A handful of species on Earth share a seemingly mysterious trait: a menstrual cycle. We're one of the select few. Monkeys, apes, bats, humans, and possibly elephant shrews are the only mammals on Earth that menstruate. We also do it more than any other animal, even though its a waste of nutrients and can be a physical inconvenience. So where's the sense in this uncommon biological process? The answer begins with pregnancy. During this process, the body's resources are cleverly used to shape a suitable environment for a fetus, creating an internal haven for a mother to nurture her growing child. In this respect, pregnancy is awe-inspiring, but that's only half the story. The other half reveals that pregnancy places a mother and her child at odds. As for all living creatures, the human body evolved to promote the spread of its genes. For the mother, that means she should try to provide equally for all her offspring. But a mother and her fetus don't share exactly the same genes. The fetus inherits genes from its father, as well, and those genes can promote their own survival by extracting more than their fair share of resources from the mother. This evolutionary conflict of interests places a woman and her unborn child in a biological tug-of-war that plays out inside the womb. One factor contributing to this internal tussle is the placenta, the fetal organ that connects to the mother's blood supply and nourishes the fetus while it grows. In most mammals, the placenta is confined behind a barrier of maternal cells. This barrier lets the mother control the supply of nutrients to the fetus. But in humans and a few other species, the placenta actually penetrates right into the mother's circulatory system to directly access her blood stream. Through its placenta, the fetus pumps the mother's arteries with hormones that keep them open to provide a permanent flow of nutrient-rich blood. A fetus with such unrestricted access can manufacture hormones to increase the mother's blood sugar, dilate her arteries, and inflate her blood pressure. Most mammal mothers can expel or reabsorb embryos if required, but in humans, once the fetus is connected to the blood supply, severing that connection can result in hemorrhage. If the fetus develops poorly or dies, the mother's health is endangered. As it grows, a fetus's ongoing need for resources can cause intense fatigue, high blood pressure, and conditions like diabetes and preeclampsia. Because of these risks, pregnancy is always a huge, and sometimes dangerous, investment. So it makes sense that the body should screen embryos carefully to find out which ones are worth the challenge. This is where menstruation fits in. Pregnancy starts with a process called implantation, where the embryo embeds itself in the endometrium that lines the uterus. The endometrium evolved to make implantation difficult so that only the healthy embryos could survive. But in doing so, it also selected for the most vigorously invasive embryos, creating an evolutionary feedback loop. The embryo engages in a complex, exquisitely timed hormonal dialogue that transforms the endometrium to allow implantation. What happens when an embryo fails the test? It might still manage to attach, or even get partly through the endometrium. As it slowly dies, it could leave its mother vulnerable to infection, and all the time, it may be emitting hormonal signals that disrupt her tissues. The body avoids this problem by simply removing every possible risk. Each time ovulation doesn't result in a healthy pregnancy, the womb gets rid of its endometrial lining, along with any unfertilized eggs, sick, dying, or dead embryos. That protective process is known as menstruation, leading to the period. This biological trait, bizarre as it may be, sets us on course for the continuation of the human race.
Nekoliko vrsta na Zemlji deli jednu naizgled misterioznu osobinu: menstrualni ciklus. Mi smo jedni od malobrojnih. Majmuni, gorile, slepi miševi, ljudi, i možda rovčice jedini su sisari na Zemlji koji menstruiraju. Takođe to činimo češće od drugih životinja, iako je to traćenje nutrijenata i može biti fizička nelagodnost. Pa, u čemu je smisao ovog neuobičajenog biološkog procesa? Odgovor počinje trudnoćom. Tokom ovog procesa, resursi tela se pametno koriste da stvore odgovarajuću okolinu za fetus, stvarajući unutrašnje utočište u kom majka neguje svoje rastuće dete. Na ovaj način, trudnoća je zadivljujuća, ali to je samo polovina priče. Druga polovina otkriva da trudnoća postavlja majku i dete u rizik. Kao i kod svih živih bića, ljudsko telo se razvilo da pospeši širenje svojih gena. Za majku, to znači da bi trebalo da svim svojim potomcima pruži jednako. Ali majka i njen fetus ne dele potpuno iste gene. Fetus nasleđuje gene i od oca, a ti geni mogu da pospešuju svoj opstanak uzimajući od majke i više resursa nego što je neophodno. Ovaj evolutivni konflikt interesa stavlja ženu i njeno nerođeno dete u biološko rivalstvo koje se odigrava unutar materice. Jedan faktor koji doprinosi ovoj unutrašnjoj borbi je posteljica, organ koji povezuje fetus sa majčinim izvorom krvi i hrani ga dok raste. Kod većine sisara, posteljica je skrivena iza barijere majčinih ćelija. Ta barijera dozvoljava majci da kontroliše dopremanje nutrijenata do fetusa. Ali kod ljudi i još nekih vrsta, posteljica zapravo prodire unutar majčinog sistema krvotoka, da bi mu direktno pristupila. Kroz posteljicu, fetus u majčine arterije upumpava hormone koji ih održavaju otvorenim kako bi stalno dobijao nutrijente. Fetus sa takvim neograničenim pristupom može da proizvodi hormone kako bi povećao majčin nivo šećera u krvi, proširio njene arterije i povisio njen krvni pritisak. Većina majki sisara može da izbaci ili vrati embrion ako je potrebno, ali kod ljudi, jednom kad se fetus poveže sa dotokom krvi, presecanje te veze može da dovede do krvarenja. Ako se fetus ne razvije dovoljno ili umre, majčino zdravlje je ugroženo. Kako raste, potrebe fetusa za resursima mogu da uzrokuju intenzivan zamor, visok krvni pritisak i stanja poput dijabetesa i preeklampsije. Zbog ovih rizika, trudnoća je uvek ogromna, ponekad i rizična investicija. Tako da ima smisla da telo pažljivo proverava embrione kako bi našlo one koji su vredni tog izazova. Ovde se menstruacija uklapa. Trudnoća počinje procesom zvanim implantacija, kada se embrion ugrađuje u endometrijum koji obavija matericu. Endometrijum se razvio da oteža implantaciju, kako bi samo zdravi embrioni mogli da prežive. Ali u tom procesu je izabrao i najinvazivnije embrione, stvarajući evolutivnu povratnu spregu. Embrion ulazi u složen, izuzetno precizan hormonalni dijalog koji transformiše endometrijum da bi dozvolio implantaciju. Šta se dešava kad embrion padne na testu? Možda će ipak uspeti da se veže, ili će delimično proći kroz endometrijum. Dok polako umire, svoju majku može učiniti podložnom infekcijama i svo vreme može da emituje hormonske signale koji remete njena tkiva. Telo ovaj problem izbegava jednostavno uklanjajući svaki mogući rizik. Saki put kada ovulacija ne dovede do zdrave trudnoće, materica se otarasi postave endometrijuma zajedno za neoplođenom jajnom ćelijom, bolesnim, umirućim ili mrtvim embrionima. Taj zaštitni proces je poznat kao menstruacija, koji dovodi do krvarenja. Ova biološka osobina, koliko god bila bizarna, je početak nastavka ljudske rase.