Eve: How the Female Body Drove 200 Million Years of Human Evolution

Bohannon tells the story of the Eve of milk: Morganucodon, or “Morgie,” who lived around 200 million years ago. She describes Morgie as a furry, whiskered creature living in a burrow and hunting insects for food while avoiding predators. When she returns, she feeds her pups milk sweated from the skin on her torso. Bohannon explains that Morgie is the reason that female humans and other female mammals breastfeed and that she was the first of humanity’s Eves to do so. While breast milk does function as human babies’ first food, there are other reasons for it. It consists largely of water but also provides minerals and a safe alternative to water—which often had parasites and other unknown pathogens—for growing human babies.

In addition, Morgie laid eggs like other early mammals and, like sea turtles and platypuses do, coated her eggs with mucus to prevent them from drying out and to protect them from fungi and bacteria. Morgie’s ancestors then developed glands secreting milk to feed their babies, which likely resembled colostrum in modern female humans. Bohannon says that the yellow-tinted milk mothers have in the first couple of weeks is rich with immunoglobins that build up babies’ immune systems and that, contrary to beliefs held by medieval women and medieval doctors such as Bartholomaus Metlinger, colostrum is not harmful or useless. Colostrum helps provide healthy bacteria for newborn babies’ digestive systems and protects the intestines from harmful pathogens. Bohannon also challenges the idea that breast milk is mostly for nutrition, stating that much of human milk is not digestible and acts primarily as a prebiotic for babies, providing them with oligosaccharides that build good bacteria in babies’ digestive systems. Also contrary to what many believe, breast milk does not provide significant protein and will not help build muscle. Bohannon then states that humans’ environments greatly impact breast milk, so each person’s breast milk will have distinctive and diverse oligosaccharides. For this reason, scientists theorize that human breast milk has evolved to accommodate urbanization and life in cities, establishing to Bohannon that human digestive systems have evolved early to accomplish this.

While Morgie, early mammals, and, in modern times, echidnas and platypuses breastfed using sweating fur patches, mammals almost all eventually developed nipples for breastfeeding. Bohannon states that nipples replaced sweating fur patches to reduce the waste of milk, with the infant’s suckling triggering lactation. She then debunks the myth that nursing people’s breasts are filled with milk and explains that the enlarged breasts are filled with fat, blood, and tissue. The suckling sends signals to the mother’s brain, inducing lactation through the production of prolactin and oxytocin, which cause milk production and the squeezing of milk into the nipples’ ducts, respectively. She then explains how the infant suckles and sometimes struggles to latch, causing frustration for both them and the mother and sometimes causing damage to the nipples, leading to the rise of lactation consultants. The evolution of the Montgomery glands, however, has reduced the chafing and other damage to nipples caused by gumming. Another evolution of the human breast is the “upsuck,” in which babies’ saliva is absorbed into the mother’s breast during breastfeeding (46). This allows the mother’s body to gain information about the infant’s health, including the presence of pathogens, and adjust the milk accordingly. The milk will then attack any pathogens in the baby’s body and provide other immunological support. This sometimes causes people to eat certain high-carb and high-fat foods as comfort foods when they are older. The nipple also has encouraged the bond between mother and child, with most mothers preferring to nurse from the left breast, allowing babies to look at the left side of the mother’s face, which is more emotionally expressive.

Bohannon then states that another evolutionary trait from Morgie is the production of cortisol in breast milk’s impact on risk taking and adventurous behaviors in children. Babies whose mothers produce breast milk with more cortisol grow up to be more risk averse, while babies whose mothers produce milk with less cortisol grow up to be more adventuresome. Bohannon explains that Morgie’s world was stressful and dangerous and that being social requires energy that must sometimes be conserved. Breast milk with higher cortisol also produces important proteins for building muscle, while breast milk with lower cortisol produces sugars that help build adipose tissue. Bohannon then asserts that the best outcome would be a balanced amount of cortisol, which helps with spatial recognition—as shown with studies in rats in moderation.

Bohannon says that the reason male humans also have nipples is that they can produce milk as well, though it is much more difficult for them and they are not as good at lactation. She uses studies of male lactation among the Aka men in the Congo. Though it is possible for cisgender men and transgender women to lactate, many will not, as many cis mothers will not. Bohannon concludes that male bodies have nipples because female bodies have nipples, and this is an important biological factor in mammalian biology. She also challenges the idea that larger breasts produce more milk, saying that there is no evidence for it and that breast size has no impact on milk production. Furthermore, she debunks the idea that breasts developed as a fertility marker. Breasts tend to grow larger during menstruation, pregnancy, and breastfeeding, when female individuals are less likely to accept sexual advances and are more likely to have sore breasts. She then says that the association of large breasts with female attractiveness is more likely tied to high estrogen and plump body types. Breasts evolved to tilt upward and be high on a female person’s chest so that after becoming bipedal, she could breastfeed her baby in multiple positions. Bohannon also states that breastfeeding and the rise of wet nurses likely led to the creation of human cities. She then explains that before the story of Noah’s ark, the first flood myth was a Sumerian story about how overpopulation in cities led the gods to create a flood. After the city became overpopulated again, the gods established mortality, birth control, wives and sex workers, and wet nurses. Bohannon then concludes that the evolution of breasts and milk created new dangers such as breast cancer, which would later give way to the development of the womb.