Milk’s survival secret: How a genetic mutation changed human diet forever

Fast-forward to today, when 65 percent of adults have lactose intolerance, and many people choose plant-based milk over animal milk.

In this piece, we investigate the origins of milk consumption among early humans and the impact of genetics and culture on lactose intolerance.

Origins of milk consumption

Many mammals, including humans, become lactose intolerant after they’ve been weaned off mother’s milk. According to a 2020 study published in Current Biology, this dates back to the Early Neolithic period, when humans learned to exploit ruminants (cows, goats, etc.) as a source of milk.

So, how did milk become such a staple in the human diet?

According to a 2015 study in Nutrients, three main factors were responsible for lactase persistence in humans: genetic mutations, the ability to develop low-lactose products (cheese and yogurt), and the adaptation of the colon microbiome, which allowed lactose-intolerant adults to overcome their intolerance.

The same study also found that mutations associated with lactase persistence only reached appreciable levels in human populations in the last 10,000 years. This is an example of recent human evolution and gene-culture co-evolution associated with the advent of agriculture.

In other words, this is an instance where human culture influenced genetic adaptation to lactose consumption.

The paradox of lactose intolerance

What motivated early humans to drink milk despite widespread lactose intolerance? The nutritional benefits of milk were a primary factor among several contributing to this.

Milk provides a rich source of calories, proteins, and fats, which were crucial for the survival of early humans in harsh environments. The nutritional benefits, therefore, outweighed the symptoms of lactose intolerance.

As humans advanced in farming and animal domestication, the need to preserve their food became evident. Consequently, various methods for preserving food were developed, including fermentation.

During the process of fermentation, lactose is broken down. Hence, products like cheese and yogurt, resulting from fermentation, offer better digestibility for people with lactose intolerance. This is backed by archaeological findings that suggest cheese-making began around 7,500 years ago in Poland.

Lastly, humans’ gut microbiome also adapted to the need for milk consumption. Some people with lactose intolerance can consume small amounts of dairy without symptoms due to bacteria in their colon that can break down lactose.

This change in the gut microbiome likely supported the ongoing consumption of milk products even before genetic lactase persistence was common.

The evolution of lactase persistence

A 2020 study in Current Biology found that the primary genetic variant causing lactase persistence in Europeans, rs4988235-A, may have arisen between 2,000 and 20,000 years ago. Interestingly, the spread of this genetic trait was remarkably rapid. 

The study’s authors found that in Bronze Age Europe, around 3,000 years ago, the frequency of the lactase persistence allele (a version of the gene responsible for digesting lactose) was still quite low—approximately five percent. However, this frequency has risen to over 90 percent in some modern European populations.

The rapid spread suggests a strong positive selection for the trait.

The authors also estimate that the advantage of having the lactase persistence trait increased by about six percent with each generation, which is considered very high for human genetic traits.

Although Europeans have the highest lactase persistence, different populations have developed lactase persistence through different genetic mutations.

Lactase persistence varies widely, being high in parts of Sub-Saharan Africa and the Middle East, intermediate (11 to 32 percent) in Central Asia, and low (less than five percent) in Native Americans, East Asians, most Chinese populations, and some African populations.

Several contributing factors, including cattle domestication, milk consumption, and genetic selection, could explain the variations in lactase persistence.

For instance, pastoralist populations in East Africa have high rates of lactase persistence but through genetic mutations different from those found in Europeans.

Interestingly, despite a long history of agriculture, China has low rates of lactase persistence due to historically low milk consumption in the region. So, what is driving the case for lactase persistence?

The case for lactase persistence

The strong selection for lactase persistence has led researchers to propose several theories explaining its adaptive advantage. One of the leading theories is the nutritional benefits.

In regions with limited food sources, milk provides a reliable source of calories and nutrients, giving lactase-persistent individuals a survival advantage.

Calcium, vital for bone health, is abundant in milk, especially cow and goat varieties. In areas with low sunlight, where vitamin D synthesis is limited, the ability to consume milk may have been particularly beneficial for calcium absorption.

Besides the nutritional benefits, milk may have been a cleaner fluid source than water, reducing the risk of waterborne diseases like cholera.

Another theory is that milk consumption may benefit the gut microbiome, improving overall health and immune function.

Modern trends in milk consumption

With the rise of alternative milk, like plant-based milk, there has been a shift in milk consumption patterns in recent years. This is particularly observed in Western countries, where, despite genetic lactase persistence, many individuals are choosing to reduce or forgo animal-based dairy products.

There has been a rise in the consumption of milk alternatives like soy, almond, and oat milk due to this trend. These products cater to lactose-intolerant individuals and those choosing to avoid dairy for other reasons.

The complex interplay of genetics, cultural aspects, and survival needs over the past 10,000 years has shaped how humans have shifted between consuming and not consuming dairy products.

Health considerations, animal welfare concerns, and environmental impact will likely shape milk consumption trends. Studies suggest the dairy industry negatively impacts air and water pollution and greenhouse gas emissions. 

Furthermore, dairy animals are often mistreated and raised under harrowing conditions to maximize milk production.

The study of human milk consumption over the years offers us an insight into the interplay between genetics, culture, environment, and survival needs.