The evolutionary origin of lactation and the composition, structures and functions of milk’s biopolymers highlight the Darwinian pressure on lactation. Lactation selected for biopolymers with considerable structural complexity that in turn provide functions from the mammary gland through the digestive system of the infant. Milk is an extensively glycosylated biological fluid whose glycan structures and functions are only recently emerging, although just one of the roles, feeding and fueling a unique microbiological community within the lower intestine of the infant is an astonishing example of evolutionary composition for biological function. Milk contains glycans, complex polymers of sugars whose stereospecific linkages are not matched by glycosidic enzymes within the mammalian infant gut. Hence, these glycan polymers travel to the lower intestine undigested. On reaching this microbe-rich environment, bacteria compete to access and ferment the sugars via different hydrolytic strategies. One specific strain of bacteria, Bifidobacteriun longum ssp. infantis, (B. infantis) is uniquely equipped genetically with a repertoire of genes encoding enzymes capable of taking up, hydrolyzing and metabolizing the complex glycans of human milk. This combination of a distinct food supply and unique genetic capability results in a profound effect on the composition and metabolic products of the entire microbial community within the lower intestine of breast-fed infants. The intestinal community is dominated by B. infantis, its metabolites and its interactions with the host. The role of B. infantis and its metabolites in shielding the infant from the growth of gram-negative enteropathogens and their endotoxins is a clear benefit and consistent with a selective advantage for this aspect of lactation evolution.