I believe the mystery of the origin of sexual modesty in man, which is totally absent in animals, is [due to our] erect posture.  For by this new posture, which Father Nature in his scheme of things probably never intended, certain posterior parts of the body at one stroke came to occupy the centre of the body, and what was naturally behind came in front.  Allied to this terrible new situation were other maladjustments chiefly affecting women, causing frequent abortions and menstrual troubles.  Anatomically, our muscles were designed and developed for the quadruped position.  The mother pig, for instance, carries its litter of pig embryos logically suspended from its horizontal spine, like wash hung on a line with its weight properly distributed.  Asking the human pregnant mother to stand erect is like tipping the wash line vertically and expecting the clothes to remain in position.  Our peritoneal muscles are badly designed for that: if we were originally bipeds, such muscles should be nicely attached to the shoulder, and the whole thing would be a more pleasant job.  Anybody with a knowledge of the anatomy of the human womb and ovaries should be surprised that they keep in position and function at all, and that there are not more dislocations and menstrual troubles.

http://www.eurotrib.com/story/2006/7/19/52415/9686