In Naturalis Historia, the Roman naturalist Pliny the Elder wrote that period blood has the power to stop whirlwinds, kill bees, and drive dogs mad…

If you feel compelled to chuckle, keep in mind that nearly 2,000 years later, menstruation is still shrouded in myth, taboo, and beliefs that are nearly as bizarre as what ‘ol Pliny was peddling.

In fact, the actual science of why people have periods is almost stranger than the myths. And while periods are part of the everyday lives of over half the population, the answer to the simple question “why do they exist?” is far from being common knowledge.

Humans — along with old world primates and certain types of bats — are one of the few species on Earth that menstruate. In these species, a drop in the levels of the hormone progesterone triggers the breakdown of the inner lining of the uterus. What follows is an extraordinary process of scar-free wound healing. The uterus can break down and regenerate every month, hundreds of times in a lifetime — all without ever leaving a scar.

This incredible process is practically unheard of in adult tissues, but, as many of us know, periods can also be debilitating. So biologists have been puzzled as to why this phenomenon should have evolved. And, more specifically, why did it only evolve in humans, old world primates, and certain bats?

Recent research suggests that the answer lies in an ancient conflict. It’s in the best interests of the father (or parent providing the sperm) for the mother (or parent providing the egg) to pour as much energy and resources into pregnancy as possible. But it’s in the best interests of the mother to balance the needs of pregnancy with maintaining their own health. In humans, this conflict has left its mark on our very DNA.

We each have two complete copies of the human genome: one paternal and one maternal. Usually, the two copies are equally active. But sometimes, the father or mother modifies their copy so that certain genes are “off” — most of the genes that are modified in this way are involved in fetal growth.

Herein lies the rub.

If both copies of those parental genes were active, the fetus would grow abnormally large. So, in the copy of the genome she provides, the mother’s body gets the hell in there and shuts those genes off.

Likewise, there are certain genes that repress growth, and if both copies of those genes were on, the fetus would be abnormally small. The father goes in and shuts those genes off. The size to which the fetus will grow is ultimately determined by this genetic tug-of-war.

This conflict has also driven the evolution of a particularly gruesome kind of placenta: the hemochorial placenta.

Some types of placentas don’t invade maternal tissues at all. But the hemochorial placenta — which all menstruating species have — burrows through the walls of the uterus and hooks into the mother’s bloodstream. Once the invasion is complete, the placenta can control the mother’s entire body by releasing hormones into her blood. So it was necessary for people with wombs to develop a defense system to mediate placental invasion.

This was particularly crucial because of another strange quirk of human biology: Human embryos are 10 times more likely than other mammals to carry an abnormal number of chromosomes. Some variation in chromosome number can be tolerated: For instance, three copies of chromosome 21 in Down Syndrome or only one copy of the X chromosome in Turner Syndrome. But in general, embryos with large parts of their genomes missing or duplicated will not be able to develop.

Why We Must Stop Calling Menstruation A ‘Women’s Issue’
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Because so many human embryos are actually inviable, our species has an incredibly high rate of miscarriage. Fifteen percent of clinically recognized pregnancies end in miscarriage, but counting pregnancies that aren’t clinically recognized, that number is actually closer to 50%. Unfortunately, society tends to tell pregnant people that something is wrong with them if they experience miscarriage, when really, a high chance of miscarriage is just part of being human.

Periods result from an adaptation to those two things: the invasive, hemochorial placenta and the prevalence of chromosomal abnormalities. One thing that humans, old world primates, and menstruating bats all have in common is a phenomenon called “spontaneous decidualization.”

Decidualization is the remodeling of the uterine lining that occurs in preparation for pregnancy. In other animals, decidualization is triggered by the presence of an embryo, but menstruating species have taken control of that process. In these species, decidualization occurs cyclically, whether there is an embryo or not.

A high chance of miscarriage is just part of being human.
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So why is this a good thing? Here’s why. Many people believe that the uterine lining thickens and changes in order to provide a hospitable environment for an embryo, but that’s not the whole story. Once the cells of the uterine lining turn into decidual cells, they gain the ability to sense whether or not an embryo is developing normally, even before it implants. If it is, the cells permit the invasion of the placenta. But if it’s not, the cells quickly self-destruct. By making the transformation into decidual cells occur cyclically, our species has made sure it’s prepared to defend against potentially harmful pregnancies before the embryo implants.

Some biologists argue that periods are just the coincidental result of combining spontaneous decidualization with cycling hormone levels. Once cells are decidualized, a drop in progesterone will trigger them to self-destruct. And, so long as pregnancy doesn’t occur, progesterone levels rise and fall each month.

It makes sense that periods might not have intrinsic benefit, because in the natural state, periods are pretty rare. Our ancestors, who were more frequently pregnant, only had about 40 periods in a lifetime. But some biologists think that menstruation does have intrinsic benefit. Because the uterus is able to repeatedly break down and rebuild itself, it may be able to learn from previous reproductive events and adapt. If this is true, it might explain why most pregnant people who experience miscarriage eventually go on to successfully conceive.

Menstruation is, in many ways, a potent symbol of the socio-cultural stronghold the patriarchy still wields. In a society that has become increasingly inured to the ubiquity of sex and violence, periods remain largely unmentionable, further complicating our long history with the uterine lining. But I tend to believe that — although he may have been oh-so-wrong in the specifics — Pliny the Elder was damn right when he wrote “over and above all this there is no limit to woman’s power.”