【翻譯練習】月經對女性的演化意義
How And Why Did Women Evolve Periods?
作者：Suzanne Sadedin
來源：https://www.forbes.com/sites/quora/2016/05/06/how-and-why-did-women-evolve-periods/#404f35157a30
跟一般認知相反的是，大多數哺乳類動物並沒有月經。事實上，月經這種現象只出現在高等靈長類和某些種類的蝙蝠身上*。而且，現代女性的經血量比任何其他動物還多，這實在非常不智：月經浪費身體養分，讓人行動不便，又把自身行蹤暴露給近處的掠食者。為了瞭解女性為何會有月經，首先必須認識一下這個你一生中一直被蒙在鼓裡、也是你至今體驗過最親密的關係：母胎連結。
懷孕難道不是件美好的事嗎？隨便找本懷孕相關書籍，總有這樣一幅畫面：準媽媽的一隻手輕撫孕肚，眼神滿溢慈愛和讚嘆。你感覺到這位媽媽願意付出一切來養育和保護她的小孩。翻開書頁，裡面多半提到懷孕是種充滿光輝的共生關係，是雌性生理學中純粹的利他主義，為孩子精心設計出完美的成長環境。
若你有過懷孕經驗，應該明白真實的懷孕過程還是有些坎坷的。的確會有那些純潔無瑕的利他主義時刻，不過卻也摻雜著好幾星期、甚至好幾個月令人天翻地覆的嘔吐、疲憊、嚴重背痛、失禁、血壓問題，還要擔心自己會不會是那 15% 可能出現足以致死的妊娠併發症的孕婦。
在大部分哺乳類動物看來，人類的懷孕簡直毫無道理可言。大部分哺乳類動物算是歡樂地度過孕期，照常靈巧閃躲掠食者和捕捉獵物，就連一次產下 12 胎的動物也行動自如。那麼，到底是什麼讓人類的懷孕如此與眾不同？答案就在人類古怪的胎盤。大部分哺乳類動物的胎盤是胎兒的一部分，胎盤只有跟母親血管的表面相連，令養分得以通過胎盤輸送給胎兒。有袋類動物甚至不會讓胎兒接觸到血液：牠們只透過子宮壁分泌出某種乳汁。只有包括靈長類和老鼠在內的少數哺乳類族群，演化出所謂的「血絨毛型」(hemochorial) 胎盤，而人類可能就得到了最難搞的血絨毛型胎盤。
人類子宮內部有一層厚厚的內膜組織，這層內膜只有生長微血管。子宮內膜擋住了母體的主要血液供應，血液無法送到新著床的胚胎。逐漸成長的胎盤其實是鑽入子宮內膜，劃開動脈壁，重新安排血管分支，讓血液直送給亟需養分的胚胎。胎盤深深嵌進周圍組織，攻佔這些組織，打出充滿荷爾蒙的動脈血液，讓血液流遍胎盤開闢出來的疆界。這些動脈只能任憑胎盤擺布，連母體也不能控制這些動脈收縮。
這麼做的用意，是讓成長中的胚胎擁有通暢無阻的管道來取得母親的血液供應。胚胎可以製造荷爾蒙，並利用荷爾蒙來操縱母體，譬如增加母體的血糖、擴張母體的動脈、升高母體的血壓，以便提供更多養分給胎盤。而胎盤也確實成功了，有些胎兒細胞會透過胎盤進入母體的血流，在母體的血液、器官、甚至大腦中生長，伴隨著母親的一生，把母親變成遺傳學上的嵌合體 (chimera)**。
胎盤的所作所為似乎有點魯莽無理。其實，這是一種對演化最有利的手足競爭。母親和胎兒的演化利益有著天壤之別：母親「想要」分配差不多平等的資源給她還活著的全部小孩，包括尚未出生的胎兒，而且一絲一毫都不分給將要死去的小孩；胎兒則是「想要」活著，能拿多少好處就盡量拿。(引號用來表示這些行為並非母親和胎兒刻意為之，而是演化傾向於讓雙方都能獲得最佳利益。)
這個關係裡還有個第三者－－父親，而父親的利益跟母親的利益又更加不一致，因為母親生的其他小孩未必是同個父親的種。藉由「基因體印記」(genomic imprinting) 這個過程，某些來自父親的胎兒基因可以在胎盤中活化，這些基因無情地利用母親來保障胎兒的幸福安康。
血絨毛型胎盤賜予胎兒和父親如此特出的大權，我們是如何得知胎盤會有這種貪婪的行徑呢？雖然從一些趨勢可以在靈長類身上看出胎盤的侵略性愈來愈強，但是確切答案仍然遺失在時間的迷霧中，畢竟子宮很難形成完整的化石。
不過，演化的結果顯而易見。一般哺乳類動物的懷孕，是一場秩序井然的事件，因為母親宰制一切，子女是生是死，全憑母親掌握：母親操控胎兒的養分供應，母親也隨時可以排掉或是再吸收胎兒。另一方面，人類的懷孕，是在委員會監督下運作的－－而且這委員會並非隨便組成，其中的成員通常有著截然不同、相互衝突的利益，他們只共享一部分的資訊。人類的懷孕就像一場拔河賽，賽事經常演變為鬥毆事件，偶爾還會公然開戰。很多有致命之虞的孕期病症，像是子宮外孕、妊娠期糖尿病和妊娠毒血症，禍首都可以追溯到這場親密賽局中的失誤。
上面介紹了這麼多，跟月經有什麼關係呢？謎底即將揭曉。
站在雌性動物的角度，懷孕永遠是龐大的投資，而且要是雌性動物的物種有血絨毛型胎盤，投資的規模還會更大。胎盤一旦長成，母親不只沒有辦法完全掌控自身的荷爾蒙分泌，還得在排出胎盤時承受大出血的風險。因此雌性動物當然會很合理地想要極其謹慎地篩選胚胎。腹中懷著虛弱、無法發育或甚至不合格的胎兒挺過孕期，這麼做根本就不划算。
此時終於有子宮內膜的戲份了。也許你之前對子宮內膜的認識，是子宮內膜如何營造出舒適宜人的環境，等待纖細的胚胎著床，然後用它充滿關愛的懷抱裹住胚胎。然而，事實全然相反。研究者 (感謝他們的好奇心) 試過把胚胎移植到老鼠身體的各部位，其轟只有一個地方最難讓胚胎生長－－子宮內膜。
子宮內膜並沒有張開它關愛的懷抱，而是化身為奪命實驗場，最堪受試煉的胚胎，才有資格生存下去。母體愈拖延胎盤侵入血流的時間，就可以花愈長時間決定是否要在免於重大損失的前提下放棄胚胎。相反地，胚胎則希望趕快把胎盤嵌入母體，動作愈快愈好，這麼做雖能讓胚胎獲取母親養分豐富的血液，不過母親也為了胚胎的存活而暴露於更高的風險之下。於是，子宮內膜變得愈來愈厚，愈來愈結實－－胎盤也隨之變得愈來愈兇悍。
然而這種發展又造成另一個問題：倘若胚胎在子宮裡死掉或半死不活，怎麼辦呢？這時必須阻止血液流到子宮內膜表面，否則胚胎就會附著在胎盤上。然而阻止血液會讓子宮內膜組織難以靈敏回應母體的荷爾蒙訊號，而且可能比較會回應近處胚胎的訊號，但胚胎必然會要求子宮內膜繼續提供養分。再者，繼續提供養分給胚胎，容易使胚胎受到感染，尤其是胚胎已經帶有死去或將死組織的時候。
對高等靈長類而言，解決辦法就是在沒有完成健康妊娠的每次排卵過後，連同將死的胚胎和所有組織，脫去子宮表層的整個子宮內膜。這個方法並不是非常明智，但確實有用，而且最重要的，只要針對原本應由胚胎在孕期中使用的化學通道動些手腳，就能輕鬆卸掉子宮內膜。也就是說，這只不過是天擇為人所知的一種效應，一種能夠有效解決類似問題，怪異而拙劣的方法。這個辦法其實沒有表面看來的那麼糟糕，因為從本質上而言，女性的月經次數算是頗少，或許一生中在泌乳停經和妊娠期之間，也就只有來幾十次月經***。
兇悍的胎盤，跟那些綜合起來讓人類獨一無二的其他特性之間有著怎樣的關聯，這點我們還無法確切得知。但是這些人類特性，確實就這樣地隨著演化一同出現了，從某種意義上，這表示古人的選擇或許是正確的。當我們「服下知識的果實」－－當我們開始探索科技的奧妙，從此人類與天真無知的動物分道揚鑣，也使得人類產生獨有的性道德意識－－或許也就在這個時刻，絕無僅有的月經、懷孕和生產之苦痛，開始加諸在女性身上。一切的一切，都算在血絨毛型胎盤的演化帳上。
*雌犬會出現外陰部出血，但此現象並非月經。象鼩從前被認為有月經，但現今咸認此現象最可能是自然流產。
**〈科學家發現子女細胞存活於母親大腦〉(Scientists Discover Children's Cells Living in Mothers' Brains，感謝 Robyn Adair 提供連結)。
***一項針對狩獵採集者已發表的估計數字為 50 次左右，但取決於數種假設，其中包括未有明顯原因、在生產之前整整 3 年皆有月經 (36 次)。
The answer to this question is one of the most illuminating and disturbing stories in human evolutionary biology, and almost nobody knows about it. And so, my friends, gather close, and hear the extraordinary tale of how the woman got her period.
Contrary to popular belief, most mammals do not menstruate. In fact, it's a feature exclusive to the higher primates and certain bats*. What's more, modern women menstruate vastly more than any other animal. And it's bloody stupid (sorry). A shameful waste of nutrients, disabling, and a dead giveaway to any nearby predators. To understand why we do it, you must first understand that you have been lied to, throughout your life, about the most intimate relationship you will ever experience: the mother-fetus bond.
Isn't pregnancy beautiful? Look at any book about it. There's the future mother, one hand resting gently on her belly. Her eyes misty with love and wonder. You sense she will do anything to nurture and protect this baby. And when you flip open the book, you read about more about this glorious symbiosis, the absolute altruism of female physiology designing a perfect environment for the growth of her child.
If you've actually been pregnant, you might know that the real story has some wrinkles. Those moments of sheer unadulterated altruism exist, but they're interspersed with weeks or months of overwhelming nausea, exhaustion, crippling backache, incontinence, blood pressure issues, and anxiety that you'll be among the 15% of women who experience life-threatening complications.
From the perspective of most mammals, this is just crazy. Most mammals sail through pregnancy quite cheerfully, dodging predators and catching prey, even if they're delivering litters of 12. So what makes us so special? The answer lies in our bizarre placenta. In most mammals, the placenta, which is part of the fetus, just interfaces with the surface of the mother's blood vessels, allowing nutrients to cross to the little darling. Marsupials don't even let their fetuses get to the blood: they merely secrete a sort of milk through the uterine wall. Only a few mammalian groups, including primates and mice, have evolved what is known as a "hemochorial" placenta, and ours is possibly the nastiest of all.
Inside the uterus we have a thick layer of endometrial tissue, which contains only tiny blood vessels. The endometrium seals off our main blood supply from the newly implanted embryo. The growing placenta literally burrows through this layer, rips into arterial walls and re-wires them to channel blood straight to the hungry embryo. It delves deep into the surrounding tissues, razes them, and pumps the arteries full of hormones so they expand into the space created. It paralyzes these arteries so the mother cannot even constrict them.
What this means is that the growing fetus now has direct, unrestricted access to its mother's blood supply. It can manufacture hormones and use them to manipulate her. It can, for instance, increase her blood sugar, dilate her arteries, and inflate her blood pressure to provide itself with more nutrients. And it does. Some fetal cells find their way through the placenta and into the mother's bloodstream. They will grow in her blood and organs, and even in her brain, for the rest of her life, making her a genetic chimera**.
This might seem rather disrespectful. In fact, it's sibling rivalry at its evolutionary best. You see, mother and fetus have quite distinct evolutionary interests. The mother 'wants' to dedicate approximately equal resources to all her surviving children, including possible future children, and none to those who will die. The fetus 'wants' to survive, and take as much as it can get. (The quotes are to indicate that this isn't about what they consciously want, but about what evolution tends to optimize.)
There's also a third player here – the father, whose interests align still less with the mother's because her other offspring may not be his. Through a process called genomic imprinting, certain fetal genes inherited from the father can activate in the placenta. These genes ruthlessly promote the welfare of the offspring at the mother's expense.
How did we come to acquire this ravenous hemochorial placenta which gives our fetuses and their fathers such unusual power? Whilst we can see some trend toward increasingly invasive placentae within primates, the full answer is lost in the mists of time. Uteri do not fossilize well.
The consequences, however, are clear. Normal mammalian pregnancy is a well-ordered affair because the mother is a despot. Her offspring live or die at her will; she controls their nutrient supply, and she can expel or reabsorb them any time. Human pregnancy, on the other hand, is run by committee – and not just any committee, but one whose members often have very different, competing interests and share only partial information. It's a tug-of-war that not infrequently deteriorates to a tussle and, occasionally, to outright warfare. Many potentially lethal disorders, such as ectopic pregnancy, gestational diabetes, and pre-eclampsia can be traced to missteps in this intimate game.
What does all this have to do with menstruation? We're getting there.
From a female perspective, pregnancy is always a huge investment. Even more so if her species has a hemochorial placenta. Once that placenta is in place, she not only loses full control of her own hormones, she also risks hemorrhage when it comes out. So it makes sense that females want to screen embryos very, very carefully. Going through pregnancy with a weak, non-viable or even sub-par fetus isn't worth it.
That's where the endometrium comes in. You've probably read about how the endometrium is this snuggly, welcoming environment just waiting to enfold the delicate young embryo in its nurturing embrace. In fact, it's quite the reverse. Researchers, bless their curious little hearts, have tried to implant embryos all over the bodies of mice. The single most difficult place for them to grow was – the endometrium.
Far from offering a nurturing embrace, the endometrium is a lethal testing-ground that only the toughest embryos survive. The longer the female can delay that placenta reaching her bloodstream, the longer she has to decide if she wants to dispose of this embryo without significant cost. The embryo, in contrast, wants to implant its placenta as quickly as possible, both to obtain access to its mother's rich blood, and to increase her stake in its survival. For this reason, the endometrium got thicker and tougher – and the fetal placenta got correspondingly more aggressive.
But this development posed a further problem: what to do when the embryo died or was stuck half-alive in the uterus? The blood supply to the endometrial surface must be restricted, or the embryo would simply attach the placenta there. But restricting the blood supply makes the tissue weakly responsive to hormonal signals from the mother – and potentially more responsive to signals from nearby embryos, who naturally would like to persuade the endometrium to be more friendly. In addition, this makes it vulnerable to infection, especially when it already contains dead and dying tissues.
The solution, for higher primates, was to slough off the whole superficial endometrium – dying embryos and all – after every ovulation that didn't result in a healthy pregnancy. It's not exactly brilliant, but it works, and most importantly, it's easily achieved by making some alterations to a chemical pathway normally used by the fetus during pregnancy. In other words, it's just the kind of effect natural selection is renowned for: odd, hackish solutions that work to solve proximate problems. It's not quite as bad as it seems, because in nature, women would experience periods quite rarely – probably no more than a few tens of times in their lives between lactational amenorrhea and pregnancies***.
We don't really know how our hyper-aggressive placenta is linked to the other traits that combine to make humanity unique. But these traits did emerge together somehow, and that means in some sense the ancients were perhaps right. When we metaphorically 'ate the fruit of knowledge' – when we began our journey toward science and technology that would separate us from innocent animals and also lead to our peculiar sense of sexual morality – perhaps that was the same time the unique suffering of menstruation, pregnancy and childbirth was inflicted on women. All thanks to the evolution of the hemochorial placenta.
*Dogs undergo vaginal bleeding, but do not menstruate. Elephant shrews were previously thought to menstruate, but it's now believed that these events were most likely spontaneous abortions.
** Scientists Discover Children's Cells Living in Mothers' Brains (Thanks to Robyn Adair for the link).
***One older published estimate for hunter gatherers was around 50, but this relied on several assumptions, including 3 whole years of menstruation before reproduction (36 periods) for no obvious reason.
作者：Suzanne Sadedin
來源：https://www.forbes.com/sites/quora/2016/05/06/how-and-why-did-women-evolve-periods/#404f35157a30
跟一般認知相反的是，大多數哺乳類動物並沒有月經。事實上，月經這種現象只出現在高等靈長類和某些種類的蝙蝠身上*。而且，現代女性的經血量比任何其他動物還多，這實在非常不智：月經浪費身體養分，讓人行動不便，又把自身行蹤暴露給近處的掠食者。為了瞭解女性為何會有月經，首先必須認識一下這個你一生中一直被蒙在鼓裡、也是你至今體驗過最親密的關係：母胎連結。
懷孕難道不是件美好的事嗎？隨便找本懷孕相關書籍，總有這樣一幅畫面：準媽媽的一隻手輕撫孕肚，眼神滿溢慈愛和讚嘆。你感覺到這位媽媽願意付出一切來養育和保護她的小孩。翻開書頁，裡面多半提到懷孕是種充滿光輝的共生關係，是雌性生理學中純粹的利他主義，為孩子精心設計出完美的成長環境。
若你有過懷孕經驗，應該明白真實的懷孕過程還是有些坎坷的。的確會有那些純潔無瑕的利他主義時刻，不過卻也摻雜著好幾星期、甚至好幾個月令人天翻地覆的嘔吐、疲憊、嚴重背痛、失禁、血壓問題，還要擔心自己會不會是那 15% 可能出現足以致死的妊娠併發症的孕婦。
在大部分哺乳類動物看來，人類的懷孕簡直毫無道理可言。大部分哺乳類動物算是歡樂地度過孕期，照常靈巧閃躲掠食者和捕捉獵物，就連一次產下 12 胎的動物也行動自如。那麼，到底是什麼讓人類的懷孕如此與眾不同？答案就在人類古怪的胎盤。大部分哺乳類動物的胎盤是胎兒的一部分，胎盤只有跟母親血管的表面相連，令養分得以通過胎盤輸送給胎兒。有袋類動物甚至不會讓胎兒接觸到血液：牠們只透過子宮壁分泌出某種乳汁。只有包括靈長類和老鼠在內的少數哺乳類族群，演化出所謂的「血絨毛型」(hemochorial) 胎盤，而人類可能就得到了最難搞的血絨毛型胎盤。
人類子宮內部有一層厚厚的內膜組織，這層內膜只有生長微血管。子宮內膜擋住了母體的主要血液供應，血液無法送到新著床的胚胎。逐漸成長的胎盤其實是鑽入子宮內膜，劃開動脈壁，重新安排血管分支，讓血液直送給亟需養分的胚胎。胎盤深深嵌進周圍組織，攻佔這些組織，打出充滿荷爾蒙的動脈血液，讓血液流遍胎盤開闢出來的疆界。這些動脈只能任憑胎盤擺布，連母體也不能控制這些動脈收縮。
這麼做的用意，是讓成長中的胚胎擁有通暢無阻的管道來取得母親的血液供應。胚胎可以製造荷爾蒙，並利用荷爾蒙來操縱母體，譬如增加母體的血糖、擴張母體的動脈、升高母體的血壓，以便提供更多養分給胎盤。而胎盤也確實成功了，有些胎兒細胞會透過胎盤進入母體的血流，在母體的血液、器官、甚至大腦中生長，伴隨著母親的一生，把母親變成遺傳學上的嵌合體 (chimera)**。
胎盤的所作所為似乎有點魯莽無理。其實，這是一種對演化最有利的手足競爭。母親和胎兒的演化利益有著天壤之別：母親「想要」分配差不多平等的資源給她還活著的全部小孩，包括尚未出生的胎兒，而且一絲一毫都不分給將要死去的小孩；胎兒則是「想要」活著，能拿多少好處就盡量拿。(引號用來表示這些行為並非母親和胎兒刻意為之，而是演化傾向於讓雙方都能獲得最佳利益。)
這個關係裡還有個第三者－－父親，而父親的利益跟母親的利益又更加不一致，因為母親生的其他小孩未必是同個父親的種。藉由「基因體印記」(genomic imprinting) 這個過程，某些來自父親的胎兒基因可以在胎盤中活化，這些基因無情地利用母親來保障胎兒的幸福安康。
血絨毛型胎盤賜予胎兒和父親如此特出的大權，我們是如何得知胎盤會有這種貪婪的行徑呢？雖然從一些趨勢可以在靈長類身上看出胎盤的侵略性愈來愈強，但是確切答案仍然遺失在時間的迷霧中，畢竟子宮很難形成完整的化石。
不過，演化的結果顯而易見。一般哺乳類動物的懷孕，是一場秩序井然的事件，因為母親宰制一切，子女是生是死，全憑母親掌握：母親操控胎兒的養分供應，母親也隨時可以排掉或是再吸收胎兒。另一方面，人類的懷孕，是在委員會監督下運作的－－而且這委員會並非隨便組成，其中的成員通常有著截然不同、相互衝突的利益，他們只共享一部分的資訊。人類的懷孕就像一場拔河賽，賽事經常演變為鬥毆事件，偶爾還會公然開戰。很多有致命之虞的孕期病症，像是子宮外孕、妊娠期糖尿病和妊娠毒血症，禍首都可以追溯到這場親密賽局中的失誤。
上面介紹了這麼多，跟月經有什麼關係呢？謎底即將揭曉。
站在雌性動物的角度，懷孕永遠是龐大的投資，而且要是雌性動物的物種有血絨毛型胎盤，投資的規模還會更大。胎盤一旦長成，母親不只沒有辦法完全掌控自身的荷爾蒙分泌，還得在排出胎盤時承受大出血的風險。因此雌性動物當然會很合理地想要極其謹慎地篩選胚胎。腹中懷著虛弱、無法發育或甚至不合格的胎兒挺過孕期，這麼做根本就不划算。
此時終於有子宮內膜的戲份了。也許你之前對子宮內膜的認識，是子宮內膜如何營造出舒適宜人的環境，等待纖細的胚胎著床，然後用它充滿關愛的懷抱裹住胚胎。然而，事實全然相反。研究者 (感謝他們的好奇心) 試過把胚胎移植到老鼠身體的各部位，其轟只有一個地方最難讓胚胎生長－－子宮內膜。
子宮內膜並沒有張開它關愛的懷抱，而是化身為奪命實驗場，最堪受試煉的胚胎，才有資格生存下去。母體愈拖延胎盤侵入血流的時間，就可以花愈長時間決定是否要在免於重大損失的前提下放棄胚胎。相反地，胚胎則希望趕快把胎盤嵌入母體，動作愈快愈好，這麼做雖能讓胚胎獲取母親養分豐富的血液，不過母親也為了胚胎的存活而暴露於更高的風險之下。於是，子宮內膜變得愈來愈厚，愈來愈結實－－胎盤也隨之變得愈來愈兇悍。
然而這種發展又造成另一個問題：倘若胚胎在子宮裡死掉或半死不活，怎麼辦呢？這時必須阻止血液流到子宮內膜表面，否則胚胎就會附著在胎盤上。然而阻止血液會讓子宮內膜組織難以靈敏回應母體的荷爾蒙訊號，而且可能比較會回應近處胚胎的訊號，但胚胎必然會要求子宮內膜繼續提供養分。再者，繼續提供養分給胚胎，容易使胚胎受到感染，尤其是胚胎已經帶有死去或將死組織的時候。
對高等靈長類而言，解決辦法就是在沒有完成健康妊娠的每次排卵過後，連同將死的胚胎和所有組織，脫去子宮表層的整個子宮內膜。這個方法並不是非常明智，但確實有用，而且最重要的，只要針對原本應由胚胎在孕期中使用的化學通道動些手腳，就能輕鬆卸掉子宮內膜。也就是說，這只不過是天擇為人所知的一種效應，一種能夠有效解決類似問題，怪異而拙劣的方法。這個辦法其實沒有表面看來的那麼糟糕，因為從本質上而言，女性的月經次數算是頗少，或許一生中在泌乳停經和妊娠期之間，也就只有來幾十次月經***。
兇悍的胎盤，跟那些綜合起來讓人類獨一無二的其他特性之間有著怎樣的關聯，這點我們還無法確切得知。但是這些人類特性，確實就這樣地隨著演化一同出現了，從某種意義上，這表示古人的選擇或許是正確的。當我們「服下知識的果實」－－當我們開始探索科技的奧妙，從此人類與天真無知的動物分道揚鑣，也使得人類產生獨有的性道德意識－－或許也就在這個時刻，絕無僅有的月經、懷孕和生產之苦痛，開始加諸在女性身上。一切的一切，都算在血絨毛型胎盤的演化帳上。
*雌犬會出現外陰部出血，但此現象並非月經。象鼩從前被認為有月經，但現今咸認此現象最可能是自然流產。
**〈科學家發現子女細胞存活於母親大腦〉(Scientists Discover Children's Cells Living in Mothers' Brains，感謝 Robyn Adair 提供連結)。
***一項針對狩獵採集者已發表的估計數字為 50 次左右，但取決於數種假設，其中包括未有明顯原因、在生產之前整整 3 年皆有月經 (36 次)。
The answer to this question is one of the most illuminating and disturbing stories in human evolutionary biology, and almost nobody knows about it. And so, my friends, gather close, and hear the extraordinary tale of how the woman got her period.
Contrary to popular belief, most mammals do not menstruate. In fact, it's a feature exclusive to the higher primates and certain bats*. What's more, modern women menstruate vastly more than any other animal. And it's bloody stupid (sorry). A shameful waste of nutrients, disabling, and a dead giveaway to any nearby predators. To understand why we do it, you must first understand that you have been lied to, throughout your life, about the most intimate relationship you will ever experience: the mother-fetus bond.
Isn't pregnancy beautiful? Look at any book about it. There's the future mother, one hand resting gently on her belly. Her eyes misty with love and wonder. You sense she will do anything to nurture and protect this baby. And when you flip open the book, you read about more about this glorious symbiosis, the absolute altruism of female physiology designing a perfect environment for the growth of her child.
If you've actually been pregnant, you might know that the real story has some wrinkles. Those moments of sheer unadulterated altruism exist, but they're interspersed with weeks or months of overwhelming nausea, exhaustion, crippling backache, incontinence, blood pressure issues, and anxiety that you'll be among the 15% of women who experience life-threatening complications.
From the perspective of most mammals, this is just crazy. Most mammals sail through pregnancy quite cheerfully, dodging predators and catching prey, even if they're delivering litters of 12. So what makes us so special? The answer lies in our bizarre placenta. In most mammals, the placenta, which is part of the fetus, just interfaces with the surface of the mother's blood vessels, allowing nutrients to cross to the little darling. Marsupials don't even let their fetuses get to the blood: they merely secrete a sort of milk through the uterine wall. Only a few mammalian groups, including primates and mice, have evolved what is known as a "hemochorial" placenta, and ours is possibly the nastiest of all.
Inside the uterus we have a thick layer of endometrial tissue, which contains only tiny blood vessels. The endometrium seals off our main blood supply from the newly implanted embryo. The growing placenta literally burrows through this layer, rips into arterial walls and re-wires them to channel blood straight to the hungry embryo. It delves deep into the surrounding tissues, razes them, and pumps the arteries full of hormones so they expand into the space created. It paralyzes these arteries so the mother cannot even constrict them.
What this means is that the growing fetus now has direct, unrestricted access to its mother's blood supply. It can manufacture hormones and use them to manipulate her. It can, for instance, increase her blood sugar, dilate her arteries, and inflate her blood pressure to provide itself with more nutrients. And it does. Some fetal cells find their way through the placenta and into the mother's bloodstream. They will grow in her blood and organs, and even in her brain, for the rest of her life, making her a genetic chimera**.
This might seem rather disrespectful. In fact, it's sibling rivalry at its evolutionary best. You see, mother and fetus have quite distinct evolutionary interests. The mother 'wants' to dedicate approximately equal resources to all her surviving children, including possible future children, and none to those who will die. The fetus 'wants' to survive, and take as much as it can get. (The quotes are to indicate that this isn't about what they consciously want, but about what evolution tends to optimize.)
There's also a third player here – the father, whose interests align still less with the mother's because her other offspring may not be his. Through a process called genomic imprinting, certain fetal genes inherited from the father can activate in the placenta. These genes ruthlessly promote the welfare of the offspring at the mother's expense.
How did we come to acquire this ravenous hemochorial placenta which gives our fetuses and their fathers such unusual power? Whilst we can see some trend toward increasingly invasive placentae within primates, the full answer is lost in the mists of time. Uteri do not fossilize well.
The consequences, however, are clear. Normal mammalian pregnancy is a well-ordered affair because the mother is a despot. Her offspring live or die at her will; she controls their nutrient supply, and she can expel or reabsorb them any time. Human pregnancy, on the other hand, is run by committee – and not just any committee, but one whose members often have very different, competing interests and share only partial information. It's a tug-of-war that not infrequently deteriorates to a tussle and, occasionally, to outright warfare. Many potentially lethal disorders, such as ectopic pregnancy, gestational diabetes, and pre-eclampsia can be traced to missteps in this intimate game.
What does all this have to do with menstruation? We're getting there.
From a female perspective, pregnancy is always a huge investment. Even more so if her species has a hemochorial placenta. Once that placenta is in place, she not only loses full control of her own hormones, she also risks hemorrhage when it comes out. So it makes sense that females want to screen embryos very, very carefully. Going through pregnancy with a weak, non-viable or even sub-par fetus isn't worth it.
That's where the endometrium comes in. You've probably read about how the endometrium is this snuggly, welcoming environment just waiting to enfold the delicate young embryo in its nurturing embrace. In fact, it's quite the reverse. Researchers, bless their curious little hearts, have tried to implant embryos all over the bodies of mice. The single most difficult place for them to grow was – the endometrium.
Far from offering a nurturing embrace, the endometrium is a lethal testing-ground that only the toughest embryos survive. The longer the female can delay that placenta reaching her bloodstream, the longer she has to decide if she wants to dispose of this embryo without significant cost. The embryo, in contrast, wants to implant its placenta as quickly as possible, both to obtain access to its mother's rich blood, and to increase her stake in its survival. For this reason, the endometrium got thicker and tougher – and the fetal placenta got correspondingly more aggressive.
But this development posed a further problem: what to do when the embryo died or was stuck half-alive in the uterus? The blood supply to the endometrial surface must be restricted, or the embryo would simply attach the placenta there. But restricting the blood supply makes the tissue weakly responsive to hormonal signals from the mother – and potentially more responsive to signals from nearby embryos, who naturally would like to persuade the endometrium to be more friendly. In addition, this makes it vulnerable to infection, especially when it already contains dead and dying tissues.
The solution, for higher primates, was to slough off the whole superficial endometrium – dying embryos and all – after every ovulation that didn't result in a healthy pregnancy. It's not exactly brilliant, but it works, and most importantly, it's easily achieved by making some alterations to a chemical pathway normally used by the fetus during pregnancy. In other words, it's just the kind of effect natural selection is renowned for: odd, hackish solutions that work to solve proximate problems. It's not quite as bad as it seems, because in nature, women would experience periods quite rarely – probably no more than a few tens of times in their lives between lactational amenorrhea and pregnancies***.
We don't really know how our hyper-aggressive placenta is linked to the other traits that combine to make humanity unique. But these traits did emerge together somehow, and that means in some sense the ancients were perhaps right. When we metaphorically 'ate the fruit of knowledge' – when we began our journey toward science and technology that would separate us from innocent animals and also lead to our peculiar sense of sexual morality – perhaps that was the same time the unique suffering of menstruation, pregnancy and childbirth was inflicted on women. All thanks to the evolution of the hemochorial placenta.
*Dogs undergo vaginal bleeding, but do not menstruate. Elephant shrews were previously thought to menstruate, but it's now believed that these events were most likely spontaneous abortions.
** Scientists Discover Children's Cells Living in Mothers' Brains (Thanks to Robyn Adair for the link).
***One older published estimate for hunter gatherers was around 50, but this relied on several assumptions, including 3 whole years of menstruation before reproduction (36 periods) for no obvious reason.
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