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The evolutionary origin of the mammary gland has been difficult to establish because little knowledge can be gained on the origin of soft tissue organs from fossil evidence. One approach to resolve the origin of lactation has compared the anatomy of existing primitive mammals to skin glands, whilst another has examined the metabolic and molecular synergy between mammary gland development and the innate immune system. We have reviewed the physiology of lactation in five mammalian species with special reference to these theories. In all species, milk fulfils dual functions of providing protection and nutrition to the young and, furthermore, within species the quality and quantity of milk are highly conserved despite maternal malnutrition or illness. There are vast differences in birth weight, milk production, feeding frequency, macronutrient concentration, growth rate and length of lactation between rabbits, quokkas (Setonix brachyurus), pigs, cattle and humans. The components that protect the neonate against infection do so without causing inflammation. Many protective components are not unique to the mammary gland and are shared with the innate immune system. In contrast, many of the macronutrients in milk are unique to the mammary gland, have evolved from components of the innate immune system, and have either retained or developed multiple functions including the provision of nourishment and protection of the hatchling/neonate. Thus, there is a strong argument to suggest that the mammary gland evolved from the inflammatory response; however, the extensive protection that has developed in milk to actively avoid triggering inflammation seems to be a contradiction.

The ramifications of nipple pain extend far beyond the act of breastfeeding, particularly for women whose pain lasts several months. Given the lack of evidence-based treatments, it is not surprising that pain is a major contributor to premature weaning. Further research into the causes of nipple pain is necessary to enable the implementation of effective interventions, thus reducing further complications such as infection and postnatal depression. Detailed pain analysis may assist in assessing the success of these interventions.

Infants of breastfeeding mothers experiencing persistent nipple pain applied significantly higher vacuum to the breast during breastfeeding despite assistance with positioning and attachment from a lactation consultant. Further investigation into the cause of the abnormally high vacuums is essential to develop successful interventions for these mother-infant dyads.