From the best selling author of Art and Physics and The Alphabet Versus the Goddess comes a provocative new book that will change our views of human sexuality and evolution. According to Leonard Shlain there is no clear and compelling explanation for the sudden emergence of glib, big-brained Homo sapiens 150, 000 years ago. In his latest book, he proposes an original thesis that variations in female sexuality changed the course of human evolution. Due to the narrowness of her bipedal pelvis and the increasing size of her infant's head, the human female began to experience high childbirth death rates. Natural selection adapted her to this environmental stress by drastically reconfiguring her hormonal cycles. Her estrus with its external signal that she was ovulating disappeared as her menses became the most florid of any mammal and it mysteriously entrained with the periodicity of the moon. These interlocking adaptations led the first women to grasp the concept of a month and make the connection between sex and pregnancy.