The Article's Conclusions
A massive influx from Germanic Europe of whole families, complete with livestock, probably numbering 200,000 people crossing the North Sea in all from about 450 CE to 550 CE, probably started closer to 407 CE, when the Roman armies left Britain after three hundred and sixty years in the face of a collapsing empire, than the ca. 1000 CE Norman conquest of popular historic myth. Germanic populations started establishing cemeteries by 410 CE. Histories and the earliest Old English literature, while somewhat patchy from that era, support this view.
Initially, the newcomers were outnumbered 5-1 or more by native Britons, but the Germanic invaders superiority in combat (and perhaps in running a society not under Roman rule in general) has since left half of British men with their Y-DNA legacy today. "People from rural England are more closely related to the northern Germans than to their countrymen from Wales or Scotland." This was not merely a case of a shift of a thin ruling elite of soldiers. Germanic genetic impact is unsurprisingly lower in Scotland and Wales, where some Celtic linguistic community also survived, is moderate in Northern Ireland (no doubt due to migrations from Britain to to Ireland much later in the historic era), and is almost nil in the Irish Republic, although it is somewhat shocking to see just how much Irish-British ethnic differences are not just religious and cultural and linguistic but are also a matter of population genetics.
The famous legend of King Arthur also originated in that era -- as a form of counter-propaganda. Historians characterize the work as a "defensive myth" created by the original Christian inhabitants (with the Holy Grail possibly symbolizing the communion cup). Perhaps the King Arthur legend is based on a mythical Celtic king who won a victory at Mount Badon around 500 A.D.
In truth, however, the army of the Britons was usually in retreat. Many fell into captivity. . . the captured Britons lived a miserable existence as "servants and maids" in the villages of the Anglo-Saxons. There were two types of grave in the cemeteries of the time: those containing swords and other weapons, and those with none. The local inhabitants, deprived of their rights, were apparently buried in the latter type of grave. . . . [T]he conquerors from the continent maintained "social structures similar to apartheid," a view supported by the laws of King Ine of Wessex (around 695). They specify six social levels for the Britons, five of which refer to slaves.
The Context of the King Arthur Myth
The context for the King Arthur myth, sometimes cast as the last defender of a Celtic pagan social order against Christianizing invaders, in accounts rich enough for readers to discern, rather than the product of Romanized Christianized British natives against invading Germanic invaders who may themselves have been pagan, is also something of a surprise.
Layers of Genetic Heritage In The British Isles
There is a strong tendency for invading populations to have more impact on Y-DNA (which is patrilineally inherited) than mtDNA (which is matrilineally inherited) or autosomal DNA (inherited from both parents, more or less equally if natural selection does not intervene). But, even if the patriline was 50% Germanic, while the matriline was 0% Germanic, once would expect the British to have 25% Germanic heritage, and given the apparently more demic than commonly understood structure of the Anglo-Saxon colonization of Britain, estimates of 50% Germanic patrilines and 30% Germanic matrilines, for a combined roughly 40% Germanic autosomal ancestry is probably a more reasonable estimate.
Tools that are designed to measure the autosomal contributions of hypothetical separate ancestral populations like Admixture, however, would probably not give you that figure because the gene pools of Germanic peoples ca. 500 CE and Celts in Britain of the same era probably had significant overlap. Both, for example, were Indo-European language speakers of somewhat related branches of that linguistic family, and language shift probably didn't happen as completely as it did at the time of Celtic settlement without some meaningful demic component. Indo-European Celtic communities achieved linguistic and cultural dominance, although their genetic impact can be debated because we don't know for certain what the genetic makeup of pre-Celtic residents of the British Isles was before they arrived, thousands of years earlier than the Anglo-Saxons.
On the other hand, it isn't entirely possible to rule out the possiblity that some of the Y-DNA attributed to Anglo-Saxon invaders with "Friscan" DNA markers in the study illustrated at the top of this post may have been the product of more peaceful earlier migrations to Britain, either through population transfers in the Roman era, or interchange of people during or before the Celtic era. The close alignment of these DNA markers with areas that kept Celtic language speaking communities into the 21st century, however, disfavors that possibility as an important factor.
Celtic conquerors, at least, who probably started to arrive in force sometime after 1,500 BCE, and their Germanic successors, however, as well as Romans, all probably had more in common with each other genetically than they did with the pre-Celtic people of Britain during the Neolithic era and early metal age associated with megaliths like Stonehenge (whose construction ceased close in time to the arrival of the Celts and whose religion probably ended with their language and culture).
The bearers of Neolithic culture to Britain (most notably farming and herding), in turn, probably in turn had replaced or greatly diluted prior Upper Paleolithic hunter-gatherer societies of Britain around the time range of 5,000 BCE to 3,500 BCE, and were probably closer genetically to later waves of invaders from the European Continent than to the Upper Paleolithic peoples of Britain who had lived there since at least the retreat of the glaciers after the Last Glacial Maximum (a repopulation that would have begun a few thousand years after 20,000 B.C.E.), and possibly was continuous from ten thousand years before that in the first wave of anatomically modern humans that arrived in Europe, who replaced the Neanderthals (the question of how that happened is for another day).
Thus, while Britain has been continuously inhabited by members of the genus Homo for many tens of thousands of years, maybe even hundreds of thousands of years or more, a huge component of the population genetics of England can be traced to migrants from just 1,600 years ago, and very little of the modern English gene pool can be traced back in Britain any further than migrants from 6,500 years ago. In between, starting at about 3,500 years ago (the Celts) and at about 1,900 years ago (the Romans), there were at least two significant infusions of migrants who changed the gene pool, although this is harder to quantify the extent of those impacts off the cuff.
The Romans, however, didn't conquer Ireland (although its non-demic Roman Catholic missionary effort in Ireland was a leading source for the Christianization, or re-Christianization in some cases, of Northern Europe after the fall of the Roman Empire), nor did they conquer Scotland (which was placed in personal union under the same monarch with England in the 18th century), and so presumably didn't leave the same kind of mark there.
It also seems plausible that the early farming techniques of the megalithic peoples of the British Isles were less advanced than that of later migrants, so in some places that were less suitable for their style of farming, the first major demographic transition may have been directly from Upper Paleolithic hunter-gatherers (perhaps admixed with outcast members of the megalithic population to some small extent) to a somewhat more agriculturally advanced Celtic food producing population.
Big Picture Observations and Caveats
My background in history, the advances of the last decade or two in population genetic and even more so in ancient DNA, the absence of excess Neanderthal admixture in Europe, and the archaeological and physical anthropology evidence, as well as the linguistic evidence all incline me to favor an interpretation of ancient history and pre-history that has only weak continuity between present European populations and the Paleolithic era, and increasingly to see significant population genetic shifts in much of Europe since then.
It is also my tendency, within the debates over Indo-European linguistic origins to see the evidence pointing towards a transition to Indo-European languages in most of Europe outside its proto-language area considerably later in history than the more "conservative" Indo-Europeanists see its arrival.
Britain is an attractive area to analyze in this regard because the amount of archaeological research done there has been exhaustive, because it was one of the later parts of Europe to experience the Neolithic revolution of food production, because the gap there between food production and reasonably credible history is narrower than in almost any other place in Europe since it was included in the literate and well documents Roman Empire, and because the linguistic and archaeological and population genetic record can fill in the gaps between the dawn of the Neolithic revolution and the arrival of the Romans pretty completely with some confidence. Its status as an island also tends to make its instances of migration and admixture likely to be less gradual than places on the continent. The English Channel was a powerful barrier to casual admixture at any significant rate for a long time even though England was connected to maritime trade routes from the earliest days of its acquisition of agriculture and probably before then.
Of course, England also has the virtue of being a fairly familiar place that I can claim some understanding of at an ethnographic level and I've been there, so the geographic is not abstract to me.
Food producing pre-history is about three thousand years older in other parts of Europe (and, of course, older than that in Anatolia and the Middle East), roughly doubling the time period between the Neolithic transition and the start of a significant written historical record.
Still, the pre-history of the British Isles is much more comparable to that of the rest of Europe than it is to that of the New World.
There is good reason to think that some other parts of Europe had more continuity between the Upper Paleolithic and the Neolithic, particularly the Cardium Pottery and Iberian Southern coast of Europe, and that some parts of Europe have likewise had more continuity between the early Neolithic and modern times, something that ancient DNA suggests was far less pronounced in the far reaches of the Danubian Neolithic, for example. The greater genetic diversity in Southern Europe relative to Northern Europe, roughly speaking divided at the olive oil-butter line, is also suggestive of these different prehistorical experiences.
Looking at population genetic principal component analysis charts of Spain v. Northern European countries, for example, it is clear that Spain is much more diverse than Northern Europe and the Portugal also has stronger genetic ties to Northern Europe than the rest of Spain. Far lower levels of lactose tolerance in Southern Europe are also suggestive of deeper ancestral roots, and the archaeology suggests that Southern Europe had fairly significant fishing and coastal forager settlements even before terrestrial food production took hold and that it experienced less discontinuity than is seen in Northern Continental Europe and the British Isles.
Much of Continental Europe, particularly further South, also simply has a more tangled population history than the British Isles.
Still, even there, the population history of Europe appears to have involved more demographic upheaval, often of the violent variety, than most anthropologists, archaeologists and ancient historians of my father's generation were inclined to imagine. Britain may be one or the more extreme examples within greater Europe, but it is hardly singular or completely exceptional either.