Humans were not initially equipped to digest animal milk, yet today, many populations can.
This evolutionary adaptation raises questions about why consuming dairy has been advantageous for certain groups.
Milk. White Poison or Healthy Drink?
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The relationship between humans and milk spans thousands of years, marked by significant changes. While plant-based milk alternatives, such as soy or almond milk, are gaining popularity for dietary or ethical reasons, the practice of drinking animal milk traces back to early farmers and pastoralists in Europe. For most of human history—roughly 300,000 years—milk consumption was rare, limited primarily to infants. However, around 10,000 years ago, the regular consumption of milk began with people who domesticated animals such as cows. Today, drinking milk is widespread in regions like northern Europe and North America but remains uncommon in other parts of the world, such as East Asia.
From a biological perspective, consuming animal milk is unusual. Milk contains lactose, a sugar that requires the enzyme lactase for digestion. While infants naturally produce lactase to digest their mother's milk, this enzyme's production typically decreases after weaning, leading many adults to struggle with digesting lactose. Symptoms such as cramps, flatulence, and diarrhea are common in those who consume milk without sufficient lactase. Unlike humans, most mammals do not retain lactase activity into adulthood.
The earliest Europeans who drank milk likely experienced significant digestive discomfort. Over time, however, genetic mutations allowed some individuals to retain lactase production throughout their lives, a trait known as lactase persistence. The emergence of this mutation, first identified in Europe about 5,000 years ago, provided an evolutionary advantage, particularly in regions where milk became a staple. Today, lactase persistence is widespread in northern Europe and some African and Middle Eastern populations, but it is relatively rare in Asia and South America.
The exact reasons why milk drinking became advantageous are not entirely understood. One hypothesis suggests that milk provided an essential source of nutrients, reducing starvation risk. However, this explanation falls short because other foods were also available. Additionally, individuals without lactase persistence could still consume processed dairy products like cheese or yogurt, which contain lower lactose levels. Archaeological evidence indicates that cheese-making practices existed in Europe as early as 7,200 years ago, long before the widespread presence of lactase persistence.
The prevalence of lactase persistence aligns with pastoralist societies—those reliant on livestock. Groups that did not raise animals, such as hunter-gatherers or plant cultivators, did not develop the trait. However, not all pastoralist societies acquired lactase persistence. For instance, Mongolian herding populations, heavily reliant on animal milk, exhibit low rates of lactase persistence, likely due to their cultural reliance on fermented milk, which is lower in lactose. Genetic mutations may not have spread widely due to historical population sizes and chance occurrences.
Some researchers speculate that milk offered benefits beyond nutrition. For example, consuming milk may have helped pastoralist communities build immunity to diseases carried by livestock. However, no single explanation fully accounts for the global patterns of lactase persistence. Factors such as milk's rich composition of proteins, calcium, and vitamin D likely contributed to its evolutionary appeal.
While lactase persistence continues to spread in certain populations, such as in parts of Chile where pastoralists have inherited the trait from European ancestors, its global significance has diminished in regions with diverse diets. In wealthier nations, where alternative food sources are abundant, the selective pressure for lactase persistence has largely disappeared.
Despite these evolutionary insights, milk consumption remains prevalent worldwide. While alternative milks are gaining traction in some regions, global milk production continues to rise, driven by increasing demand in developing countries, particularly in Asia. Interestingly, this growth occurs even in populations with low lactase persistence, suggesting that the benefits of milk outweigh its potential digestive challenges. Organizations like the United Nations encourage adopting dairy animals like llamas in developing regions to make milk more accessible.
Ultimately, while the evolutionary forces shaping lactase persistence may have slowed, milk remains a crucial part of human diets. Whether as a source of nutrition or cultural significance, it continues to be integral to human societies worldwide.