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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