Evolutionary development of the mammary gland has driven mammalian survival.
Human milk has a choice of 200 – 250 unique oligosaccharides which form its third largest solid component. These oligosaccharides are partially or not digested by the infant and provide a carbon source for the intestinal flora of Bifobacterium Infantis. Genomic sequence of this strain has revealed 700 unique genes and selective pressures for evolution may be expected to continue.
Also see Pediatric Research 2015 January Vol. 77 (1) p. 220 Walker WA and Lyengar RS (Open access Harvard Medical School)