Stone Age Adults Couldn't Stomach Milk, Gene Study Shows
for National Geographic News
|February 26, 2007|
Milk wasn't on the Stone Age menu, says a new study which suggests the vast majority of adult Europeans were lactose intolerant as recently as 7,000 years ago.
While cow's milk is a mainstay in the diet of modern-day Europeans, their ancestors weren't able to digest the nutritious dairy product after childhood, according to DNA analysis of human skeletons from the Neolithic period.
The study was led by Joachim Burger of the Institute of Archaeology at Mainz University in Germany.
The findings supports the idea that milk drinkers became widespread in Europe only after dairy farming had become established there—not the other way around.
Most mammals lose their ability to digest milk after being weaned, but some humans can continue to benefit from the calcium-rich, high-energy liquid.
This is because they carry a mutation that lets them continue producing lactase, the gut enzyme needed to break down the milk sugar lactose, in adulthood.
Lactose tolerance is most common in people of European origin, especially those from the northern and central areas of the continent. It is relatively rare in Asian and Native American populations.
High levels of lactose tolerance among these European groups are thought to reflect an evolutionary advantage. Early farming communities that could digest milk could consume the liquid during otherwise poor harvests, for instance.
Some scientists argue this adaptation was previously very rare in humans, spreading only after the introduction of farming to Europe.
Others say prehistoric populations were already split between those who could and couldn't drink milk as adults. This split, the researchers say, determined which groups became dairy farmers.
Burger's team analysed the DNA of well-preserved Stone Age skeletons from locations in northern and central Europe.
Bones dated to between 5800 and 5200 B.C. were tested for a genetic marker associated with the production of lactase.
The team says it found no trace of the lactase gene, indicating that people from the period weren't yet able to drink milk.
The findings are reported today in the online early edition of the journal Proceedings of the National Academy of Sciences.
The study suggests that the lactase gene spread rapidly in the human population only after dairy livestock were introduced to Europe about 8,000 years ago, Burger says. (Related: "Goats Key to Spread of Farming, Gene Study Suggests" [October 10, 2006].)
"I think it's a very old mutation that was completely useless before farming started," he said.
But then the gene suddenly became useful, and its presence in the population quickly grew through natural selection, Burger said.
"People who had cows, goats, or sheep and were lactose resistant had more children, and those children survived infant mortality and years of poor harvests," he said.
The legacy of this evolutionary process is very apparent in the DNA of northern and central Europeans today, Burger notes.
In parts of Sweden, he says, 100 percent of people carry the lactase gene, whereas the average figure for the whole country is about 90 percent.
In Scandinavia, Holland, Britain, and Ireland, he added, "you can say most of the people are the descendents of dairy farmers." (See a map of Europe.)
Milk tolerance also exists in southern and eastern European populations, while certain prehistoric farming communities in North Africa and the Middle East also developed the trait, scientists say.
But in other populations the lactase gene is largely absent.
"All over the world most people can't drink milk when they're adults," Burger said. "It's only some populations in northern Africa and Europeans that can."
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