Why We BleedMenstruation is one of the rarest things mammals do — and the reason is a quiet war fought inside the womb

📖 The Story

Because menstruation is so central to human life, we assume it's the mammalian default. It isn't — it's a rarity. Most female mammals build up a uterine lining during their cycle and, if no pregnancy occurs, quietly reabsorb it back into the body. No shedding, no blood, no mess. They recycle. The menstruating species are the exceptions that throw the lining away instead, expelling it through bleeding when the cycle ends without a pregnancy.

The membership of the "menstruation club" is the first big clue to the mystery. These species don't form a tidy branch of the mammal family tree — primates, bats, an elephant shrew, and a desert rodent are not close cousins. That scattered pattern means menstruation didn't evolve once and get inherited; it appeared independently, several times over, in lineages that otherwise have little in common. In evolution, when the same odd trait keeps reappearing in unrelated groups, it's usually a sign that some shared pressure keeps pushing different animals toward the same solution.

So the real question isn't "why do we bleed?" It's "what problem are these particular animals solving that cows and cats are not?" And to answer that, we have to look not at the bleeding, but at what the womb does before the bleeding ever starts.

📖 The Story

The key process is called decidualization: ordinary cells of the womb lining transform into specialised "decidual" cells that can nourish, cushion, and control an implanting embryo. In the great majority of mammals, this only happens reactively — the lining waits, and decidualizes only once an embryo lands and chemically announces itself. The mother builds the nursery after the guest knocks.

In humans and the other menstruating species, it works the other way around. The lining decidualizes spontaneously — every cycle, driven purely by the mother's own hormones, whether or not any embryo exists. The womb pre-builds the nursery, fully furnished, on a recurring schedule. If a pregnancy happens, the preparation is used. If it doesn't, all that specialised tissue can't be maintained once hormone levels fall — so it breaks down and is shed. That shedding is the period. Menstruation, in other words, is the downstream consequence of a womb that prepares for pregnancy proactively instead of waiting to be asked.

This reframes the whole phenomenon. The bleeding is not the adaptation; "spontaneous decidualization" is. To understand why some animals would evolve to do all that expensive preparation on spec, every single month, we have to look at what makes the relationship between a human mother and her embryo so unusually fraught.

📖 The Story

Mother and fetus share most of their interests — both "want" a healthy baby — but not all. A fetus carries genes from two parents, and it is selected to extract as much investment from the mother as it can, even beyond what's optimal for her long-term survival and future children. Biologists call this tension maternal–fetal conflict, and in humans it is unusually intense, because the human placenta is one of the most aggressively invasive in the entire animal kingdom — the fetal tissue (trophoblast) drives deep into the lining and hijacks the maternal blood supply.

This is where spontaneous decidualization earns its keep. By transforming the womb lining in advance, on her own terms, the mother turns it into a thick, controlled, defensive barrier — a fortress wall built before the siege begins. The decidualized tissue both supports a pregnancy and regulates it, reining in how far the fetal cells can invade. The leading hypothesis (developed by Deena Emera, Roberto Romero, and Günter Wagner) is that menstruating species evolved to pre-build this maternal wall precisely because their embryos invade so aggressively. The mother fortifies first, and asks questions later.

It rhymes with a theme that runs through this whole series — the same evolutionary logic we met in "The Invention of Sex" and in the story of the placenta's viral origins. Reproduction is never a simple act of cooperation. Even the bond between a mother and her unborn child is, at the cellular level, a negotiated truce between parties with not-quite-identical interests.

📖 The Story

Humans produce a striking number of genetically abnormal embryos — far more than many other animals — and carrying a non-viable pregnancy is enormously costly, in time, energy, and risk. Researchers including Jan Brosens and colleagues have proposed that the decidualized lining doesn't just defend against invasion; it also biosenses embryo quality. The freshly prepared tissue mounts a finely tuned response that, in effect, screens an arriving embryo: a healthy one triggers the signals that let implantation proceed, while a failing one fails to elicit them and is rejected early.

On this view, menstruation is partly a reset button. Rather than investing months in a pregnancy likely to fail late and catastrophically, the body runs a fresh evaluation each cycle: build the lining, test any candidate, and — if there's no viable pregnancy — clear everything out and start again next month. The monthly bleed is the cost of running that recurring quality-control loop. It's a far cry from the old idea (going back to some mid-century thinking) that menstruation exists to "cleanse" the womb of infection; the modern picture is almost the reverse — a sophisticated screening system, not a wash cycle.

Importantly, the bleeding itself is still best understood as a byproduct, not the purpose. The adaptive thing is the spontaneous preparation and its screening power; menstruation is what that system looks like from the outside when a cycle ends empty. Nature didn't design the period. It designed the fortress and the inspector — and the period is the rubble cleared away when neither was needed this month.

📖 The Story

Evolution rarely keeps an expensive trait around for no reason — and menstruation is expensive. Each cycle sheds tissue and blood, and with the blood goes iron, which the body must constantly work to replace. Across populations, that recurring drain is a key contributor to widespread iron deficiency and anaemia among menstruating people. The fact that this cost persists is itself evidence for the argument running through this feature: the bleeding survives not because it helps, but because it is bolted to something that does — the spontaneous preparation and screening of the womb.

What's striking is the mismatch between how central menstruation is to human life and how little attention science gave it. For centuries it was treated less as a physiological process to be understood than as a thing to be hidden, feared, or moralised about — surrounded by purity taboos and old wives' tales in culture after culture. The result is a strange knowledge gap: a process experienced by roughly half of all humans for a large part of their lives, left chronically under-researched, with even basic questions like "why does it happen?" missing from most textbooks.

That silence is exactly the gap Book Three's final chapter is about. The same trait this feature explains in deep-time, evolutionary terms is the one that modern femtech — period trackers, diagnostics, menstrual science — is finally treating as a serious, designable, fundable problem. Understanding why we bleed is the biology. Refusing to keep it in the dark is the next chapter of the human story.