A blow of blood

Menstruation is the correct term for the strange phenomenon that women experience in a cycle of about once a month, in which they service the wall of the uterus that comes out as bleeding through the vagina. This phenomenon is not obvious from an evolutionary point of view.

Menstruation is the correct term for the strange phenomenon that women experience in a cycle of about once a month, in which they service the wall of the uterus that comes out as bleeding through the vagina. This phenomenon is not obvious from an evolutionary point of view. There is a terrible waste in it - a considerable amount of blood and tissue is thrown away in this process. If we add to this the reality in which the ancient humans lived when the menstrual cycle developed - a world full of predators and diseases, the thought of dripping blood and filling a delicate opening of the body with dead tissue once a month seems disastrous. Every woman, and every man who lives with a woman, can testify to how uncomfortable, limiting and debilitating this process is. So why, evolution, why?

One of the common misconceptions is that the menstrual cycle (plus or minus) and menstruation are simply the way the mammalian reproductive system works. But anyone who has raised a representative of the mammal department in their life (hamster, dog, cat) knows that this is not true. In most mammals, the uterine wall begins to thicken in honor of a fetus only after fertilization has occurred and it is the fetus that gives the signal for the beginning of the construction of the uterine wall. This method seems to be much more effective than having a menstrual cycle. In fact, the only mammals that have the mechanism of menstruation are most primates, some species of bats, and the elephant tusker. According to the phylogenetic distribution it can be seen that the trait does not derive from a common ancestor but developed in each group separately. The primates (and us) acquired the trait relatively late in their evolutionary development.

A phylogenetic tree showing the distribution of the trait of having menstruation in placental mammals and the genetic lines to which it belongs. Species/phylogenetic lines that maintain menstruation are colored in pink, species/phylogenetic lines that do not maintain menstruation are colored in black. The cases where the situation is unknown are not hypocritical. The phenomenon of menstruation appears to have appeared independently three separate times in placental mammals. Opossum (Monodelphis), the rightmost group, is an external group for comparison. The species containing the meniscal phenomenon split to the left at the base of the phylogenetic tree. The first group that diverges includes the elephant hornet (Elephantulus myurus). The second bifurcating branch includes the bats and the large group painted in pink on the left side of the figure includes the primates with the leftmost being the human (Homo sapiens).

So why have a period? Throughout scientific history, several attempts have been made, more successful and less successful, to answer the question. One version claims that it is a way to expel pathogens (disease agents) that have entered the female reproductive system along with the male seminal fluid. To explain this there are several failures, the first is that it is not so effective to get rid of pathogens only once a month, not to mention a pregnancy situation in which the pathogens remain with the fetus until birth since menstruation does not occur in the case of fertilization. But another fallacy that can be examined scientifically is that there was and the explanation is correct, and the purpose of menstruation is to get rid of pathogens, we would expect more frequent menstruation in very sexually active women and the gradual disappearance of menstruation in nuns, for example. this is not happening. Additionally, we would expect to see menstruation in most biological species since all females are exposed to the same pathogens that come with the semen. This explanation was probably a response to the mannotoxin idea that during menstruation women release toxins that harm plants, an idea that reinforced the whole issue of impurity during menstruation and was, of course, proven to be false.
Another explanation holds that it is more effective to get rid of unused uterine wall tissue than to preserve it for a long time. This sounds logical until you remember what happens in the other placental mammals that do not menstruate. They also do not preserve the uterine wall for a long time, they do not build it at all until there is fertilization. This is a much more effective regulatory mechanism. A final explanation is that humans have relatively aggressive embryos that penetrate deep into the mother's tissue and the uterus prepares the uterus for such stress. But this does not constitute proof that menstruation strengthens the "stiffness" of the uterus in general.

A study published on November 7, 2011 in the journal Bioessays by researchers Emera, Romero and Wagner offers a new idea. They claim that the phenomenon that requires an explanation is the thickening of the uterine wall every month and not the secretion of this wall.

All mammals prepare a special tissue designed to receive the embryo(s), the difference is that most mammals prepare this tissue only after the embryo itself has given the signal for its creation, while most primates have a spontaneous tissue construction that occurs even without the existence of an embryo. In mice, for example, thickening of the uterine wall can be induced by mechanical stimulation of scratching the uterine wall. The mice will enter a simulated pregnancy, the wall of the uterus will thicken, and it will be secreted only after the levels of progesterone, the hormone that maintains the pregnancy, have decreased. So it's not that mice don't have the ability to get rid of a thick, unused uterine wall, they just don't build it if it's not going to be used.

So why is the process spontaneous in humans?

The answer offered by the researchers is completely evolutionary and includes a conflict between the mother and the fetus. The mother and the fetus are in a relationship of conflict: the mother's interest is to survive the pregnancy in the best possible way so that she can get pregnant again, and thus her body tries to conserve resources for the long term. The fetus, on the other hand, benefits from taking as much as it can from its mother, sometimes to the point of harming the mother. For example, the fetus affects the mother's hormone levels to weaken the response to insulin, so that less sugar is taken up by the mother's cells, and more sugar remains available for the fetus.

Within mammals there is variation in the degree to which the placenta formed by the fetus penetrates the wall of the mother's uterus. In some species the contact is completely superficial (epithelochorial) and in others the fetus is particularly penetrating and penetrates the blood vessels of the mother (hemochorial). The fetus of humans is more invasive and so are the fetuses of the other mammals that menstruate.

This is a hint. It is possible that menstruation is actually a means of self-defense. The women build a thick and rich uterine wall to create a layer of isolation and protection between them and the greedy fetus and its selfish placenta. In species where the embryo is penetrative, waiting for fertilization will be too late, the uterine wall must be built as a preventive measure before the chance of fertilization. So, if there is no fertilization, the universal reaction of falling progesterone levels occurs and the lining of the uterus is shed.

Bonus! Another process that takes place is that the layer of the uterine wall is a detector of the quality of the fetus, identifies chromosomal abnormalities and enables spontaneous abortion at an early stage.

The authors of the article also have a suggestion for how this mechanism could have developed: there was environmental pressure, created by the embryo, which led to the construction of a thick uterine wall in response to the implantation of the embryo. Later this feature was strengthened by additional mutations. According to this explanation it is possible to predict that in species in which spontaneous uterine wall construction does not occur, the fetal chain will lead to an increase in cAMP (a component of intracellular signaling) in the uterine wall which will contribute to its thickening. The researchers claim that in mammals with spontaneous uterine construction, the hormonal activation has reached its maximum value. That is, if these models are correct, we would expect to find an increase in the amount of cAMP-responsive factors in response to progesterone in menstruating species, but not in non-menstruating species such as mice. This experiment still remains to be performed.

The results of such experiments may shed light on the transition from induced to spontaneous uterine formation and thus answer questions concerning the evolutionary advantages of menstruation. Identifying and diagnosing the normal state will help find the genes that lead to the abnormal state. Such information may help solve problems of the reproductive system that include abnormal functioning of the uterine wall in response to the processes of the menstrual cycle and pregnancy such as growth of uterine tissue outside the uterus itself (endometriosis), uterine cancer, preeclampsia and repeated losses of pregnancies.

Sources:
Excellent blog PHARYNGULA by PZ Myers. Highly recommended for all lovers of evolution and development.

The excellent podcast Skeptically speaking (Chapter 149) which is recommended to anyone who likes to have misconceptions and common beliefs related to science shattered.

And of course, the article itself:
Emera D, Romero R, Wagner G (2011) The evolution of menstruation: A new model for genetic assimilation: Explaining molecular origins of maternal responses to fetal invasiveness. Bioessays 34(1):26-35

More on the blog on similar topics:

why sex? On the development of sexual reproduction

William Hervey and the discovery of the blood circulation