Lactation appears to be an ancient reproductive feature that pre-dates the origin of mammals. A cogent theory for the evolution of the mammary gland and lactation has been provided by Olav Oftedal [1
]. The features of current mammals were gradually accrued through radiations of synapsid ancestors, and the mammary gland is hypothesized to have evolved from apocrine-like glands associated with hair follicles (Figure ). Oftedal suggests that these glands evolved from providing primarily moisture and antimicrobials to parchment-shelled eggs to the role of supplying nutrients for offspring. Fossil evidence indicates that some of the therapsids and the mammalia-formes, which were present during the Triassic period more than 200 million years ago, produced a nutrient-rich milk-like secretion.
Simplified representation of evolution of extant Mammalia and lactation.
The capacity to supply fluid and perhaps nutrients to eggs would be promoted and enhanced by incorporation of antimicrobials into the fluid. These may have been antimicrobials already produced in the skin, as in amphibian skin, and evolutionary pressure would probably have fostered the incorporation of molecules such as lysozyme and iron-binding proteins into the secretion, components that are prevalent in milk. The disaccharide lactose (galactose β1–4 glucose) is contained in all milks, except for those of some marine mammals. Its synthesis is catalyzed in the mammary gland by lactose synthetase, an enzyme that is a complex of β1–4-galactosyl transferase and the regulatory subunit α-lactalbumin. Because α-lactalbumin evolved from lysozyme before the division of amniotes into synapsids and sauropsids (see Figure ), the capacity to produce lactose was an ancient trait that preceded its utility in milk synthesis. It is likely that early milks primarily contained antimicrobial oligosaccharides and the prevalence of lactose as a component of milk arose only when α-lactalbumin was produced in sufficient quantity.
With the synthesis of lactose, these modified secretions would have provided nutrients to the egg. The evolution of the casein family of milk proteins in particular would provide calcium, phosphate and protein to hatchlings. Fossil records suggest that caseins were present during the Triassic, because the extensive bone and tooth development evident in the relevant species at stages before independent feeding would have required delivery of ample calcium. Given this evolutionary scenario, the composition of mammary secretions during early lactation in monotremes and marsupials is likely to be similar to that of the primitive milk of mammalian predecessors. The milk then converts to a more nutrient-rich source during later stages of lactation. The evolution of placenta-based reproduction displaced the function of milk as a source of water and nutrients for the egg, leading to secretion of a complex milk throughout lactation in eutherians (Figure ).
Milk also enhances the survival of offspring by satisfying other needs, for example, by promoting immunological competence and endocrine maturation in the neonate [6
]. In this regard, milk seems to provide for the immediate and long-term needs of the offspring. These needs can be highly species-specific. There are also behavioral and 'psychological' aspects of suckling and nurturing between mother (dam) and offspring that produce bonds that promote neonate survival. This is an aspect of lactation that is independent of the chemical and physical characteristics of milk.