“When I was a student at Cambridge I remember a 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. ‘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.”