Very, very few mammals do. So why do humans? PZ Myers looks at new research and finds a very persuasive answer. But it's complicated to summarize so you'll have to click through and let PZ do his thing on your brain.
Why do women menstruate? | Pharyngula
Menstruation is a peculiar phenomenon that women go through on a roughly monthly cycle, and it’s not immediately obvious from an evolutionary standpoint why they do it. It’s wasteful — they are throwing away a substantial amount of blood and tissue. It seems hazardous; ancestrally, in a world full of predators and disease, leaving a blood trail or filling a delicate orifice with dying tissue seems like a bad idea. And as many women can tell you, it’s uncomfortable, awkward, and sometimes debilitating. So why, evolution, why?
One assumption some people might make is that that is just the way mammalian reproduction works. This isn’t true! Most mammals do not menstruate — they do not cycle their uterine linings, but instead only build up a thickened endometrium if fertilization occurs, which looks much more efficient. Of the mammals, only most primates, a few bats, and elephant shrews are among the lucky animals that menstruate, and as you can see from the phylogeny, the scattered diversity of menstruating mammals implies that the trait was not present ancestrally — we primates acquired it relatively late.
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