The following is an excerpt from Milk: The Biology of Lactation by Michael L. Power and Jay Schulkin.
The young mother hurries across the hot, dry sands, her four legs carrying her quickly toward her nest and the four precious eggs within, her tail swaying side-to-side to aid her balance. The time is 280 million years ago, in the middle of the Permian period of Earth’s history, a time when the continents of Laurasia and Gondwana were colliding and merging to form the super- continent Pangaea. The warm, wet rainforests of the Carboniferous period were rapidly shrinking and even vanishing, and the landscape was changing to a more arid environment, dominated by conifers and including large swaths of desert. Life on land was becoming more difficult, as water became scarce. Hence the mother’s hurry. She had needed to leave her nest to forage and find water, but that left her eggs vulnerable to dehydration in the hot, dry conditions. She could not leave them for long.
She reaches her nest and all is well. No predators have found her eggs, and she was not gone so long that their water loss was critical. These eggs are not like the bird eggs of
The mother eases herself onto her eggs and begins to gently rub her chest against them. This action stimulates a prolactin surge from her pituitary gland, which both calms the mother and stimulates the glands on her chest to begin to make a watery secretion. The physical contact with her eggs also causes a surge in oxytocin from her pituitary, which further stimulates these glands, ejecting the watery secretion onto her eggs. The fluid bathes the eggs, which begin to absorb the vital water, but also additional compounds, such as calcium, phosphate, sodium, and other minerals. Proteins in the fluid might also enter the egg, to be metabolized and used for growth, but perhaps also to perform important developmental functions, acting as growth factors and hormonal signals. Perhaps there are complex sugars (oligosaccharides) in the fluid. The fluid probably contains little if any lipid (fat), though the mother does secrete lipid from glands on her skin to aid her in combating water loss, so it is possible. There are also compounds in the fluid that are poisonous; not to the eggs and the developing embryos inside, but to bacteria, molds, and fungi. After all, the eggs and the mother’s skin are now covered in a warm, moist solution, perfect for microbial growth that would be dangerous for the health of the mother and her eggs. One of these molecules likely was the bactericidal protein lysozyme, which today is found in many animal secretions, including mucus, tears, saliva, and, yes, milk.
For despite her reptilian looks, this young mother is a synapsid, an off- shoot of the vertebrate phylogenetic tree that diverged from the reptiles about 300 million years ago. Her descendants 80 to 100 million years later will be the stem mammals, the ancestors of all mammals alive now on Earth. In that sense she could be considered to be the mother of all mammals, and the watery secretion she deposited onto her eggs the rudimentary beginning of lactation and milk.
Fast-forward 280 million years and watch as a young mother is handed her newborn. She cradles it in her arms and brings its questing mouth to her breast. The neonate instinctively roots with its mouth, finds the nipple, and mother and child settle in for their first nursing session. The infant’s suckling action stimulates the mother’s nipples, sending nerve signals to her brain resulting in a surge of prolactin and oxytocin from her pituitary, stimulating milk production in the mammary gland, and initiating milk let down. Milk begins to be expressed from her nipples, eagerly consumed by the suckling babe. The milk flowing into the baby’s mouth is a complex biochemical soup containing all that the baby needs to grow and develop. Nutrients of course; water, protein, sugar, fat, minerals, and vitamins, all the necessary ingredients for life are contained within this marvelous white fluid. But also so much more. Immunoglobulins that carry the mother’s immunization and disease history to her baby, enabling its immature immune system to combat infectious disease. Hormones and growth factors that regulate growth and development, signals the baby will eventually produce on its own, but supplemented and in some cases wholly provided by mom in early life. Compounds to combat bacterial and fungal infections of the intestinal tract, such as lysozyme mentioned above. But also compounds such as oligosaccharides that encourage the growth of beneficial microbes in the gut, to produce the vitally important gut microbiome that contributes to our health and well-being. And newly discovered, a host of RNA molecules that have the potential to affect actual gene expression of the baby. Milk has come a long way since its ancient origins perhaps 300 million years ago. As our technological abilities to measure biological compounds increases, our understanding of the complexity and importance of milk continues to expand.
The time is ripe for a book that takes a broad comparative view of the evolution of lactation and how breastfeeding and mother’s milk affect the health and development of infants. Biological science has progressed to where we are beginning to understand how epigenetic effects and complex regulatory signaling translate the blueprint provided by our DNA into the development of an independent, living being. The base of knowledge is finally sufficient to begin to examine milk from the perspective of regulatory and developmental biology; to look beyond its acknowledged nutritional importance and to investigate milk as a mechanism by which a mother profoundly influences her baby’s growth, development, and future health.
Michael L. Power is a senior research associate at the American College of Obstetricians and Gynecologists and an animal scientist at the Smithsonian National Zoological Park. Jay Schulkin is the senior director of the research department at the American College of Obstetricians and Gynecologists and a faculty member at Georgetown University’s Department of Neuroscience and the University of Washington’s Department of Obstetrics and Gynecology. Power and Schulkin are the coauthors of The Evolution of Obesity and The Evolution of the Human Placenta. Their latest book, Milk: The Biology of Lactation, is available now.