How the Woman Got Her Period

The title sounds like one of Kipling’s “Just So” stories, but this article from theoretical biologist Suzanne Sadedin gives scientific answers, told in plain language that can be funny in places. She introduces the story of why humans menstruate and other animals don’t by focusing on the relationship between a pregnant woman and her fetus, which she says we’ve been lied to about all our lives. On a biological level, it’s not all sweetness and love.

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.

The story is about natural selection and humans competing with each other to pass along their genes. You are familiar with that concept, but this story pits mother and fetus and even father against each other, because they all have their specific interests, which are not necessarily those of the other two in the family. Read it all at Quora. -via Metafilter

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