Breaking Taboo: Understanding the Evolution of Menstruation

There are currently about two billion women on the planet who menstruate every month. Despite this being a normal biological process, menstruation is still often considered taboo. Although the hormonal processes leading up to menstruation are well studied and understood, few researchers have considered the evolutionary question of how “the period” came to be.

Theories on the evolution of menstruation range from its acting as protection from male pathogens to its flushing out old sperm before the next ovulation. Another theory put forward by evolutionary biologists is based on the concept of a “choosy uterus” that selectively accepts only high quality embryos at the time of implantation, and eliminates any defective embryos via menstruation.

However, according to Vernon Thomas, professor emeritus in the Department of Integrative Biology, the ability of the uterus to screen for defective embryos appears limited at best. He suggests that screening for defective embryos is unlikely to be enough to drive the evolution of menstruation because it’s associated with failed reproduction.

Thomas is instead proposing a different theory altogether: that menstruation is tied to the same vital process that allows the placenta to be safely delivered after giving birth without causing maternal hemorrhage. 

The developing fetus is reliant on the mother for oxygen and nutrition. To provide these vital resources, a full quarter of a mother’s blood supply is diverted to the developing child through the placenta. When the fetus is ready to be born, the mother’s body needs to turn off this massive blood flow. This is especially critical in humans and other Old World primates because the placenta erodes the walls of the maternal blood vessels, and makes direct, intimate, contact with the maternal blood.  Shutting off this blood flow is no simple task, however.  Prior to delivery, the amount of the hormone progesterone produced by the placenta begins to drop, ultimately arresting blood flow to the placenta by constricting the uterine blood vessels.  This allows the child and placenta to be delivered without the mother experiencing a potentially fatal blood loss.

In a non-pregnant woman, progesterone is also produced by the ovaries and helps increase blood flow to the uterus in preparation for pregnancy. But, asks Thomas, “What if pregnancy does not occur?  How do you shut off the blood supply?”

Again, progesterone levels play a critical role. If an egg is not fertilized and implanted in the uterus, the amount of progesterone drops, and constriction of the uterine blood vessels occurs. “But instead of giving birth,” says Thomas, “this process leads to a cessation of blood flow and the nearly monthly shedding of the lining of the uterus.”

While this monthly shedding is often viewed as inconvenient, it an important by-product of the potentially life-saving sensitivity of their uterine blood vessels to progesterone withdrawal.

Interestingly, the need to regularly “shut off” the uterine blood supply may have emerged relatively recently.

“Historically, women were unlikely to menstruate frequently. Instead, early hominids would cycle between pregnancy and lactation,” explains Thomas. “Menstruation is really an artifact of human cultural evolution.” 

As societies became more organized, females experienced more and better nutrition, and lived longer. Then reproduction became less frequent and menstruation became the norm – a switch that happens even today when Old World primates are moved to captivity.

Having put forth this alternate view on the evolution of menstruation, Thomas emphasizes that further investigation is required to support it. He suggests that researchers could test if blood flow is shut off in the same way prior to both menstruation and placental shedding – but this might ultimately be difficult to test as only a very small number of species menstruate, and none to the full extent that humans do.  That means that for now, says Thomas, the occurrence of menstruation in humans “remains a testament to the evolution of successful pregnancy.”

Read the full study in the journal BioEssays.

Read about other CBS Research Highlights.