Analyzing DNA from four ancient skeletons and comparing it with thousands of genetic samples from living humans, a group of Scandinavian scientists reported that agriculture initially spread through Europe because farmers expanded their territory northward, not because the more primitive foragers already living there adopted it on their own.

The genetic profiles of three Neolithic hunter-gatherers and one farmer who lived in the same region of modern-day Sweden about 5,000 years ago were quite different a fact that could help resolve a decades-old battle among archaeologists over the origins of European agriculture, said study leader Mattias Jakobsson, a population geneticist at Uppsala University in Sweden.

The hunter-gatherers, from the island of Gotland, bore a distinct genetic resemblance to people alive today in Europe's extreme north, said Jakobsson, who reported his findings in Friday's edition of the journal Science. The farmer, excavated from a large stone burial structure in the mainland parish of Gokhem, about 250 miles away, had DNA more like that of modern people in southern Europe.

"People have known for some time that agriculture spread from the Middle East to Eastern Europe and northward and westward," Jakobsson said. "But it's been difficult to determine if people migrated and brought farming with them, or if local hunter-gatherers changed their practices."

The study joins a growing body of work, assembled over the last decade, that aims to settle lingering debates over early human history by examining ancient DNA.

One such controversy is how agriculture, which emerged 11,000 years ago in the Middle East, spread through Europe over the course of several thousand years. It's a subject that has fascinated archaeologists for decades because the shift to farming fueled "the storing of goods and the beginning of money and all of that stuff," said population geneticist Joachim Burger of the Johannes Gutenberg University in Mainz, Germany, who wasn't involved in the new study. "It's the origin of our civilization."

While artifacts like pottery and stone tools make clear that some ancient people were hunter-gatherers and some were farmers, scientists haven't been able to say with certainty whether the migration of people or the spread of ideas pushed farming practices north.

To figure that out, Jakobsson said, "you really need to get into the genetics of those human remains."

The first wave of studies of ancient genes focused on mitochondrial DNA, the 13 genes that control the energy sources of cells and are passed directly from mother to child.

Jakobsson and his colleagues examined a wider selection of so-called nuclear DNA, the kind inherited from both parents that contains instructions to build and control all aspects of an organism. They were able to read nearly 500 million of the As, Cs, Gs and Ts extracted from the ancient cells.

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Ancient DNA sheds light on spread of European farming