Milking our evolutionary advantage

CHINESE scientists have genetically modified 300 cows to enable them to produce human-like breast milk.

The new milk — created by inserting human DNA into the animals’ cells — has a strong taste, but it contains key nutrients and boosts the immune system.

Not every mother produces enough milk for her baby. Hopefully, cow-derived mother’s milk will soon be available in supermarkets and prevent shortages.

A huge change is taking place in world food consumption. Beef did not feature in the traditional Chinese diet, but food preferences are changing in the most populous country. Demand for meat is soaring there. Cattle ranching is set to become widespread and will increase milk production. Humans are unique in being able to drink milk.

There’s a problem: adult Chinese can eat yoghurt and cheese, but get sick if they drink milk. It’s no surprise it hardly features in their cuisine. Few Asians can digest milk and alcohol.

Nor is allergy to milk confined to China — only a minority of people world-wide can drink it. Nineteen out of every 20 Swedes can, but only one in 10 Sicilians. The reason is that lactose, one of the constituents of milk, is indigestible. Babies produce an enzyme to break lactose down into other proteins, which their systems can deal with. When a baby is weaned, the enzyme is no longer required and the body stops producing it. Milk then becomes indigestible.

We northern Europeans are unusual in continuing to produce the enzyme beyond babyhood. This is due to a genetic mutation, which we picked up thousands of years ago when herding cattle for their meat. Individuals carrying the modified gene were able to digest milk as adults and this conferred a survival advantage on them. They prospered at the expense of their non milk-drinking peers and so, over the generations, being able to consume milk became the norm. Nigerian herders developed a similar ability, but, in parts of the world where cattle raising was not widely practised, the change did not occur.

Although pigeons and flamingos also feed their young on it, milk is the great mammalian invention. It helped furry creatures to become the most sophisticated animals on Earth. We humans have a further string to our bow; we are able to drink the milk of other species. Cows and goats are the suppliers in our part of the world, but, elsewhere, horses, camels, reindeers, sheep, yaks and water-buffaloes are pressed into service.

The geneticist Steve Jones, outlining the evolution of milk-drinking on RTÉ’s Mooney Show, speculated on the possible origins of a related digestive change: the capacity to imbibe alcohol. People in many parts of the world become ill if they drink this poisonous substance. Jones said that our Western ability to do so may have its origins in dirty water. Obtaining clean water was not a problem when we were hunter-gatherers. We were constantly on the move, so wells and ponds did not become contaminated with our wastes. When humans opted for a settled lifestyle, that changed; water sources close to habitations became polluted, mainly by sewage leaching through the ground. Clean water was almost impossible to obtain in medieval cities and towns; people died of dysentery and other water-borne diseases. Outbreaks of cholera are common in unsanitary areas even today.

Beer contains alcohol, which is poisonous to the lethal organisms present in dirty water. It was safer to drink beer wherever water quality might have been compromised.

Even so, our digestive systems had to change so that they could deal with alcohol. Youthful first encounters with this potentially nasty substance are almost always unpleasant; most of us became sick when we drank it for the first time. If we persisted in doing so, our bodies soon manufactured appropriate enzymes so that we could drink modest amounts. However, when we overdo things, we still develop hangovers. The Chinese drank tea, the boiled water for which was relatively sterile and there was no need to resort to using alcohol.

Evolution, clearly, is not a one-way process; local environment and lifestyle also drive genetic change.


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