No clear and compelling explanation currently exists for the sudden emergence of big-brained Homo sapiens 150,000 years ago. Here, Leonard Shlain proposes an original thesis that argues that profound changes in female sexuality hold the key to this mystery. According to Shlain, bipedalism, narrow pelvises, and enormous fetal heads precipitated a crisis for our species. Mothers faced a grave death threat in childbirth. To compensate, women lost estrus...