Image Credit: W Carter, CC BY-SA 4.0, Image Cropped

Milk, cheese and ice cream…if these words make your mouth water rather than induce a panicked search  for the nearest bathroom, you’re one of humanities’ recently evolved lactose-tolerant specimens.

When mammals are born, their mothers give them milk enriched with the ideal nutrients to sustain growth and development. Evolutionary diversity being what it is, mammals have evolved a plethora of creative ways to deliver that milk to their babies. For blue whales, this means producing about 200 liters of milk a day that is high enough in fat not to immediately dissolve in the water before it reaches the calf. In monotremes (platypuses and echidnas), the milk does not come from nipples but rather from mammary glands embedded in the skin beneath the fur. Another fascinating case are marsupials such as kangaroos and wallabies, some of which can produce two different kinds of milk based on the age and needs of their joeys. 

Once the offspring is ready for weaning and developed enough to feed on other food sources, they normally lose the ability to properly digest lactose. This also applied to humans – until a few thousand years ago, when some populations gained the ability to continue producing the enzyme lactase after weaning. Lactase breaks down the main carbohydrate in milk, lactose, into easily digestible fragments. But how and why did this “lactase persistence” develop?

Humans started domesticating animals rather than just killing wild animals for their meat and skin around 10,000 years ago. Evidence of humans processing milk into dairy products such as cheese – which makes it more digestible to lactose-intolerant people – can be found as early as around 8,000 years ago in Anatolia, Turkey.

In humans, lactose tolerance is a trait that is especially interesting for evolutionary biologists – and there are some controversial theories.

It’s a bit of a chicken-and-the-egg story: did lactose tolerance develop before or after humans started producing dairy? If so, was genetic drift responsible for randomly giving some populations the advantageous lactose-tolerance-gene, and then later on when they held cattle, they could drink the milk without problems? Or, and this is better supported in the literature, did humans who were able to drink their cattle’s milk have a selective advantage over lactose-intolerant people, and were better able to pass on their genes to their offspring?

There are certainly many advantages to drinking milk, especially for early humans. In times of droughts or famine, individuals who were able to drink milk without getting diarrhea obviously had a huge advantage.

In addition, milk improves calcium absorption which in turn may have helped supplement the diets of human populations living in higher latitudes during the winter months, when sunlight is scarce. The microbiome in our guts also benefits from milk, introducing bacteria that keep improve our immune system. In comparison to water, milk is relatively safe to drink – diseases like cholera or typhoid that are commonly spread through drinking water were probably less problematic for pastoral populations.

In different populations, different mutations on the genome seem to be responsible for lactase persistence in humans – and differing percentages of the population carry the mutation. As the lactase persistence mutations have mostly been studied in especially high-incidence areas, there are likely more to be found in other regions. In Northern Europe, humans are especially good at digesting lactose – almost 100% of people are able to – whereas in parts of Central Asia for example, lactose can only be digested by less than 10% of humans.

So, whether you can properly digest lactose or not is a gentle reminder that your ancestors had to survive in a much more dangerous, hostile environment, where one food item could mean the difference between life or death. Thankfully, nowadays lactose-intolerance is no longer disadvantageous. Humans today can get their calcium fill in a variety of ways, everything from lactose-free cow milk to plant-based “milk” products.

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Eva Paulus is an evolutionary marine biologist who just started her PhD at the University of Konstanz. Follow her on Twitter @Deep_Sea_Dirndl, or read more of her writing on her Ecology for the Masses profile.