My Big Fat European Family: What Genomics Tell Us About Shared Ancestors

Last month, a trio of engineers debuted an app that allows Icelanders to determine if they’re actually related to a potential date. Why, you ask? Because the entire population of Iceland, roughly 320,000 people, derives from a single family tree, and it’s very possible to bump into a former flame at a family gathering.

The case of Iceland is an extreme one, but the idea that we are all distant cousins, in the scope of human history, is well accepted. A new study, published today in the journal PLOS Biology, explains this degree of relatedness in modern-day Europeans.

The study reveals that just about any two random people from anywhere in Europe, even those living on opposite sides of the continent, share hundreds of genetic ancestors from only 1,000 years ago. In fact, a person living in the United Kingdom shares a chunk of genomic material with someone living in Turkey 20 percent of the time.

Researchers from the University of California, Davis and the University of Southern California studied genomic data for 2,257 Europeans from a massive database of genome-mapped individuals known as the Population Reference Sample. They measured ancestral ties going back 3,000 years by analyzing long segments of genome, passed down from generation to generation, shared by individuals.

Distant relatives share these long blocks of genome because they have both inherited them from common ancestors. First cousins share about one-fourth of their genome, inherited from a shared set of grandparents. Second cousins share just one-sixteenth of their genome, thanks to the same pair of great-grandparents. The researchers detected 1.9 million of these shared DNA sequences within the data pool, and then used their varying lengths to infer how long ago the shared ancestors lived.

These shared chunks of genome become shorter and shorter between more distant relatives because DNA strands undergo recombination, shuffling our genetic makeup around, with each successive generation. For example, a shared block of genome is shorter between second cousins than it is between first cousins. The longer a shared segment, the more recent the common ancestor.

As we might expect, the numbers of shared genetic ancestors dramatically decrease as geographic distance (in this case, across Europe) increases. This means that people who live near each other are more likely to be related to each other than those who don’t. For example, someone living in England will have a higher degree of relatedness to a fellow Briton than he would with someone from Germany. Researchers found that two modern Europeans living in neighboring populations, for example two adjacent countries, share between two and 12 genetic ancestors from the last 1,500 years.

This pattern can be seen in historically small or more isolated populations too, where fewer possible ancestors exist. Such is the case on the Italian and Iberian peninsulas—areas least affected by Slavic and Hunnic migrations between the fourth and eighth centuries—where people share more ancestors with each other than people in most other regions of Europe. Additionally, those living in Western Europe are also somewhat less related to each other than people living in Eastern Europe, a historically tight-knit region in terms of population.

However, some findings deviate from this genealogical norm. The researchers found that people from the United Kingdom shared more recent ancestors with people living in Ireland than with other UK residents. Recent ancestry also tied Germans more closely with Polish people than with other Germans. These instances likely reflect human migration in recent centuries, as smaller populations moved into larger ones.

Although this study looked only at European lineage, the researchers suggest that such patterns probably exist in the rest of the world. In any case, such research in human history brings us closer to learning more about the most recent common ancestor of all modern humans, which scientists believe who, according to mathematical models, might have walked the Earth roughly as early as 3,500 years ago (PDF). This common ancestor, a product of the intermixing of once-isolated population groups, could have lived much earlier than this if remote populations managed to prevent its members from mating with far-flung explorers, but the recent paper’s finding seems to support the idea that distant populations converged relatively recently when compared to the long history of ancient humans.

Marina Koren | | READ MORE

Marina Koren is a staff writer at The Atlantic. Previously, she was a digital intern for Smithsonian.com.