Britain has a long history of invasions: over the past two millennia, various armies from the Romans to the Anglo-Saxons conquered the bulk of the British Isles. A new genetic analysis of the country has revealed which invading force had the greatest impact on its DNA.

The UK can be split into 17 genetically distinct groups

Britons share the most DNA with people from France and Germany — countries which were home to the Angles and Saxons that moved into the British Isles after Roman rule collapsed in the 4th century. And despite broad similarities, the UK can be split into 17 distinct genetic groups that correspond to modern regions, according to a paper published in Nature that analyzed genetic data from more than 2,000 individuals in the UK and more than 6,000 across Europe.

"It’s really the first time that scientists have looked in great detail within a country at patterns of genetic variation," said Peter Donnelly, a professor of statistical science at the University of Oxford and one of the paper's lead authors.

The genetic data was collected from Caucasians living in rural areas. Scientists selected people whose grandparents were all born within 80 kilometers of each other. "Because we inherit DNA from our parents," said Donnelly, "and they inherit their DNA from their parents... If was as if we were able to sample DNA from the population at the time the grandparents were born, and that’s roughly in the late 1800s."

A map of the UK showing genetic clusters. Each combination of shape and color represents a distinct group. Image credit: Stephen Leslie.

This method allowed the scientists to focus on early influences on the British population and ignore the upheavals in the 20th century caused by increasing urbanization. Once the genetic data had been compiled — a process that took 20 years — it was then sorted into clusters based on differences and similarities in the DNA.

Of all the forces that invaded Britain, only the Anglo-Saxons had a lasting genetic effect, the study showed. Individuals in the study shared almost 30 percent of their DNA with people from what is now Germany, while those in the south and central parts of England shared around 40 percent of their DNA with people from the French region.

what happened after the romans left?

This data also helps solve the dispute over what happened in the 4th century when the Romans left Britain and the Anglo-Saxons moved in. "What we know from the historical evidence is that society collapsed," said archaeologist Mark Robinson, an author on the paper. "What was Roman Britain essentially became a failed state with warlords fighting each other and economic decline."

Historians have suggested the Anglo-Saxons that subsequently moved in purged the Romano-British people, a theory supported by the sudden shift in culture such as the changing in pottery to Germanic styles. However, the shared genetics between individuals in central England and Europe suggests that the integration was more peaceful. "It is not genocide or complete disappearance of Britons," said Robinson.

That’s in contrast to the Danish Vikings who ruled the same central swath of Britain from around the 9th century but left little genetic mark on the population. This suggests their power came from a ruling military elite, rather than a large-scale movement of settlers.

"genetics tells us about the vast bulk of people."

"It’s interesting when you think about the sources of information we have for this kind of history," said Donnelly. "All of those sources [such as the archaeological and written records] predominantly tell us about the successful and ruling elite in society…. But what genetics tells us about is what’s happening to the vast bulk of people, to the masses, rather than to the elite."

Not only did the data align with historical events, but also political and cultural divides. For example, the population of Orkney (a set of small islands off the northeast coast of Scotland) was found to be the most genetically distinct from the rest of the UK, with a quarter of the residents' DNA coming from Norwegian ancestors. This fits the historical record, since Orkney was settled by Norwegian Vikings around 800 AD, and remained part of their country for the next 600 years.

However, even certain parts of mainland Britain — such as Wales, on the Western flank of the country — were found to be distinct from the rest of the UK. The Welsh are also the closest genetic match to the first settlers in Britain following the end of the last Ice Age, around 10,000 years ago.

century-old genetic divides "almost exactly match the modern county boundaries"

Clear genetic divides were also found between populations living very close to one another. Genetics differences between neighboring Devon and Cornwall, for example, "almost exactly match the modern county boundaries," according to Donnelly.

The power of politics over genetics is most clearly shown in the data from the center and south of England. Historically, this area had tended to be brought under the rule of a single invading force — first by the Romans, for example, and then later by the Anglo-Saxons. Robinson suggests that this is reflected in the fact that the largest genetically cohesive in the study (containing almost half the individuals analyzed) is found in this region. Because the invading forces broke down geopolitical boundaries, says Robinson, it "homogenized genetic signals in that area."

"This [sort of genetic changes] doesn't need people to move a long way away, " explains Robinson, it just needs a little bit of movement over many generations.