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Modern Europeans Share a Number of Recent Genetic Ancestors

NEW YORK (GenomeWeb News) – Modern Europeans, even ones who live in different regions of the continent, share a number of genetic common ancestors from the past 2,000 years, according to a PLOS Biology paper that appeared online yesterday.

Drawing on genome-wide data from about 2,250 Europeans, Peter Ralph and Graham Coop from the University of California, Davis, calculated that a pair of present-day Europeans who live in neighboring regions share between two and 12 genetic common ancestors from the last 1, 500 years, and near the tune of 100 genetic common ancestors from the last 2,500 years.

"What's remarkable about this is how closely everyone is related to each other. On a genealogical level, everyone in Europe traces back to nearly the same set of ancestors only a thousand years ago," said Coop, an associate professor at UC Davis, in a statement. He added that that had been theorized, but now there was DNA-based evidence to support the idea.

Ralph, who is now at the University of Southern California, and Coop noted, though, that there were regional differences in the number of shared genetic ancestors. For example, low levels of common ancestry were found in both the Italian and Iberian peninsulas, perhaps reflecting historical events or barriers like the Pyrenees.

To examine the recent genetic ancestry of Europe, Ralph and Coop turned to the European subset of the Population Reference Sample dataset and pulled out data from 2,257 people from 40 populations who had been genotyped at half a million SNPs.

If a pair of people shared a common ancestor, they both may have inherited stretches of their genomes from that common ancestor, regions called "identical by descent." Such IBD regions are often broken up over time as they are passed on to new generations and undergo recombination — the longer ago a shared ancestry was, the more broken up the shared bits of DNA may be.

Long, shared segments can be used to infer common, recent ancestry. "By observing the number of shared genomic blocks, we learn about the degree to which [two individuals'] genealogies overlap, or the number of common ancestors from which both individuals have inherited genetic material," Ralph and Coop wrote.

Using fastIBD, the duo found that the 2,257 Europeans in their dataset shared 1.9 million IBD segments, with any given pair sharing 0.74 segments, on average. Further, it noted that IDB blocks had a mean length of 2.5 cM and that their density was generally constant throughout the genome, with some distribution differences near centromeres.

The populations of modern European countries, the researchers found, show statistically significant intra-country heterogeneity. This, they added, is perhaps not surprising as modern countries are fairly new constructs. Italian samples, however, appeared to share few genetic common ancestors from the past 2,500 years with other European populations. Further, ancestral variations seen within countries seemed to reflect a geographic continuum.

Continent-wide, most individuals tended to share more IBD blocks with other members of their population. However, individuals in the UK shared more blocks, on average, with individuals from Ireland, while Germans shared more blocks with Polish individuals, a pattern, the researchers noted, that could be due to asymmetric migration from a smaller population into a larger one.

By grouping the 40 populations into five larger European regions — east, north, west, the Iberian and Italian peninsulas, and Turkey and Cyprus — Ralph and Coop examined broader sharing of IBD blocks. The average number of shared blocks, they noted, decayed exponentially with distance.

Pairs of Eastern Europeans exhibited rather high levels of IBD sharing, similar to levels seen in much smaller populations. This, the researchers said, indicated that Eastern Europeans have a bit of their ancestry drawn from a small population that expanded over a large geographic area — and it also is consistent with the migration period that occurred between the fourth century and the ninth century, during which the Huns moved into Eastern Europe, Germanic groups extended into the western Roman Empire, and Slavic populations expanded.

By looking at the distribution of IBD blocks, the researchers calculated — assuming a mean human generation time of 30 years — when pairs of individuals last had a genetic common ancestor. While only members of the same population likely shared genetic common ancestors within the past 500 years — with some exceptions for neighboring countries like the UK and Ireland — between 500 years and 1,500 years ago, people generally shared tens to hundreds of genetic common ancestors. Stretching further back into history, pairs of European individuals likely shared hundreds of common ancestors, and perhaps more.

"The overall picture is that everybody is related, and we are looking at only subtle differences between regions," Coop said.

There were, the researchers noted, some differences, particularly on the Italian peninsula and the Iberian peninsula.

People from the Italian peninsula shared little recent common genetic ancestry with people from other areas — and what shared ancestry there was dated back past 2,500 years ago — except for nearby Balkan populations. They, the researchers found, shared a small but number of common ancestors within the last 1,500 years or so. Additionally, populations within Italy shared few genetic common ancestors with each other.

"This suggests significant old substructure and large population sizes within Italy, strong enough that different groups within Italy share as little recent common ancestry as other, distinct modern-day countries, substructure that was not homogenized during the migration period," the researchers wrote.

Similarly, populations in Spain and Portugal had low numbers of recent, common ancestors with other European groups, but they have higher levels of IBD block sharing within their population than the Italians. This suggested to the researchers that the lower level of shared ancestry on the Iberian peninsula is due to geographic isolation.

Meanwhile, the highest rates of IDB within a population was found in people who spoke Albanian, sharing about 90 ancestors in the last 500 years and about 600 ancestors between 500 years and 1,500 years ago. Ancestors of modern-day Albanian speakers, Ralph and Coop wrote, likely drew from a small, cohesive population.

Overall, the researchers noted that Europeans appear to be closely genealogically related in a short time span. This finding, they added, "lends substantial support to models predicting close and ubiquitous common ancestry of all modern humans."

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