Why Do Humans Drink Animal Milk?

Dairy milk has competition. Alternative “milks” made from plants like soya or almonds are increasingly popular. These alternatives are often vegan-friendly and can be suitable for people who are allergic to milk, or intolerant of it. The runner-up in the 2018 series of The Apprentice (UK) ran a flavoured nut milk business.
But the rise of alternative milks is just the latest twist in the saga of humanity’s relationship with animal milk. This relationship dates back thousands of years, and it has had a lot of ups and downs.
When you think about it, milk is a weird thing to drink. It’s a liquid made by a cow or other animal to feed its young; we have to squirt it out of the cow’s udders to obtain it.

In many cultures it is almost unheard of. Back in 2000, China launched a nationwide campaign to encourage people to consume more milk and dairy products for health reasons – a campaign that had to overcome the deep suspicions of many older Chinese people. Cheese, which is essentially milk that has been allowed to go off, can still make many Chinese people feel sick.
Milk is poured at a dairy farm in Russia. Compared to humanity’s 300,000-year history, drinking milk is a new habit (Credit: Getty)
Set against the 300,000-year history of our species, drinking milk is quite a new habit. Before about 10,000 years ago or so, hardly anybody drank milk, and then only on rare occasions. The first people to drink milk regularly were early farmers and pastoralists in western Europe – some of the first humans to live with domesticated animals, including cows. Today, drinking milk is common practice in northern Europe, North America, and a patchwork of other places.
Baby food
There is a biological reason why drinking animal milk is odd.
Milk contains a type of sugar called lactose, which is distinct from the sugars found in fruit and other sweet foods. When we are babies, our bodies make a special enzyme called lactase that allows us to digest the lactose in our mother’s milk. But after we are weaned in early childhood, for many people this stops. Without lactase, we cannot properly digest the lactose in milk. As a result, if an adult drinks a lot of milk they may experience flatulence, painful cramps and even diarrhoea. (It’s worth noting that in other mammals, there aren’t any lactase-persistent adults – adult cows don’t have active lactase, and neither do cats or dogs, for example).
So the first Europeans who drank milk probably farted a lot as a result. But then evolution kicked in: some people began to keep their lactase enzymes active into adulthood. This “lactase persistence” allowed them to drink milk without side effects. It is the result of mutations in a section of DNA that controls the activity of the lactase gene.
“The first time that we see the lactase persistence allele in Europe arising is around 5,000 years BP [before present] in southern Europe, and then it starts to kick in in central Europe around 3,000 years ago,” says assistant professor Laure S├ęgurel at the Museum of Humankind in Paris, who co-authored a 2017 review of the science of lactase persistence.
The lactase persistence trait was favoured by evolution and today it is extremely common in some populations. In northern Europe, more than 90% of people are lactase persistent. The same is true in a few populations in Africa and the Middle East.
But there are also many populations where lactase persistence is much rarer: many Africans do not have the trait and it is uncommon in Asia and South America.

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