Funny, told a buddy once that an evolutionary theory on the female bust was that men are attracted to it because it looks (cleavage anyway) something like a butt, doubling a woman's chance of attracting a mate. He said funny, as a boob man, I see a woman's butt and think, put a couple of nipples on each cheek and we're talking.
(off topic, better explanation of the bust: unlike apes, human faces, mouths, are relatively flat. By protruding, a woman's breast is able to feed a baby without smothering that baby.)