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Dairy is both new and ancient to our species. It is ancient in the form of breastfeeding our young. It is new in that we now continue drinking milk into old age.

That being said, some of us are more or less adapted to digesting lactose, the primary sugar found in milk. What is the reasoning behind this variation? It largely has to do with how certain populations around the world adapted to their cultural implementation of pastoralism and dairy consumption.

It’s truly fascinating how the cultural decisions of our human ancestors thousands of years ago affect our everyday lives. What is it that we do today that will affect the lives of future human generations?

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Lactose Intolerance and its Link to Human Evolution

The story of lactose intolerance and lactase persistence is a story of human evolution.

Human evolution did not stop once we reached our modern form, 300,000 years ago. It continued and we are still evolving today. Statements like these often elicit vivid imagery of people merging with technology and becoming a race of super-humanoids. While scenarios like that may not be as far into the future as we think, the evolutionary processes we’ve been experiencing are a little more subtle. 

Evolution is generally a long and gradual process with intermittent bouts of more rapid change. It takes time for species to expense with old traits and acquire new ones. This is partly because the environments around them do not change all that rapidly. Now and again you may find a dramatic shift in climate due to a volcanic eruption or rapid cooling episode, whereby some species will go extinct and certain populations will evolve to adapt to these new environments. However, species often find themselves in states of equilibrium with their environments for long periods.

The human environment is one that has changed rapidly in the past 12,000 years or so. Not by natural means, but by cultural intervention. We have culturally constructed a world that is vastly different from that of our prehistoric ancestors. So much so, that the natural world is no longer the main driver in our evolution - culture is. 

Like a species of fish adapting to warmer ocean temperatures, we are adapting to this novel environment we’ve created. This is called gene-culture coevolution, which means that our biology and culture are in a conversation where they influence each other’s trajectory - changes in one affect the other. Milk consumption and the evolution of lactase persistence is a prime example of this.

The Introduction of Milk to the Human Diet

If we look at what human culture was like before 12,000 years ago, we see that our technology and behaviors were very different. We lived in nomadic groups of less than a few hundred individuals and were essentially restricted to stone age tools. Importantly, we hunted for wild game and foraged for things like fruit, vegetables, tubers, and honey.

All that changed once we learned to domesticate plants and animals. This was a huge cultural shift with many downstream effects, like the ability to live in permanent locations, population growth, and eventually increased rates of infectious disease. It also led to downstream effects on our genetic makeup, particularly due to the introduction of a new nutrient source - milk.

Prehistoric humans could not extract milk from the wild cattle they often hunted, but once they learned to domesticate them, that all changed. When and where did this happen?

It is widely believed that the origin of plant and animal domestication dates to roughly 12,000 years ago in what’s known as the fertile crescent - an area in the Middle East spanning modern-day Iraq, Israel, Jordan, and several other countries. The earliest evidence we have of cattle domestication specifically comes from Anatolia, dating to 10,500 years ago [1].

But just because we have evidence of domestication does not mean that humans were milking these animals, they could have simply been eating them. The earliest evidence of actual milk consumption comes also comes from Anatolia, and dates to around 9,000 years ago [2]. There, archaeologists have found ceramics with residue of milk proteins and lipids. Another form of dairy, cheese processing, likely started 1,000 years after that [3].

So, it wasn’t long after cattle domestication that dairy consumption became a new avenue to acquire some extra calories. After many more thousands of years, this snowballed into the widespread market for milk and dairy products we see around us today. 

Aside from it being sourced from a nonhuman animal, the biggest difference between the milk consumption of prehistoric people and post-agricultural revolution people, is that we now drink milk as adults. This is where we begin to see the role of evolution come into play.

Lactose Intolerance and Lactase Persistence

The primary source of nutrition for newborn humans, like most mammals, is their mother’s breast milk. Also like most mammals, prehistoric humans were weaned off of that breast milk by around one year of age. Throughout adolescence and adulthood, milk was largely absent from their diets. The human genome took this into consideration.

There are structural genes that code for things like eye color, skin pigmentation, bone size, etc. Then, there are regulatory genes, which code for actions, like turning on or off other genes, or certain physiological mechanisms. When it comes to dairy consumption, regulatory genes are most important. Here’s why.

Lactose is the sugar found in milk. It consists of two sugar molecules, glucose and galactose, linked together. To be absorbed and utilized by the body, lactose must be broken down into its constituent sugars by an enzyme called lactase, which is produced in the small intestine. Specific regulatory genes code for the production of that enzyme. If those genes are turned on, then you can digest and absorb lactose. If not, then you may experience pain and discomfort. This is what we call lactose intolerance. 

Since prehistoric humans stopped consuming milk after about one, their genes that coded for lactase were turned off. Evolutionarily, it wouldn’t make sense to continue spending energy and resources on an enzyme that no longer has a function. This became problematic for people who introduced dairy into their adult diets. This trait is called lactase non-persistence (LNP). 

Today, most people around the globe still fall into the category of LNP. However, some populations express the opposite trait - lactase persistence. In these populations, the genes that code for lactase do not turn off, allowing these people to continue breaking down and properly digesting lactose throughout adulthood. So where are these populations and why? 

The Population Genetics of Lactase Persistence

Research suggests that LP is most common in northern Europe [4], and the prevalence of this trait within different populations is as follows:

• Northern Europe (Swedes and Danes) = >90%

• Southern Europe and the Middle East (Spanish, French, and pastoralist Arabs) = 50%

• Non-pastoralist Asians and Africans (Chinese and West African agriculturalists) = 5-20%

• Pastoralist Africans (Tutsi and Fulai) = 90% and 50%, respectively

A team of researchers recently constructed an interactive map showing this distribution across the globe.

As you can see, the bluer the dot, the higher the rate of LP within that population. Note the large cluster of blue in northern Europe and to a lesser degree in East Africa.

There are commonalities and differences among these populations. What’s generally common amongst them is that they either practice pastoralism or are descendants of populations that did practice it. This means that people with cultural histories of drinking milk are better adapted to digesting it.

What’s different between these populations are the specific genes involved in their respective adaptations. The first study to really consider this was performed by geneticist Sarah Tishkoff and colleagues [4]. The gene that was known to be associated with LP in Europeans up until this study was a single spot on chromosome 2 at location 13910:T. They looked beyond Europe to investigate the genes of three African populations (Tanzanians, Kenyans, and Sudanese). They found that the gene associated with LP in these populations was at a different location on chromosome 2, 14010:C. 

This study showcases what’s known as convergent evolution. This is when different populations evolve the same adaptation to similar selective pressures independently from one another. 

According to Tishkoff and colleagues, “These data provide a marked example of convergent evolution due to strong selective pressure resulting from shared cultural traits—animal domestication and adult milk consumption.”

Since this study, researchers have found even more genetic variants associated with LP, with five we’re now aware of [6].

• 13.910:T (combined with −22.018:A) = Eurasia, North Africa, and Central Africa

• 13.915:G = Middle East

• 13.907:G = East Africa (Ethiopia and Sudan)

• 14.009:G = East Africa (Ethiopia and Sudan)

• 14.010:C = East Africa (Kenya and Tanzania) and South Africa

The Evolutionary Advantages of Drinking Milk 

Drinking milk was advantageous for many human populations around the globe. This is supported by the fact that the genetic regions associated with lactase persistence have gone through some of the strongest selective pressures in the past 10,000 years, “providing a selective advantage of up to 4%–5% per generation” [7]. 

In a literature review, researchers came away with four main advantages that dairy consumption conferred to ancient humans [5].

1. Lifelong access to nutrient-rich milk

2. Lifelong access to carbohydrates and a fluid source, which would’ve been critical to pastoralists living in hot, arid environments

3. Lactose enhances calcium absorption, which may be compromised by low vitamin D synthesis in the high-latitude (northern) environments of Europe

4. Human consumption of bovine milk may accelerate reproductive maturation or physical growth, or contribute to larger adult size, possibly due to milk’s stimulatory effects on insulin-like growth factor-I.

Final Thoughts

Given that dairy provided these advantages to certain human populations in the past, does this mean you should consume it too? Not necessarily. This is a great example of why knowing your more immediate cultural history can inform your dietary decisions. 

If you come from a culture with a long line of dairy consumers, like those in Northern Europe or East Africa, you may not struggle with lactose intolerance. If you don’t, then incorporating dairy into your diet may cause pain and discomfort. The decisions of your ancestors play a role in shaping who you are today, and your ability or inability to digest milk is a simple example of this interconnectedness.

So, I want to leave you with a question. What cultural traits do we have today that will influence the DNA of future generations? Let me know what you think in the comment section below.

References: 

[1] MacHugh, D., et al. 2017. “Taming the past: ancient DNA and the study of animal domestication.” Annual Review of Animal Biosciences 5:329-51

[2] Evershed, R., et al. 2008. “Earliest date for milk use in the Near East and southeastern Europe linked to cattle herding.” Nature 455:528-31

[3] Salque, M., et al. 2013. “Earliest evidence for cheese making in the sixth millennium BC in northern Europe.” Nature 493:522-25

[4] Tishkoff, S., et al. 2007. “Convergent adaptation of human lactase persistence in Africa and Europe.” Nature Genetics 39:31-40.

[5] Anguita-Ruiz, A., et al. 2020. “Genetics of Lactose Intolerance: An Updated Review and Online Interactive World Maps of Phenotype and Genotype Frequencies.” Nutrients 12(9):2689.

[6] Ségurel, L. and Bon, C. 2017. “On the Evolution of Lactase Persistence in Humans.” Annual Review of Genomics and Human Genetics 18:297-319.

[7] Misselwitz, B., et al. 2019. “Update on lactose malabsorption and intolerance: pathogenesis, diagnosis and clinical management.” Gut 68(11):2080-2091.

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