It is white, it is liquid and a large percentage of the population drinks it regularly: it is milk. There are many different types of milk: cow milk, goat milk, breast milk, soy milk, almond milk and so on. This article, however, will mainly be about the type of milk most commonly found in supermarkets: cow milk.
What is milk?
The term 'milk' comes from the Proto-Germanic word 'meluks'. Many things that have nothing to do with milk but looks vaguely the same has also been known as milk throughout history - for example latex is also known as 'rubber milk'.
Milk is a white nutrient-rich liquid food produced by mammals, and it is the main source of nutrition for infant mammals - like human babies.
Milk contains a special kind of sugar called lactose. As babies, our bodies produce a special enzyme called lactase that allows the body to break down lactose and digest it. However, after babies are weaned from their mothers's milk, they stop producing lactase as there is no longer a need for it. Without lactase, the lactose in milk cannot be properly digested, which leads to cramps, flatulence and even diarrhea.
The first people who began to drink milk regularly were early farmers in western Europe about 10.000 years ago. They were some of the fist humans to live with domesticated animals such as cows. Drinking cow milk could then be a source of nutrition, and even a way to get a safe alternative to drinking dirty and contaminated water.
Since the adults had been weaned from their mothers' milk, they most likely had a lot of stomach problems at first. But, thanks to evolution, some people began to continue producing lactase enzymes throughout adulthood. This happened due to a mutation in the section of DNA that controls the activity of the lactase gene and resulted in them being able to keep drinking milk without side effects. Being able to continue producing lactase as an adult is known as "lactase persistence".
The lactase persistence trait is dominant, meaning the chance of passing it off to one's kids is high, and the trait is therefore very common in some populations. In Northern Europe for example, more than 90% of people are lactase persistent! The same is the case in some populations in Africa and the Middle East.
However, there are many places where lactase persistence is rare: many Africans doesn't have it, and the trait is uncommon in both Asia and South America.
People who aren't lactase persistent can still eat a small amount of lactose without negative effects. This means drinking small amounts of milk is fine. However, if milk is processed into butter, yoghurt, cheese or the like, the percentage of lactose is lowered, which means it is easier for people who aren't lactase-persistent to digest it. For example, there is less than 10% as much lactose in butter or hard cheese like cheddar than there is in milk.
It seems people invented cheese rather fast after starting to drink milk. Archaeologists have found about 6000 years old pottery from Northern Europe that is used in the process of cheese making. There has also been a find of similar pottery that is 7200 years old, but the interpretation of it's use has been questioned. However, both finds are well before lactase persistence became normal in Europeans. The first time lactase persistence is documented in Europe is about 5000 years ago in southern Europe and 3000 years ago in central Europe.
There is a clear pattern between which populations developed high levels of persistence and who didn't. Logically, those who raised livestock developed the persistence, while the hunter-gatherers and those who cultivated plants but not animals didn't. However, not all groups raising and relying on animals developed lactase persistence. For example, the herding people of Mongolia have some of the lowest lactase persistence rates even thought they rely heavily on animal milk for food. It is still a big mystery for scientists why some populations developed the persistence and others didn't.
Possible theories is that part of it might be because too few people in the populations developed - and passed on - the mutation, or that people fermented or otherwise processed the milk, which lowers the lactose content a lot and thus the need to be able to produce lactase.
Milk: yes or no?
Recently, there has been a lot of debate about drinking milk, and many alternative types of milk has been introduced. Despite this however, global milk production has increased every year since 1998, and the Dairy Research Network IFCN has estimated that the milk demand will rise with 35% by 2030. However, this rise is worldwide, and in some countries milk's popularity is falling. Milk consumption in the US has decreased over the past decade, and studies found drinking milk is generally more popular with older people than younger.
Interesting enough though, the main part of the growth in demand for milk is in Asia, where most people aren't lactase persistent.
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