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  • 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 it's 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 bloodstream.

  • 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.

A handful of species on Earth share a seemingly mysterious trait: a menstrual cycle.

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B2 中上級

TED-ED】女性にはなぜ生理があるのか? (【TED-Ed】Why do women have periods?)

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    ming7154 に公開 2021 年 01 月 14 日
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