Hitting the Books: The genetic fluke that enabled us to drink milk

It may not contain our recommended daily allowance of Vitamin R but milk — or “cow juice” as it’s known on the streets — is among the oldest known animal products repurposed for human consumption. Milk has been a staple of our diets since the 9th century BC but it wasn’t until a fortuitous mutation to the human genome that we were able to properly digest that delicious bovine-based beverage. In her latest book, Life as We Made It: How 50,000 Years of Human Innovation Refined — and Redefined — Nature, author Beth Shapiro takes readers on a journey of scientific discovery, explaining how symbiotic relationships between humans and the environment around us have changed — but not always for the better.

Excerpted from Life as We Made It: How 50,000 Years of Human Innovation Refined—and Redefined—Nature by Beth Shapiro. Copyright © 2021. Available from Basic Books, an imprint of Hachette Book Group, Inc.

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The first archaeological evidence that people were dairying dates to around 8,500 years ago — 2,000 years after cattle domestication. In Anatolia (present-day eastern Turkey), which is pretty far from the original center of cattle domestication, archaeologists recovered milk fat residues from ceramic pots, indicating that people were processing milk by heating it up. Similar analyses of milk fat proteins in ceramics record the spread of dairying into Europe, which appears to have happened simultaneously with the spread of domestic cattle.

It’s not surprising that people began dairying soon after cattle domestication. Milk is the primary source of sugar, fat, vitamins, and protein for newborn mammals, and as such is evolved expressly to be nutritious. It would not have taken much imagination for a cattle herder to deduce that a cow’s milk would be just as good for him and his family as it was for her calf. The only challenge would have been digesting it—without the lactase persistence mutation, that is.

Because lactase persistence allows people to take advantage of calories from lactose, it also makes sense that the spread of the lactase persistence mutation and the spread of dairying would be tightly linked. If the mutation arose near the start of dairying or was already present in a population that acquired dairying technology, the mutation would have given those who had it an advantage over those who did not. Those with the mutation would, with access to additional resources from milk, more efficiently convert animal protein into more people, and the mutation would increase in frequency.

Curiously, though, ancient DNA has not found the lactase persistence mutation in the genomes of early dairy farmers, and the mutation is at its lowest European frequency today in the precise part of the world where dairying began. The first dairy farmers were not, it seems, drinking milk. Instead, they were processing milk by cooking or fermenting it, making cheeses and sour yogurts to remove the offending indigestible sugars.

If people can consume dairy products without the lactase persistence mutation, there must be some other explanation as to why the mutation is so prevalent today. And lactase persistence is remarkably prevalent. Nearly a third of us have lactase persistence, and at least five different mutations have evolved—all on the same stretch of intron 13 of the MCM6 gene—that make people lactase persistent. In each case, these mutations have gone to high frequency in the populations in which they evolved, indicating that they provide an enormous evolutionary advantage. Is being able to drink milk (in addition to eating cheese and yogurt) sufficient to explain why these mutations have been so important?

The most straightforward hypothesis is that, yes, the benefit of lactase persistence is tied to lactose, the sugar that represents about 30 percent of the calories in milk. Only those who can digest lactose have access to these calories, which may have been crucial calories during famines, droughts, and disease. Milk may also have provided an important source of clean water, which also may have been limited during periods of hardship.

Another hypothesis is that milk drinking provided access to calcium and vitamin D in addition to lactose, the complement of which aids calcium absorption. This might benefit particular populations with limited access to sunlight, as ultraviolet radiation from sun exposure is necessary to stimulate the body’s production of vitamin D. However, while this might explain the high frequency of lactase persistence in places like northern Europe, it cannot explain why populations in relatively sunny climates, such as parts of Africa and the Middle East, also have high frequencies of lactase persistence.

Neither this hypothesis nor the more straightforward hypothesis linked to lactase can explain why lactase persistence is at such low frequency in parts of Central Asia and Mongolia where herding, pastoralism, and dairying have been practiced for millennia. For now, the jury is still out as to why lactase persistence has reached such high frequencies in so many different parts of the world, and why it remains at low frequencies in some regions where dairying is economically and culturally important.

Ancient DNA has shed some light on when and where the lactase persistence mutation arose and spread in Europe. None of the remains from pre-Neolithic archaeological sites—economies that relied on hunting and gathering—have the lactase persistence mutation. None of the ancient Europeans from early farming populations in southern and central Europe (people believed to be descended from farmers spreading into Europe from Anatolia) had the lactase persistence mutation. Instead, the oldest evidence of the lactase persistence mutation in Europe is from a 4,350-year-old individual from central Europe. Around that same time, the mutation is found in a single individual from what is now Sweden and at two sites in northern Spain. While these data are sparse, the timing is coincident with another major cultural upheaval in Europe: the arrival of Asian pastoralists of the Yamnaya culture. Perhaps the Yamnaya brought with them not only horses, wheels, and a new language, but an improved ability to digest milk.

The mystery of lactase persistence in humans highlights the complicated interaction among genes, environment, and culture. The initial increase in frequency of a lactase persistence mutation, regardless of in whom it first arose, may have happened by chance. When the Yamnaya arrived in Europe, for example, they brought disease—specifically plague—that devastated native European populations. When populations are small, genes can drift quickly to higher frequency regardless of what benefit they might provide. If the lactase persistence mutation was already present when plague appeared and populations crashed, the mutation’s initial increase may have happened surreptitiously. When populations recovered, dairying was already widespread and the benefit to those with the mutation would have been immediate. By domesticating cattle and developing dairying technologies, our ancestors created an environment that changed the course of our own evolution.

We continue to live and evolve in this human-constructed niche. In 2018, our global community produced 830 million metric tons (more than 21 billion US gallons) of milk, 82 percent of which was from cattle. The rest comes from a long list of other species that people domesticated within the last 10,000 years. Sheep and goats, which together make up around 3 percent of global milk production, were first farmed for their milk in Europe around the same time as cattle dairying began. Buffaloes were domesticated in the Indus Valley 4,500 years ago and are today the second largest producer of milk next to cattle, producing around 14 percent of the global supply. Camels, which were domesticated in Central Asia 5,000 years ago, produce around 0.3 percent of the world’s milk supply. People also consume milk from horses, which were first milked by people of the Botai culture 5,500 years ago; yaks, which were domesticated in Tibet 4,500 years ago; donkeys, which were domesticated in Arabia or East Africa 6,000 years ago; and reindeer, which are still in the process of being domesticated. But those are just the most common dairy products. Dairy products from more exotic species—moose, elk, red deer, alpacas, llamas—can be purchased and consumed today, and rumor has it that Top Chef ’s Edward Lee is working out how to make pig milk ricotta, should one want to try such a thing.