Why do human females have permanently prominent breasts?
You may be interested in Breast asymmetry, sexual selection, and human reproductive success.

Abstract:


  Breasts of human females are large compared to those of closely related primate species,
  and they can thus be hypothesized recently or currently to have been subject to directional
  sexual selection. Here we show that (1) large breasts have higher levels of fluctuating
  asymmetry than small breasts, (2) breast fluctuating asymmetry is higher in women without
  children than in women with at least one child, (3) breast fluctuating symmetry is
  a reliable predictor of age-independent fecundity, and (4) breast fluctuating symmetry
  appears to be associated with sexual selection. These conclusions were similar in studies
  from two cultures differing in fecundity and brenstfeeding traditions (Spain; New Mexico,
  U.S.A.). Choosy males that prefer females with symmetrical breasts may experience a
  direct fitness benefit in terms of increased fecundity and an indirect benefit in terms of
  attractive or fecund daughters. 


Some quotes from the text:


  The size of breasts in human females is large compared to that of other primate
  species, and there has thus been a recent evolutionary change in breast size.
  Human breasts are characterized by their large size and rapid development prior
  to and during puberty, when growth of other body parts also reaches a high level.
  They can thus be considered costly structures in terms of energy use. …
  Heavy investment in breast development during puberty may seem puzzling
  because the functional significance of breasts is far from obvious. There is no
  or little relationship between breast size and production and composition of milk
  in current industrial societies (reviews in Anderson 1983; Cant 1981; Low et al.
  1987), and breasts have therefore been hypothesized to constitute a deceptive signal
  used by women to attract preferred mates (Low et al. 1987). In prehistoric
  human populations, conditions may have been different, when mother's milk
  was the sole source of nutrition for young children and when resources often
  may have been more limited than in present industrial societies. In this study
  we show that large breasts tend to be more asymmetric than small breasts. In
  prehistoric societies, where resources were likely to be less available or more unevenly
  distributed among women, breast asymmetries are likely to have been much
  larger than in present industrial societies with relatively easy access to resources. …
  
  If the relationship between breast size and fluctuating
  asymmetry is robust, we hypothesize that a directional preference for large, symmetrical
  breasts, as apparently found in some cultures (Ellis 1954), can be maintained
  in the population because selection for large breasts will be balanced by
  selection against asymmetrical breasts
  
  We also show that breast symmetry is a reliable predictor of age-independent
  fecundity, and that choosy males that use breast symmetry as a cue in their mate
  choice will experience an advantage in terms of reproductive success. Breast symmetry
  can thus be considered a reliable signal used in intersexual communication.