The Standing Invitation

Posts Tagged ‘Lactose

Domesticated Animals

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Here at the S I we like nothing more than a nice glass of milk after a long day of studying. But drinking milk is, in evolutionary terms, a very strange thing to do.

Fact of the day: most of the peoples on Earth are unable to digest milk. Surprising, isn’t it? Note the ‘s’ in ‘peoples’; that is where the clue is.

All mammals* are able to produce milk, but humans are alone on Earth in drinking it as adults. The problem is the carbohydrate lactose, a disaccharide sugar formed by a reaction between glucose and galactose in the mammary gland. Lactose is unique to milk, being found nowhere else in the body. As such, it requires special chemical equipment to break it down and digest it.

Lactose is broken down in the small intestine by an enzyme called lactase, which, in most mammals, is produced only when the mammal is very young; production tails off shortly after birth. After lactase production shuts down, it is impossible to digest lactose in the small intestine; one becomes ‘lactose intolerant’. In this event, lactose digestion occurs downstream, as it were, in the large intestine, where decomposition by intestinal bacteria fills the gut with unwanted gas and water. Naturally, people with lactose intolerance tend to avoid milk.

But I can drink milk. If you are reading this, there is a good chance that you can too. What makes this possible? The answer is surprising.

The ability to drink milk is an evolutionary phenomenon, and it occurred extremely recently. Only ten thousand years ago, before the agricultural revolution, people with a tolerance to lactose were the minority. This made sense in the terms of evolutionary economy: since your mother will stop producing milk, why continue to produce a costly enzyme to digest it?

Nevertheless, there was variation. Some people continued to produce lactase a little longer than the others. This variation was meaningless noise until the domestication of the cow, when suddenly it became a real advantage. For the first time, people other than children were able to access the energetic and nutritional powerhouse that is cow’s milk.

In times of scarcity this additional food source was a matter of life or death. Children who were able to drink cow’s milk later in life were more likely to survive than those who stopped producing lactase early. That’s one hell of a selection pressure. Effectively, in those parts of the world where cows were domesticated, the lactose-tolerant outbred the intolerant. In short, we evolved.

‘Domestication’ is the process by which a wild animal becomes accustomed to an agricultural environment. We think of cows as being domesticated by humans, but what the story of milk tells us is that it works both ways. On a world map, the presence of milk-producing cows is tracked by a detectable change in human genetic makeup.

We domesticated the cows to produce milk more or less consciously. But without realising it, we simultaneously domesticated ourselves to consume it.


Harold McGee, McGee on Food and Cooking

Richard Dawkins, The Ancestor’s Tale

* Well, half of them.


Written by The S I

September 19, 2011 at 11:59 pm