“That Time” in Evolutionary History

Sidestepping the sitcom clichés, the PMS jokes and the scores of euphemisms like “surfing the crimson tide,” most women would agree that when it comes right down to it, their monthly period can be two feet south of a pain in the neck. For some women their period is a mere inconvenience; for others it’s a downright nuisance. Fortunately for them, the marvels of modern science allow today’s women to do away with menstruation almost completely. Continuous doses of contraceptives, by shot, patch or pill, can safely eliminate a woman’s cycle, not to mention the associated bloating, cramping and irritability. Moreover, studies suggest that reducing the number of lifetime periods might actually help prevent ovarian and breast cancer by sparing women from monthly hormone surges.

Nonetheless, other women celebrate what their sisters consider a “curse” as a beautiful, natural phenomenon. Google “menstruation” and you’ll find “Moon Diary” menstrual calendars, Wikipedia entries, educational websites, even a digital museum – The Museum of Menstruation and Women’s Health. But love it or hate it, there’s one question that has defied both men and women alike. No one really knows why women menstruate.

Between the irritability and mood swings, which certainly aren’t selling points for the feminine sex appeal, and the loss of anywhere from one to 55 teaspoons of blood and tissue, itself a potential advertisement to nearby predators, one would imagine that even Mother Nature would have steered natural selection in the opposite direction. So what is the advantage of monthly bleeding? To make sense of what appears to be an otherwise maladaptive trait, present-day biologists, anthropologists, veterinarians and gynecologists have turned to evolutionary theory, statistical analysis, physiology and their own clinical experience and insight, continuing what is now a 2,000-year-old investigation.

Back When Menstrual Blood Made Dogs Go Mad

As early as the second century A.D., physician and philosopher Galen put forth that menstruation rids the female of excess blood that accumulated in the body. Some 200 years later, Aristotle proposed that menstrual fluid was the inaugural matter from which babies arose. In the seventh century, St. Isidore of Seville illustrated in his “Etymologiae,” “On contact with this gore, crops do not generate, wine goes sour, trees lose their fruit, iron is corrupted by rust, copper is blackened. Should dogs eat any of it they go mad.”

The eccentric proposals seeking to explain this phenomenon were manifold: That menstruation was a means to sexually satisfy women, thus safeguarding their virginity; that menstrual fluid itself served as a confirmation of sexual potency; that it helped to detoxify the blood poisoning women were prone to; and that it was a defect, wherein menstrual fluid leaked through the weakest parts of the body.

We have since come a long way in our thinking, even in high school sex-ed: We know today that early in the menstrual cycle, the endometrium (the protective tissue that lines the uterus) thickens in response to ovarian hormones in preparation for a possible pregnancy. During this thickening, delicate blood vessels infiltrate the endometrium in anticipation of supplying a fertilized egg (or blastocyst) with oxygen and nutrients through the bloodstream. When a woman does not get pregnant, the same muscular contractions that occur during childbirth instead sever the endometrial tissue and vessels from the uterine wall. The ensuing bloodshed marks the beginning of the next menstrual cycle. Still, this only explains the how of menstrual bleeding; why it happens at all remains unanswered.

In 1993, the scholastic volley of contemporary hypotheses was borne into the limelight when biologist Margie Profet at the University of California, Berkeley, posited that menstruation is a defense against sperm-borne pathogens. Profet proposed that the specialization of uterine arteries, the low coagulability of menstrual blood and the hemorrhaging of the uterine lining promote bleeding, an adaptive design to deliver disease-killing white blood cells to the uterus and rid the body of potentially infected tissue.

Three years later, anthropologist Beverly Strassmann at the University of Michigan, Ann Arbor, systematically picked apart Profet’s antipathogen theory, debunking it altogether. Strassmann’s counter-theory suggested that shedding the endometrium every month is energetically less costly than maintaining the expensive tissues all the time; menstrual blood loss is merely a side effect. Specifically, Strassmann calculated that over the course of four months, an average woman would require an extra six days’ worth of food to maintain her endometrium continuously. Long before supermarkets and drive-thru restaurants, such economics could have weighed heavily on a woman’s survival.

Caution: Blastocyst Implantation Prep May Cause Menstruation

In 1998, former veterinarian and University of Liverpool professor Colin Finn sought to dispel the notion that menstruation is part of an adaptive design. Finn argued that the specialization of uterine tissues is a necessary step in anticipation of the blastocyst implanting. Menstruation is merely a consequence, not an advantageous evolutionary strategy, of the disposal of this tissue in preparation for the cycle to continue.

But as early as 1961, Miami gynecologist Thad H. Doggett had begun toying with an evolutionary theory, in a Journal of the Florida Medical Association article. In 1995 – the twilight of his career – Doggett published his monograph, “Menstruation: Origin and Evolutionary Significance,” a treatise on the precise mechanics and evolutionary history of menstruation.

According to Doggett, certain upper primates (the Catarrhini, which includes humans, Old World monkeys and humanoid apes) menstruate because their uterine tissue is too specialized for oxygen delivery to be reabsorbed (as it is in other mammals). In fact, he argues, menstruation arose in response to the high oxygen demands of our super-advanced central nervous system and our Mr. Peabody brains, the same grey matter that gives us poetry, hand-eye coordination, calculus, shivering, the atom bomb and the ability to walk and chew bubble gum at the same time.

All embryonic tissue requires oxygen, but neural tissue needs a constant oxygen supply or it can suffer irreparable damage.  In humans and fellow Catarrhini, the blastocyst penetrates deep into the nourishing endometrium within a week of fertilization. As early as day 21, the central nervous system begins to develop. This intimate association gives our brainy embryo a plentiful supply of oxygen earlier than any other primate, mammal or vertebrate. And, of course, we have humongous brains to show for it.

So Which Is Culprit, Blastocysts or Big Brains?

The public and scientific courts of opinion are still out on the matter. Now retired, Finn still sees Doggett’s conclusions as overly imaginative. “What I have tried to dispel is the notion that menstruation has evolved by adaptive selection to answer some specific human need,” Finn explains. “I am not happy with the idea of associating the development of the nervous system with the evolution of menstruation. I cannot see the connection. The nervous system has evolved very gradually over millions of years and as far as I can see there is no great jump between mammals – especially primates – with menstruation and those without.”

Funnily enough, in most respects, Doggett’s theory closely parallels Finn’s. They both believe women menstruate because the endometrial tissue is just too specialized to be reused. Doggett just takes it a little further, theorizing that reproductive tissues co-evolved with bigger brains throughout the mammalian line. And this relationship, he writes, “accounts for the enlargement of the brain in primates and the sudden appearance of the human family that is revealed in the fossil record.”

Strassmann, who is currently investigating the merit of what she calls the Doggett hypothesis, admits that her team “found it to be interesting and well worth testing,” but there’s no official word yet on their findings.

Until they do, the Doggett hypothesis at the very least serves as a reminder that there remains at least one mystery we’ve yet to shed light on. Period. In the meantime, we’ll just have to keep our fingers crossed that the human race can withstand another 2,000 years of mood swings. It may take that long to puzzle out what seems, at first, to be the bleeding obvious.

Regardless, Do It In Style! (AKA Shameless Plugs For Inkette Endorsed Period Gear)

We’re completely in love with all of Chronicle Book’s Vinnie’s Tampon Cases.

Vinnie’s Vinyl Retro Tampon Case. Long enough to hold your checkbook, some coins, and of course, your tampons. This case also urges you to “know your flow” with a handy period chart with all the days of the months. $11.75

Vinnie’s Mini Canvas Retro Tampon Case Toiletry Holder Bag. Wallet sized. Small enough to tag along everywhere. Price: $4.99

Vinnie’s Original Canvas Retro Tampon Bag.Take it on the go! Pencil bag sized. Price: $8.99

Upgrade your period panties from ratty to happy with this purpose-built pair from the California National Organization for Women.