The (R)evolutionary Story of Neohormones and Human Milk

As a lactation professional, you know a lot about human milk—its components, its importance, and how to help parents make and deliver it. But how much do you know about milk’s history?

The evolutionary story of human milk—both how it evolved and how it evolves us—is the topic of the second presentation in Laurel Wilson’s Lactation Education Resources conference series “Human Milk: Leaps Beyond Nutrition."

Wilson, IBCLC, RLC, BSc, CLE, CLD, CCE, is a TedX and international perinatal speaker, author, and consultant for perinatal professionals, specializing in epigenetics and human milk. Below are key take-aways from her talk on human milk’s fascinating story.

Milk is evolutionary. Milk as we know it has been around for quite a long time on an evolutionary scale—around 310 million years. However, the components of milk have been evolving all along, with many being much newer. For example, casein has only been around for about 170 million years.

This ability of milk to evolve to precisely suit each species is what makes it such a powerful evolutionary tool.

“Every mammal has evolved over time to create milk that is specifically designed to help its babies thrive in the environment they are born into,” Wilson says. “It has components in it that help each species to expand in its particular evolutionary niche. For humans, this really meant evolving our brains, both intellectually and emotionally.”

This brain development is what has allowed humans to expand our niches all over the world.

Neohormones are the key to milk’s evolutionary power. Modern Eutherian (or placental) mammals evolved to have mammary glands and the ability to lactate because of the evolution of very special class of hormones. Called “neohormones,” these are relatively recently arrivals on the evolutionary scene.

Neohormones are the hormones involved in mammalian fertilization, pregnancy, lactation, and parental behavior. Many of their names will be familiar: estrogen, oxytocin, prolactin, cortisol, and insulin are all neohormones.

In terms of inventions, Wilson says neohormones can be compared to the wheel—few other evolutionary leaps have been so revolutionary.

Neohormones facilitated the enormous biological success of mammals by allowing for a prolonged perinatal period and lactation. “[Neohormones] allow mammals to invest long periods of time with their progeny to encourage survival,” Wilson explains. “Even more than that, with the addition of the magical mammary gland and milk, mammals were able to thrive across all continents.”

Oxytocin is a neohormone superstar. You already know that the neohormone oxytocin is the hormone of love, labor, and letdown. But actually, oxytocin is involved in every aspect of the human reproductive cycle.

Oxytocin causes contractions during labor that facilitate birth. Then, it cues parental behaviors of bonding and protection. Over time, the child models these behaviors—they learn to play and interact with others, which helps them practice for finding a partner. Oxytocin then plays a key role in mate selection and sexual behavior, which produces pregnancy … and the cycle begins all over again, with oxytocin again playing a starring role in yet another birth.

Neohormones are an important part of milk itself. Neohormones not only lead to lactation and milk production, they are also a key ingredient in milk. Neohormones in human milk play a vital role in infant development by causing epigenetic changes, turning on and off genes. (For more on epigenetics and lactation, enroll in Wilson’s conference, and see our recent blog post. This explains much of why human infants who receive human milk have different health outcomes than those who don’t.

“[N]eohormones, delivered through the milk to the newborn, help to shape the baby’s development, key specific hormone release, and activate genes that lead to health and developmental success,” Wilson says.

Neohormones drive future fertility via milk. One key example is the neohormone relaxin. Relaxin is found in human milk, but not in cow’s milk. It’s at its highest levels in colostrum, and declines gradually over time in mature milk.

The relaxin in human milk has a significant epigenetic effect on the development of male and female gonads in neonates, and the receipt of relaxin in infancy via milk is thought to have a significant impact on uterine development later in life. Relaxin in the milk begins right away to send signals to the developing female and eventually affects development of the uterus and cervix, ultimately shaping the child’s fertility.

Several animal studies have illustrated the impact of species-specific milk on individuals’ later fertility. Research shows, for example, that if a female piglet does not get its parent’s colostrum at the proper time, it will have a shortened fertile period and be able to produce fewer offspring as an adult.

“While these studies have not been completed on humans, one can only assume that there are similar and perhaps marked impacts, especially with the rise in infertility treatments around the world as we medicalize birth and feed humans altered cow’s milk in the critical [first] 1,000 days of development,” Wilson notes.

Neohormones and milk raise big questions Neohormones help explain why human milk feeding and formula feeding is such an important topic—the choice affects not only the individual infant, but over time, the evolution of the entire species.

Mammalian milk, according to Wilson, is “ridiculously specific” to each type of mammal. What happens when a large percentage of the individuals in a population do not receive this species-specific milk?

The impacts are significant and diverse. For example, when large numbers of individuals in the population receive formula instead of human milk, and that affects gene expression, what happens to future generations? “How is infant formula influencing evolution of our species through gene transcription?” Wilson asks.

Or, when parents feed formula and do not lactate, what happens to the 700+ genes that are usually turned on through the lactation process … and what impact does that have on parental behavior, bonding, and future social behavior of the offspring? “How are we evolving parenthood by how we feed?” Wilson asks.

There is much more to know about neohormones. Dive deeper by registering for Wilson’s multi-part conference presentation through Lactation Education Resources. Learn more here.