The history of Europe is written in its people's DNA.
The Huns and the Slavs made incursions into Eastern Europe about 1,500 years ago. Migrants moved from Ireland to England in recent centuries. Populations in Italy and Spain have been comparatively stable.
None of this is breaking news. But scientists were able to see it anew by examining the patterns of genes in 2,257 people now living in 40 countries on the continent.
It's surprising "how much past history was still evident in the patterns we've seen," said Peter Ralph, a computational biologist at USC who reported the findings Tuesday in the journal PLOS Biology.
Ralph and his former postdoctoral advisor, Graham Coop, a geneticist at UC Davis, conducted their analysis by looking at the Population Reference Sample data. The data include language and country-of-origin information for several thousand European people, along with DNA sequences covering 500,000 locations on the genome that are known to vary from person to person.
Coop and Ralph used computer programs to ferret out sequences that were identical or nearly identical and used the matches to figure out who was related to whom.
Following the DNA trails they encountered, the researchers were able to confirm that Europeans living near one another were more closely related than Europeans living farther apart.
They were also able to put a time frame on the genetic relatedness they saw, by examining the length of the DNA segments the people shared.
The DNA sequences you have in common with each of your parents are quite long, the chunks you share with each of your four grandparents are half as long, and the bits you share with each of your eight great-grandparents are half as long again.
"The longer ago an ancestor is, the shorter the chunk is likely to be," Ralph said.
Out of the roughly 3 billion base pairs in the human genome, Coop and Ralph looked for shared segments that were on the order of millions of base pairs long; statistically, matches any shorter could be a chance result. They calculated that these segments could reveal shared ancestry stretching back about 100 generations, or 3,000 years.
It turned out that pairs of people living as far away from each other as Britain and Turkey shared the DNA chunks 20% of the time, the researchers discovered. That meant that each pair would have to have had at least thousands of common ancestors over the last 1,000 years, they wrote.
Going back a few thousand years, Coop and Ralph were able to show that everyone on Earth is related to everyone else.
"It's a nice illustration of how interconnected human ancestry is," said John Novembre, a professor of human genetics at the University of Chicago who was not involved in the research. "It's been expected theoretically, but here they show it empirically."
Coop and Ralph also looked at the length of the shared DNA segments to link genetic differences among populations to historical events and trends. Among other things, they found that modern-day Italians were not as closely related to one another as people in other countries were. That could reflect relative stability in the region that is now Italy, they said.
The researchers found more recent shared ancestry — that is, longer chunks of shared DNA — among some people from southeastern Europe. That could be a genetic signature of the influxes of Huns and Slavs into the region around the time of the fall of the Roman Empire, they wrote.
John Hawks, a paleoanthropologist at the University of Wisconsin in Madison who was not involved in the research, said that although the new research was interesting, there were many "key insights" about humanity's past still waiting to be revealed in the human genome.
"Europe happens to have a really good genetic sample and really good match of geography to genetic variation," Hawks said. "In some other parts of the world, like Africa, things are not so simple."
Researchers have examined the genomes of African Americans to try to learn about their recent history, Ralph said.
The technique could also prove useful in studying other living things, he added: A survey of humpback whale DNA could shed light on population sizes in the pre-whaling era, for instance, or the genes of a particular kind of tree might tell scientists something about how that species has responded to glaciation in the past.