Seeking to explain the sudden emergence of big-brain Homo sapiens, Shlain proposes that profound changes in female sexuality hold the key to this mystery. As the female body compensated for certain physiological changes, women's reproductive cycles also changed. Heavy menses allowed women to discover the dimensions of time and the relationship between sex and pregnancy. Throughout Shlain offers carefully reasoned and certain to be controversial discussions on subjects such as menses, orgasm, masturbation, menopause, circumcision, male aggression, the evolution of language, homosexuality, and the origin of marriage.