our upright posture, it has been suggested, it what lead to the development of "secondary sexual signals" on our chest

Interestingly, in a photograph of cleavage (if properly cropped to exculde extranious indicia), it can be very difficult to distinguish whether said cleavage is frontal or buttocks. I would google to provide examples for a url here, but my spouse might misconstrue the purely scientific purpose of such (g)oogling.

suggesting that human female (frontal) mammalia evolved as shape to mimic the posterity.

perhaps related to the fact that in humans, uniquely, the more typical (ahem) entry angle is frontal rather than posterier. which fact in turn perhaps relates to changes in pelvic structure need to accommodate the loadbearing stresses unique to bipedal rather than quadrapedal locomotion.

How's that for tasteful obscurantism?