Evolution of lactation
Every biology student quickly learns that the presence of functioning mammary glands is a hallmark of mammals. As outlined in his recent review, Oftedal (2012) provides support for the idea that primitive lactation began as a reproductive advantage in synapsids (the direct ancestors of mammals), likely during the Pennsylvania period. The capacity of ‘modern’ mammary glands to provide secretions with both vital nutrients (proteins, fats and carbohydrates) and health benefits (immunoglobulins, lysozyme, etc.) likely evolved from apocrine-like glands that were associated with hair follicles. It is believed that secretions from these glands provided moisture and antimicrobial agents for the parchment-like shelled eggs laid by these ancestors. Fossil evidence suggests that some therapsids and mammalian forms present during the Triassic period produced milk-like secretions. Evolutionary pressure likely promoted the incorporation of molecules such as lysozyme or iron-binding lactoferrin into these secretions. Certainly the capacity to prevent desiccation of the eggs and protection from microbial attack would have been highly beneficial.
The disaccharide lactose appears in all milks, with the exception of some marine mammals. Its synthesis depends on the enzyme lactose synthetase, which is active as a complex of β1-4-galactosyl transferase and the hormonally regulated milk protein α-lactalbumin. It is known that α-lactalbumin evolved from lysozyme prior to the evolution of sauropsids (leading to birds and reptiles). Thus, the ability to produce lactose appeared before it was utilized as a milk component. It is likely, then, that many of these early milk-like secretions contained antimicrobial oligosaccharides but no lactose. Interestingly, the cellular biosynthesis of lactose in the modern mammary gland also acts to promote hydration of suckling young because of the capacity of lactose contained within secretory vesicles to attract water via osmosis, thus effectively diluting mammary secretions. It is also likely that the presence of lactose improves milk let-down and the supply of milk to the suckling young because of the production of less viscous secretions. Indeed, experiments in transgenic mice have demonstrated that when the α-lactalbumin gene is silenced, the pups fail to thrive because of a failure to productively nurse (Stinnakre et al. 1994).