Menstruation, for example, is not the gentle, autumnal-sounding process of “shedding” an endometrial lining that it is usually described as. When no embryo implants, the uterus releases chemical signals summoning immune cells to come in from the bloodstream and devour its thick endometrial lining, which quickly becomes a killing field, with the debris pouring out of the vagina. Fortunately, during most of human existence, thanks to frequent pregnancies and lengthy periods of lactation, human females probably endured very few menstrual periods during their lifetimes.
