Actually, I read a really intriguing article on this topic a while ago. In most other primates, the buttocks are the female body part that the males are attracted to—in many primates the buttocks even swell or change color during ovulation to show that the females are fertile. Some scientists postulate that when humans became bi-pedal, and our butts were no longer eye-level, females evolved swollen breasts to mimic the curve of the butt, since that was more visible as we stood upright. However, since we no longer have visible oestrus (ovulation) the breasts are always enlarged, which the male brain perceives as a sign of fertility. It’s also interesting to note that in other cultures, however, the breasts are far less eroticised than in Western culture. There are many parts of the world where it is still considered perfectly fine for women to bare their breasts to nurse their babies in any public space; in the United States, this would still result in some raised eyebrows, since breasts are considered erotic. In other countries (brasil, for example) a woman’s large butt and hips are still considered as sexy or even sexier than breasts.