I may have got this wrong, but I recall Desmond Morris once suggesting that the breasts evolved once we started walking on two feet. Before then, females would attract males by simply sticking their bottoms in the air. Once we became bipeds, this obvious signal was not so.... well, obvious. So women, being very clever, grew their breasts larger and made them permanent to imitate a pair of bum cheeks (not exactly Desmond's words)... and when you think about it chaps, the view 'from behind' is not too dissimilar to the ladies' cleavage view from the front! Plausible?