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

It may not contain our recommended daily amount of vitamin R, but milk – or “cow’s juice” as it is called on the streets – is among the oldest known animal products converted for human consumption. Milk has been a staple in our diet since the 9th century BC, but it was only by a random mutation in the human genome that we were able to properly digest this delicious bovine beverage. In his latest book, The Life We Made: How 50,000 Years of Human Innovation Has Ennobled – and Redefined – Nature, author Beth Shapiro takes readers on a journey of scientific discovery, explaining how symbiotic relationships between people and the environment around us have changed – but not always for the better.

Basic books

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

The first archaeological evidence that humans produced milk dates back approximately 8,500 years – 2,000 years after the domestication of cattle. In Anatolia (present-day eastern Turkey), which is quite far from the original cattle domestication center, archaeologists have found remnants of milk fat from ceramic pots, indicating that people processed milk by heating. Similar analyzes of milk fat proteins in ceramics record the spread of dairy farming to Europe, which seems to have occurred simultaneously with the spread of domestic cattle.

Not surprisingly, people started milking soon after the cattle were domesticated. Milk is the primary source of sugar, fat, vitamins and protein for newborn mammals and as such has evolved explicitly to be nutritious. It would not take much imagination for a cattle breeder to conclude that cow’s milk would be just as good for him and his family as for her calf. The only challenge would be to digest it – without a mutation in lactase resistance, that is.

Because lactase persistence allows people to utilize calories from lactose, it also makes sense that the spread of the lactase persistence mutation and the spread of dairy products would be closely related. If the mutation originated near the beginning of dairy farming or was already present in a population that had acquired milk production technology, the mutation would give 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 proteins into more humans, and the mutation would increase.

Interestingly, however, ancient DNA did not find a mutation in lactase persistence in the genomes of early farmers, and the mutation is today at the lowest European frequency in the part of the world where dairy farming began. The first milk producers, it seems, did not drink milk. Instead, they processed milk by boiling or fermenting it, making cheeses and sour yogurts to remove harmful indigestible sugars.

If people can consume dairy products without a lactase resistance mutation, there must be some other explanation for why the mutation is so prevalent today. And lactase persistence is extremely prevalent. Nearly a third of us have lactase persistence, and at least five different mutations have evolved – all on the same portion of the intron 13 of the MCM6 gene – that make humans lactase resistant. In any case, these mutations have become high in the populations in which they have evolved, indicating that they provide a huge evolutionary advantage. Is the fact that you can drink milk (in addition to eating cheese and yogurt) enough to explain why these mutations were so important?

The clearest hypothesis is that, yes, the benefit of lactase-related stability is lactose, a sugar that represents about 30 percent of the calories in milk. Only those who can digest lactose have access to these calories, which could have been key calories during hunger, drought and disease. Milk could also be an important source of clean water, which could also be limited during periods of difficulty.

Another hypothesis is that drinking milk enabled access to calcium and vitamin D in addition to lactose, whose supplement helps calcium absorption. This could benefit certain populations with limited access to sunlight, as ultraviolet radiation from sun exposure is necessary to stimulate vitamin D production in the body. However, although this may 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 a high frequency of lactase persistence.

Neither this hypothesis nor the simpler lactase-related hypothesis can explain why lactase persistence is so low in parts of Central Asia and Mongolia where livestock, animal husbandry, and dairy farming have been practiced for millennia. For now, the jury still does not know why lactase persistence has reached such high frequencies in so many different parts of the world, and why it is still at a low frequency in some regions where dairy is economically and culturally important.

Ancient DNA shed little light on when and where the lactase persistence mutation originated and spread across Europe. None of the remains from pre-Neolithic archaeological sites – economies that relied on hunting and gathering – have a lactase persistence mutation. None of the old Europeans from the early agricultural populations in southern and central Europe (people believed to be descended from farmers who spread to Europe from Anatolia) had a lactase persistence mutation. Instead, the oldest evidence of a lactase persistence mutation in Europe comes from a 4,350-year-old individual from Central Europe. At about the same time, the mutation was found in one individual from present-day Sweden and at two locations in northern Spain. Although these data are scarce, the time coincides with another major cultural upheaval in Europe: the arrival of the Asian Yamnaya culture breeders. Perhaps Yamnaya brought with her not only horses, wheels, and a new tongue, but also an improved ability to digest milk.

The mystery of lactase persistence in humans highlights the complicated interaction between genes, the environment and culture. The initial increase in the frequency of the lactase persistence mutation, regardless of when it first occurred, could have occurred by accident. When Yamnaya arrived in Europe, for example, they brought diseases – especially plague – that devastated indigenous European populations. When populations are small, genes can be quickly switched to a higher frequency, no matter what benefit they may provide. If the lactase persistence mutation was already present when the plague occurred and when the population collapsed, the initial increase in the mutation may have occurred secretly. By the time the populations recovered, dairy farming was already widespread and the benefit to those with the mutation would be immediate. By domesticating cattle and developing dairy technologies, our ancestors created an environment that changed the course of our own evolution.

We continue to live and develop in this niche built by people. In 2018, our global community produced 830 million metric tons (more than 21 billion US gallons) of milk, of which 82 percent was from cattle. The rest comes from a long list of other species that humans have domesticated over the last 10,000 years. Sheep and goats, which together make up about 3 percent of world milk production, were first bred for their milk in Europe at about the same time as cattle milk production began. Buffaloes were domesticated in the Indus Valley 4,500 years ago and today are the second largest milk producer next to cattle, producing about 14 percent of the world’s supply. Camels, which were domesticated in Central Asia 5,000 years ago, produce about 0.3 percent of the world’s milk supply. People also consume horse milk, which was first milked by the 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 domestication. But these are just the most common dairy products. Dairy products from more exotic species — moose, elk, deer, alpaca, llama — can be bought and consumed today, and rumors say that Edward Lee of Top Chef is working on how to make ricotta from pork milk and try something like that.

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Naveen Kumar

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