Menstruation
Most female mammals have an estrous cycle, yet only ten primates species, four bats species, the elephant shrew, and the spiny mouse have a menstrual cycle. The relatedness of these groups suggests that four distinct evolutionary events have caused menstruation to arise.
Culture and menstruation
Menstruation in synchrony with the moon is widely assumed in myths and traditions as a ritual ideal. The idea that menstruation is—or ideally ought to be—in harmony with wider cosmic rhythms is one of the most tenacious ideas central to the myths and rituals of traditional communities across the world. One of the most thoroughgoing analyses of primitive mythology ever undertaken was that of the French anthropologist Claude Lévi-Strauss, who concluded that, taken together, the indigenous myths of North and South America expressed men's worry that, unless women's periods were carefully monitored and synchronised, the universe might descend into chaos.