These dramatic contrasts between baboons, chimps and bonobos seem to preclude any idea of a linear trajectory from primates to humans with respect to sexuality. Fisher proposes that sex drive evolved so that we can mate with anybody. However to conserve energy, the co-evolution of attraction allows us to have an individual romantic sexual focus. Finally, attachment allows us to tolerate and emotionally sustain our romantically selected sexual partners. As with primates, these three systems vary from society to society and from individual to individual.
Variation can continue throughout the life cycle.
Research is beginning to indicate how hormones, brain chemistry and structures produce these variations. Since ninety-seven percent of all mammals do not have attachment in their emotional behavior, this is what, for Fisher, is a distinguishing characteristic of humans. In the evening question and answer, Barbara Smuts further distinguished humans by their monogamous relationships in a heterosexual environment which unlike most of the animal world occurs within social groupings. Helen Fisher predicted that in the next millennium we would see increased freedom of expression of our sexuality. Fisher's assertion that the more we understand the brain, the more we appreciate culture led perfectly into Don Pfaff's lecture.