Scientists still stumped by the evolution of human breasts: all other primate females have bosoms when pregnant or lactating, staying flat-chested for the rest of their lives. Only women accumulate fat around their milk glands for life and we don't know how that happened or why.

See the source article by following the link below:
Scientists still stumped by the evolution of human breasts: all other primate females have bosoms when pregnant or lactating, staying flat-chested for the rest of their lives. Only women accumulate fat around their milk glands for life and we don't know how that happened or why.