Not that this makes Menstruation any more fun to deal with, but as always, now that I know the scientific reason behind the function, I feel better about this monthly ordeal. From Pharyngula:
Menstruation is a peculiar phenomenon that women go through on a roughly monthly cycle, and it’s not immediately obvious from an evolutionary standpoint why they do it. It’s wasteful — they are throwing away a substantial amount of blood and tissue. It seems hazardous; ancestrally, in a world full of predators and disease, leaving a blood trail or filling a delicate orifice with dying tissue seems like a bad idea. And as many women can tell you, it’s uncomfortable, awkward, and sometimes debilitating. So why, evolution, why?
A new paper by Emera, Romero, and Wagner suggests an interesting new idea. They turn the question around: menstruation isn’t the phenomenon to be explained, decidualization, the production of a thickened endometrial lining, is the key process.
All mammals prepare a specialized membrane for embryo implantation, the difference is that most mammals exhibit triggered decidualization, where the fertilized embryo itself instigates the thickening, while most primates havespontaneous decidualization (SD), which occurs even in the absence of a fertilized embryo. You can, for instance, induce menstruation in mice. By scratching the mouse endometrium, they will go through a pseudopregnancy and build up a thickened endometrial lining that will be shed when progesterone levels drop. So the reason mice don’t menstruate isn’t that they lack a mechanism for shedding the endometrial lining…it’s that they don’t build it up in the first place unless they’re actually going to use it.
So the question is, why do humans have spontaneous decidualization? The answer that Emera suggests is entirely evolutionary, and involves maternal-fetal conflict.
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