menstruation
The evolutionary origins of menstruation have been based on two theories: the capacity to get rid of pathogens brought into the uterus by spermatozoa and the relative energy conservation of menstruation compared to its absence.

Why do mammals not menstruate as opposed to humans?
In an evolutionary perspective, menstruation is an ancestral feature of mammals that many populations have abandoned, but not ours. The uterus doesn't become colonized by sperm-borne pathogens and doesn't become ill as a result of menstruation. Maintaining a distinct endometrium is impossible in other animals. In some mammals with a placenta, a fertilized embryo attaches to the endometrium only superficially. In humans and other menstruating species, implantation is deep and invasive, requiring an especially complex lining to develop in preparation for implantation.While other mammals are able to reabsorb the lining that adorns their fertile wombs, the volume of tissue in humans is expelled instead.

Most female placental mammals have a uterine lining that builds up when the animal begins ovulation and further increases in thickness and blood flow after a fertilized egg has successfully implanted. The human uterine lining is thickened during each cycle as a defense against trophoblast penetrating the endometrial wall, regardless of whether an egg becomes fertilized or successfully implants in the uterus. This produces more unneeded material per cycle than in non-menstruating mammals, which may explain why the extra material is not simply reabsorbed as it is in those species. In essence, menstruating animals treat every estrous cycle as a possible pregnancy by thickening the protective layer around the endometrial wall, while non-menstruating placental mammals do not begin the pregnancy process until a fertilized egg has implanted in the uterine wall.

Varying views on the evolution of overt menstruation in humans and related species
There are varying views on the evolution of overt menstruation in humans and related species and the evolutionary advantages of losing blood associated with dismantling the uterine lining rather than absorbing it, as most mammals do. The reason is likely related to differences in the ovulation process.]It is speculated that menstruation is not a trait that provides any evolutionary advantage. Instead, it is a side effect of spontaneous decidualization, which evolved in some placental mammals due to its advantages over non-spontaneous decidualization Spontaneous decidualization allows for more maternal control in the maternal-fetal conflict by increasing selectivity over the implanted embryo. This may be necessary in humans and other primates, due to the abnormally large number of genetic disorders in these species. Since most aneuploidy events result in stillbirth or miscarriage, there is an evolutionary advantage to ending the pregnancy early, rather than nurturing a fetus that will later miscarry. There is evidence to show that some abnormalities in the developing embryo can be detected by cells in the uterus. This triggers epigenetic changes that prevent the formation of the placenta, which prevents the embryo from implanting and leaves it to be removed in the next menstruation. This failsafe mode is not possible in species where decidualization is controlled by hormonal triggers from the embryo. This is sometimes referred to as the "choosy uterus theory," and it is theorized that this positive outweighs the negative impacts of menstruation in species with high aneuploidy rates and hence a high number of "doomed" embryos.

Other menstrual theories:
Other theories on the evolution of menstruation suggest that shedding or reabsorbing the endometrial lining is energetically advantageous to the female. The advantage of shedding over re-absorption may be that sperm-borne pathogens are removed from the uterus. A more parsimonious explanation, however, is that the endometrium in primates has developed into too large of a structure to be completely reabsorbed by the uterus wall. According to theories, the main reason for either reabsorbing or shedding the endometrial lining is to save energy. It has been calculated that, when implantation fails, a cyclical regression and renewal of the endometrium are energetically less costly than maintaining it in the metabolically active state required for implantation. In the regressed state, oxygen consumption in human endometrial cells declines nearly sevenfold. The metabolic rate is at least 7% lower, on average, during the follicular phase than during the luteal phase in women, which signifies an estimated energy savings of 53 MJ over four cycles, or nearly six days’ worth of food.

Why do human females shed the endometrial lining instead of resorbing it?
It has also been hypothesized that shedding the endometrium may be an effective way to get rid of sperm-based pathogens. The accompanying bleeding, Profet hypothesizes, "delivers immune cells into the uterine cavity that can combat pathogens." Another hypothesis is that the endometrial microvasculature is designed to provide the blood supply to the endometrium and the placenta, and that external bleeding appears to be a side effect of endometrial regression that arises when there is too much blood and other tissue for complete reabsorption. The relatively large blood loss as seen in humans and chimpanzees can be attributed to the large size of the uterus relative to adult female size and to the design of the microvasculature in the uterus wall.



Decidualization
There is no definite answer as to why menstruation evolved in women, but the main theory that I would agree with the most is Emeras' argument that menstruation is an  inevitable consequence of spontaneous decidualization. Decidualization is the development of a stromal reaction in the endometrium of the uterus that resembles an inflammatory tissue reaction and occurs during the luteal phase. If fertilization fails, progesterone drops and the stromal tissue is shed with bleeding, i.e., menstruation. Therefore, menstruation is a result of endometrial preparation evolving to become independent of any action of the embryo. Menstruation may have co-evolved as a protective mechanism against maternal-fetal rejection.