Why do women have periods?

A handful of species on Earth share a seemingly mysterious trait: a menstrual cycle. We're one of the select few mammals on Earth that menstruate, and we also do it more than any other animal, even though it's a waste of nutrients, and can be a physical inconvenience. So where's the sense in this uncommon biological process? TED-Ed describes the history and evolution of menstruation. [Directed by TED-Ed, narrated by Susan Zimmerman].