The two lineages "don't look like the complete set of ancestors necessary to build the modern gene pool," Burger said.
The puzzle highlights how little we know about where modern Europeans came from.
"Another, unidentified factor must come in, maybe an additional migration" or genetic mutation, he said.
Archaeologist Ron Pinhasi, of University College Cork in Ireland, agreed that more ancient migrations might await discovery.
Pinhasi, who was not involved in the new study, said the report "raises the possibility that the modern European genetic structure has been shaped by a series of subsequent migrations [or] dispersals during prehistoric and historic times."
Where Did Europe's First Farmers Come From?
Though study author Burger is placing his bets on southeastern Europe—specifically parts of what are now western Hungary and southwestern Slovakia—no one knows where the early immigrant farmers came from.
To pinpoint the pioneers' origins, Burger, Pinhasi, and others are working on a separate project, which also uses genetic material from skeletal remains.
According to Burger, it's possible that the first farmers in Europe were part of a vast chain of farming populations that stretched perhaps as far as the ancient Near East, including Anatolia (now Turkey) and Mesopotamia (roughly present-day Iraq)—where agriculture is thought to have been born about 11,000 years ago.
Pinhasi recently reached the same conclusion—detailed in a separate study published in the journal PLoS ONE in August—by comparing skulls from hunter-gatherers and early farmers found at sites from Europe to the Near East.
More evidence that these far-flung farming populations were genetically similar should be forthcoming, he said. And that would confirm "that agriculture was certainly introduced into Europe from western Anatolia."
SOURCES AND RELATED WEB SITES