It's not related to food or sustenance. In other primate species, females develop round, swollen buttocks during estrus. These indicate sexual receptivity, and males have co-evolved an attraction to them. As human females became potentially fertile year-round, and fully bipedal, they evolved enlarged, permanently rounded breasts (and buttocks, too) which are visually prominent when standing upright, and they act as similar sexual signals to human males. Our attraction to breasts is one of those instincts we've inherited from our hominid ancestors. The article discusses it in more detail.
http://brainmind.com/sexevolution.html
Yeah, no, receptivity and fertility don't enter into it for me.
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