No evidence exists for the sudden emergence of big-brained Homo sapiens 150,000 years ago, but Leonard Shlain proposes that profound changes in female sexuality explain the mystery. According to Shlain, the narrow bipedal pelvis and the increasing size of human infants' heads led to increased death rates among women during childbirth. Nature responded to this crisis by gradually but radically altering the human female hormone cycle, allowing women to make the connection between sex and pregnancy, thereby gaining power over their sexuality. Once men understood this connection, the concept of paternity was born, and with it, the struggle to maintain that paternity by controlling women's reproductive choice.