Schraf writes:
It is possible that the preference for larger breasts (cleavage) came about after we humans started to walk more upright and also started to have sex facing each other much of the time.
Breasts that are large and bulbous enough to resemble a buttocks, in other words.
Gee, Schraf, last time I saw you the subject was breasts!
I've heard the above theory, and it makes some sense in terms of sexual display. But if that were the case, then why would Africans generally regard breasts as "mommy parts"?--bipedalism long predates any African diaspora, so any cleavage-as-butt-crack response would have evolved there first.
I think it's more likely--or at least partly--cultural. Ancient cultures by and large were more nonchalant about bare breasted women. I suspect the repressive mores and fashions of pre-modern centuries helped to sexualize the breast.
Besides, in my manly heart, I know that boobies are not booties, and a shapely bootie is more important to female attractiveness than an ample bosom.