The results confirm decade-old mathematical models, but will nevertheless come as a surprise to Europeans accustomed to thinking of ancient nations composed of distinct ethnic groups like "Germans," ''Irish" or "Serbs."
"What's remarkable about this is how closely everyone is related to each other," said Graham Coop of the University of California, Davis, who co-wrote the study published Tuesday in the journal PLoS Biology.
Coop and his fellow author Peter Ralph of the University of Southern California used a database containing more than 2,250 genetic samples to look for shared DNA segments that would point to distant shared relatives.
While the number of common genetic ancestors is greater the closer people are to each other, even individuals living 2,000 miles (3,220 kilometers) apart had identical sections of DNA that can be traced back roughly to the Middle Ages.
The findings indicate that there was a steady flow of genetic material between countries as far apart as Turkey and Britain, or Poland and Portugal, even after the great population movements of the first millennium A.D. such as the Saxon and Viking invasions of Britain, and the westward drive of the Huns and Slavic peoples.
The study did find subtle regional variations. For reasons still unclear, Italians and Spaniards appear to be less closely related than most Europeans to people elsewhere on the continent.
"The analysis is pretty convincing. It comes partly from the enormous number of ancestors each one of us have," said Mark A. Jobling, a professor of genetics at the University of Leicester, England, who wasn't involved in the study.
Since the number of ancestors each person has roughly doubles with each generation, "we don't have to go too far back to find someone who features in all of our family trees," he said.
Jobling cited a scientific paper published in 2004 that went so far as to predict that every person on the planet shares ancestors who lived just 4,000 years ago.
Experts say the study's findings need to be compared with what we know about population movements in Europe and elsewhere from other fields, including archeology and linguistics.
"Although, as the authors note, the approach is inherently 'noisy' (i.e. error-prone), it still does give results for European populations that are in reasonable agreement with historical expectations," said Mark Stoneking, a professor evolutionary anthropology at the University of Leipzig, Germany, who also wasn't involved in the study. "It would be interesting to see this applied in situations where we don't have such good historical information."
Coop and Ralph said the findings might change the way Europeans think about their neighbors on a continent that has had its fair share of struggle and strife.
"The basic idea that we're all related much more recently than one might think has been around for a while, but it is not widely appreciated, and still quite surprising to many people, even scientists working in population genetics, including ourselves," they said in an email to The Associated Press. "The fact that we share all our ancestors from a time period where we recognize various ethnic identities also points at how we are like a family -- we have our differences, but are all closely related."
Just don't expect news of closer family ties to prompt a surge of brotherly love in Europe or elsewhere.
"There have been many studies that we've been involved in showing that groups which are fighting each other furiously all the time are actually extremely closely genetically related. But that's never had any impact on whether they continue to fight each other," Jobling said.
"So for example Jewish and non-Jewish populations in the Middle East are extremely similar genetically, but to tell them they are genetic close relatives isn't going to change their ways."
Author's FAQ on the study: http://gcbias.org/european-genealogy-faq/
Copyright 2013 The Associated Press.