Mammalian milks usually contain a few percent of carbohydrate. The milk of most of the Eutheria, with a few exceptions, contains lactose (Gal(β 1-4)Glc) as the dominant saccharide; lactose accounts for more than 80% in the carbohydrate fractions. The milk or colostrum also contains other free oligosaccharides, called milk oligosaccharides, which have N -acetylglucosamine, galactose, fucose and/or sialic acid residues as well as a lactose unit in reducing end. Lactose serves as an energy source for the young, while it is thought that milk oligosaccharides are anti-infection factors against pathogenic microorganisms. Why did mammals select lactose as an energy source for their youngs? This issue should be discussed from a phylogenetic aspect as well as from nutritional and physiological aspects. The answers are as follows from the latter aspects: the half osmolarity as two equivalent monosaccharides, the growth stimulation for beneficial colonic bacteria such as Bifidobacterium and provision of galactose as a material for brain development. From the phylogenetic aspect, one should understand how lactation was acquired during the evolution from Amniotes to extant mammals. Free lactose is synthesized within lactating mammary glands from UDP-galactose (donor) and glucose (acceptor) by a transgalactosylation catalysed by lactose synthase. This enzyme is a complex of a β4 galactosyltransferase I and α-lactalbumin, one of the milk proteins. The acquisition of α-lactalbumin is a key for the presence of lactose and oligosaccharides containing the reducing lactose unit in milk. It is believed that α-lactalbumin evolved from lysozyme, which is an enzyme which cleaves the bond in peptidoglycans of bacterial cell walls. When α-lactalbumin first appeared, its concentration would have been very low. As it is assumed that the primitive glands contained most of the glycosyltransferases which are found in the mammary glands of mammals today, these enzymes would have catalyzed the synthesis of oligosaccharides from lactose. Lactose would have unable to accumulate; thus the early secretions would have continued much higher concentrations of oligosaccharides than of lactose. The milk oligosaccharides would have served to inhibit the attachment of pathogenic microorganisms in the infant colon. The molecular evolution from lysozyme, an antipathogen, to α-lactalbumin, a milk protein, should have had an advantage to provide milk oligosaccharides, the antipathogen, to the infants, and thus this gene expression survived and defeated the natural selection. The high content of lactose would be caused by the significant increase of α-lactalbumin expression within lactating mammary glands of eutherians. On the other hand, the acquisition of intestinal neutral lactase in eutherians provided an efficient mechanism for the digestion of lactose. Lactose therefore became a significant energy source for most eutherian young while milk oligosaccharides continued to serve mainly as anti-microbial agents for them.