New Theory: Women Menstruate To Protect Ourselves From Babies
A Series of Fallopian Tubes
Why humans (and most primates, some bats, and elephant shrews) menstruate is a question that many have attempted to answer in some way… some explanations more convincing than others. The downsides to menstruation (the process of shedding the endometrium of the uterus that was built up in anticipation of the possibility of the implantation of a fertilized egg); “throwing away a substantial amount of blood and tissue,” “leaving a blood trail or filling a delicate orifice with dying tissue” in a “world full of predators and disease,” not to mention the menstrual cycle’s “uncomfortable, awkward, and sometimes debilitating” symptoms; seem to necessitate a strong explanation for the evolution of the feature. Not only is it hard to find a theory that explains why just a few mammals find it more evolutionarily advantageous to menstruate, it’s difficult to come up with a theory that explains why all those other mammals haven’t found it more efficient as well.
Says PZ Meyers:
There are many explanations floating around. One is that it’s a way to flush out nasty pathogens injected into the reproductive tract by ejaculating males — but that phenomenon is ubiquitous, so you have to wonder why only a few species bother. Another explanation is that it’s more efficient to get rid of the endometrium when not using it, than to maintain it indefinitely; but this is a false distinction, because other mammals don’t maintain the endometrium, they just build it up in response to fertilization. And finally, another reason is that humans have rather agressive embryos that implant deeply and intimately with the mother’s tissues, and menstruation “preconditions” the uterine lining to cope with the stress. There is, unfortunately, no evidence that menstruation provides any boost to the ‘toughness’ of the uterus at all.
However, new research has a variation on that last explanation that Meyers finds quite plausible. In a nutshell, humans, other primates, bats, and elephant shrews menstruate in order to protect the mother from the particularly intense demands on her system from the growing fetus akin to those species.
The mother and fetus have an adversarial relationship: mom’s best interest is to survive pregnancy to bear children again, and so her body tries to conserve resources for the long haul. The fetus, on the other hand, benefits from wresting as much from mom as it can, sometimes to the mother’s detriment. The fetus, for instance, manipulates the mother’s hormones to weaken the insulin response, so less sugar is taken up by mom’s cells, making more available for the fetus.
Within the mammals, there is variation in how deeply the fetus sinks its placental teeth into the uterus… The most invasive are hemochorial, and actually breach maternal blood vessels. Humans are hemochorial. All of the mammalian species that menstruate are also hemochorial.
That’s a hint. Menstruation is a consequence of self-defense. Females build up that thickened uterine lining to protect and insulate themselves from the greedy embryo and its selfish placenta.
All mammal species require tissue for the fertilized egg to implant in, but the thickening of uterine walls in most mammals is triggered by the fertilized embryo, like Bilbo Baggins getting an unexpected party from some very rowdy dwarves. In species where the embryo/mother connection is particularly invasive and has a large impact on the mother’s health, the theory states that is these species have found it evolutionarily advantageous to thicken the endometrium preemptively on a regular basis (much like a wiser Bilbo Baggins getting everything ready for his long awaited 111th birthday party). And like any party, if no one shows up, all the decorations you put up can’t just stay on the uterine walls.
Researchers Emera D, Romero R, and Wagner G have even proposed an explanation for exactly how menstruation would have evolved according to their theory of why it is actually advantageous, which Meyers elaborates on. It certainly seems like a great argument to support the safety of birth control options that actually eliminate menstruation to me.
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