When I was a student at Cambridge I remember an anthropology professor holding up a picture of a bone with 28 incisions carved in it. "This is often considered to be man's first attempt at a calendar" she explained. She paused as we dutifully wrote this down. 'My question to you is this - what man needs to mark 28 days? I would suggest to you that this is woman's first attempt at a calendar. It was a moment that changed my life. In that second I stopped to question almost everything I had been taught about the past. How often had I overlooked women's contributions?