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  • Humans have been getting their periods since, well, forever.

  • So you might think that's just something that comes along with being a mammal.

  • Butit's not. Few mammals get periods every month, or at all.

  • Like, that bleeding thing that female dogs do? It's not menstruation.

  • It comes from the vagina, not the uterus, and happens before ovulation, rather than after.

  • As far as true menstruators go, there are only a few non-human primates,

  • some species of bats, elephant shrews, and an African mouse. That's it.

  • Which just seems cosmically unfair. And it begs the question: why?

  • After all, you'd think bleeding once a month would be a really good way to attract predators,

  • not to mention a waste of perfectly good reproductive tissue.

  • Well, scientists have had several ideas over the centuries.

  • Some of which have been less great than others.

  • But they seem to finally be getting somewhere when it comes to understanding

  • what menstruation really is and why it happens.

  • And that's not only great because it adds to our understanding of our bodies,

  • it could also help doctors treat conditions that stem from the reproductive system.

  • Now, part of the reason menstruation is so poorly understood is very human.

  • It's that people throughout history have been quite squeamish about periods.

  • Even talking aboutthat time of the month,”

  • like today, in the 21st century, is considered taboo in some circles.

  • So we still use euphemisms around the world to describe

  • the fact that half of all people between puberty and middle age

  • have blood and other fluids flow from their vaginas for a few days each month.

  • But we here at SciShow are not too squeamish to talk about, well, most things.

  • So we are going to talk aboutthat time of the month”,

  • Aunt Flo, the crimson tide, shark week, whatever you want to call menstruation.

  • Blood. Folks. There's blood. And also some other stuff.

  • And cramps. And weird poops. So thanks for that, nature.

  • In humans, and those other animals we mentioned,

  • menstruation is a month-long cycle regulated by hormones.

  • Technically, it starts with the bloody part.

  • The lining of the uterus, or the endometrium, is shed and expelled,

  • so a mixture of blood, endometrial cells, mucus, and other tissue

  • flows out of the vagina for three to seven days.

  • Over this period, hormones slowly trigger the ovaries to produce around

  • five to twenty tiny sacs called follicles, each of which contains an immature egg cell.

  • And after a few days of maturing,

  • just one or two of those follicles grows in diameter and becomes dominant.

  • It gets ready to release a mature egg cell, while the rest disintegrate.

  • At that point, a sudden surge in luteinizing hormone prompts the dominant follicle

  • to release its now-mature egg so it can begin its journey down the fallopian tube.

  • So, great, you've got an egg!

  • Next, the body creates a nice place for it to land.

  • In this part of the process, the spent follicle

  • turns into a tiny hormonal powerhouse called the corpus luteum,

  • which releases hormones that act on the lining of the uterus.

  • They tell it to differentiate into the types of tissues needed

  • for a fertilized embryo to implant and grow; a process called decidualization.

  • In non-menstruating mammals, this part of the cycle only happens

  • when a fertilized embryo attaches to the wall of the uterus.

  • But in humans, that process happens every month,

  • and it starts before the egg is even fertilized.

  • Which is why scientists call what we do spontaneous decidualization.

  • And now, many think that's ultimately why we menstruate.

  • See, after all that hard work, if the egg doesn't get fertilized and implant,

  • the corpus luteum shrinks and dissolves.

  • And since the corpus luteum stops producing

  • the hormones that maintain that thick, decidualized uterine lining,

  • the uterus sheds the extra bits, kicking off another crimson tide.

  • Before we knew all this, of course,

  • people had some interesting ideas about the menstruating body.

  • And byinteresting,” we meanpretty preposterous mythology

  • at the expense of, well, half of humanity.

  • For example, scholars in ancient Rome thought that contact with menstrual blood

  • could turn crops barren, kill hives of bees, and drive dogs crazy.

  • Even when people who considered themselves scientists

  • first started studying menstruation in earnest, they did a pretty lousy job of it.

  • Like, in 1920, a doctor coined the term menotoxin

  • to denote the supposedly toxic substance in menstruating people's sweat.

  • He, because, of course it was a he,

  • observed that flowers wilted when handled by a menstruating nurse,

  • so clearly, she was emitting some terrible stuff.

  • That led to the actual hypothesis from scientists that that's why people menstruated:

  • to get rid of awful toxins that had built up inside them.

  • Research into menotoxin continued for decades.

  • Exceptthere's no such thing as menotoxin.

  • Scholars today think that the whole idea came about

  • and persisted simply because of ingrained misogynistic biases.

  • Still, this whole idea that menstruation was related to something toxic

  • could be one reason why the first sort-of plausible,

  • but ultimately-debunked, hypothesis was that menstruation evolved to flush out pathogens.

  • Though, this time, they were thought to be sperm-borne pathogens.

  • The idea, proposed in a 1993 paper, hinged on the fact that

  • bacteria from the genital tract are known to cling to the tails of sperm cells.

  • And if sperms are squirmy little pathogen carriers,

  • then menstruation could have evolved to

  • dislodge those hitchhikers and flush them out of the uterus.

  • But, sperm-borne pathogens are a thing in species that don't menstruate, too.

  • And in 1996, a scientist analyzed all of the data they could find

  • regarding pathogens in the uterus and periods, and found that

  • neither the number nor abundance of them decreased following menstruation.

  • So that kind of put a nail in that coffin,

  • and the entire notion that menstruation has anything to do withcleaningthe uterus.

  • Instead, that scientist proposed that monthly menstruation evolved for a much simpler reason:

  • because it's more energy and resource-efficient.

  • The idea here was that we know the uterine lining

  • has to be somewhat ready before implantation can happen.

  • And species could keep their linings in a perpetual state of readiness,

  • and therefore be ready to get pregnant whenever. Woo hoo!

  • The problem is, maintaining that lining all the time would take a lot of energy.

  • It's actually less costly, from an energy standpoint,

  • to grow a new, implant-ready lining each month.

  • So, we evolved to do that instead.

  • That does sound logical. But it's ultimately not a great explanation for menstruation,

  • mostly because no species of mammal we know of has a constantly-ready uterus.

  • As far as we can tell, every non-egg-laying mammal that has ever lived

  • has had a menstrual-like cycle where the lining builds and recedes.

  • Others are just way better at absorbing the extra tissue they make.

  • Presumably because, without decidualization, they don't make as much of it.

  • So, that brings us back to the big question:

  • Why do humans have periods?

  • Why did we go from decidualizing when an egg implants to doing it spontaneously?

  • The current hypothesis is this:

  • that menstruation is a byproduct of the evolutionary struggle between the fetus and its host.

  • And yes, the wordhostkind of makes it sound like fetuses are little parasites

  • because they kind of are.

  • They're a somewhat genetically-distinct organism that's dependent on another,

  • and whose wants and needs might not line up perfectly

  • with those of the parent they're inside of.

  • And depending on the species, fetuses vary in just how parasitic they are.

  • There are around four thousand mammals, including us,

  • that deliver nourishment to their young before birth through a placenta,

  • an organ attached to the wall of the uterus.

  • All placentas are temporary.

  • They grow at the beginning of pregnancy, and are expelled immediately following birth.

  • But that's pretty much where the similarities end.

  • More to the point, there's a lot of variability when it comes to the invasiveness of a placenta,

  • or how deeply it attaches itself to the uterine wall.

  • A horse or pig's placenta, for example,

  • only superficially squishes up against the uterine wall.

  • Dog and cat placentas go a bit deeper,

  • but still don't make direct contact with the maternal bloodstream.

  • But lucky us, we have what's called a hemochorial placentation.

  • It's the most invasive kind of placenta, where the fetal tissue

  • burrows through and actually erodes some of the maternal endometrial tissue

  • to make direct contact with the host's bloodstream.

  • And scientists think that spontaneous decidualization

  • could have evolved to mediate this relationship.

  • There are two main hypotheses for how that would work.

  • The first is that, since spontaneous decidualization

  • is driven by the host's hormones instead of signals from the fetus,

  • it could help the host keep a bit of control over their resources.

  • If the uterine lining is wholly responsive to the fetus,

  • there's not much standing in the way of that little parasite becoming too attached,

  • taking too much from the host, or destroying too much of the host's tissues.

  • And that's especially important if you have an already-invasive placenta.

  • The second hypothesis is that instead,

  • or in addition to preventing the fetus from exploiting its parent,

  • decidualization helps people deliver more healthy babies.

  • See, researchers know that human embryos have particularly high rates of aneuploidy,

  • which is when there are too few or too many copies of any specific chromosome.

  • Aneuploidies don't often keep an embryo from implanting and growing into a full-term baby,

  • but most of them would prevent that baby from surviving.

  • It therefore makes sense for the uterus to have

  • some way of detecting that kind of genetic red flag early on,

  • so it can terminate the pregnancy earlier and save resources.

  • At least, from a purely evolutionary perspective.

  • Miscarriages can be so hard, no matter what biology says.

  • But ultimately, spontaneous decidualization may help make those calls.

  • This is what's known as the choosy uterus hypothesis.

  • And there does seem to be some evidence for it.

  • Like, research suggests that shortly after they burrow into the uterine wall,

  • embryos with lethal aneuploidies are more metabolically active than healthy ones.

  • Probably since they're doing their best to stay alive

  • despite having a lethal number of chromosomes.

  • It's thought that there's something special about the decidualized cells

  • in the outer layers of the endometrium that allows them to sense this overactivity.

  • And when they do, they basically slough off, leading to early miscarriage.

  • The remaining piece of the puzzle is understanding exactly how

  • the endometrium being decidualized prior to implantation makes a difference

  • when it comes to choosing embryos.

  • And if we figure that out, it wouldn't just explain why we menstruate,

  • it could help treat many cases of infertility and recurrent pregnancy loss.

  • In the end though, there's more work to be done, scientists are somewhat confident

  • that our monthly crimson tide is a side-effect of spontaneous decidualization.

  • And that, in turn, evolved because of the delicate evolutionary balance between

  • being able to get and stay pregnant and not get totally ransacked by the fetus.

  • Now, they want to understand how spontaneous decidualization actually evolved.

  • That should help quench our curiosity about

  • why the heck half of us get a monthly visit from Aunt Flo.

  • Nobody really likes her, we just tolerate her because we have to, am I right?

  • And it should also help us better understand how the reproductive system works in general,

  • so we can find ways to help people whose uteruses and ovaries

  • don't work as well as they'd like or make them sick.

  • Thanks for watching this episode of SciShow!

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