Thanks for the public post, and a good question!

The recent studies have looked at rates of random mutations in the different genetic strings, and have tried to model how long ago these mutations occurred. The 2015 study in Nature, which argued that the Anglo-Saxon migration could be identified in modern genetics, did so because they found traces of shared DNA in modern N. Germany and SE Britain (GER3) that appeared to have drifted apart for about 1200 years, suggesting to the authors a divergence in the early middle ages as a result of the Anglo-Saxon migrations. The same study looked for parallels between modern British and Scandinavian genetics, and didn't find strong parallels; they concluded that the Viking invasion had little impact on Britain's modern genetic profile.

Except wait, 1200 years ago -- weren't the Anglo-Saxon migrations 1500 years ago? Are the models that imprecise? It turns out, the study authors had to stretch their conclusions to make their evidence fit with the Anglo-Saxon migrations in the fifth or sixth centuries. They decided -- to fit the data to their hypothesis -- that the migrant Anglo-Saxons didn't intermarry with the local population until 200+ years after their arrival. As a consequence, the genetic split (they argued) didn't happen until a few centuries after the initial migrations, the -1200 years before today that their models predicted as most likely. This theory of non-intermarriage isn't completely out there, but it's very unlikely (one well-publicized article has argued that Anglo-Saxons didn't marry locals; but almost all the other evidence suggests that they absolutely did). To me, it feels like a stretch -- an attempt to make the data tell a story that's not there. The genetic divergence between N. Germany and the modern British population there study found happened in the 8-9th century, not the 5-6th. All this is buried in the supplementary files attached to the published report, but didn't make it into the news articles.

A recent study that came out a month ago has highlighted precisely this issue, and argues that it makes more sense to see this common bit of genetic heritage as resulting from the Viking invasions. This brings us much closer to the -1200 year estimate for the migration date. The earlier study's authors, you see, had assumed that Vikings and Anglo-Saxons would each be most closely related to the modern populations of the countries from which they had originated -- that Anglo-Saxons would be most closely related to modern Germans, while Vikings would be found in modern Scandinavians. They didn't question whether the N. German genetics they found crossing the North Sea in the Viking Age might have been the Viking invaders they struggled to find when they looked at modern Denmark. To know more, we'll have to do tests on skeletons from medieval cemeteries -- something that's becoming increasingly possible, thanks to improving methods for sequencing full human genomes in ancient (degraded) human bone.

Of course, this all assumes that these shared genetic markers come from single migration events. What if migration were ongoing? In fact, it was: a study published last winter of ancient bones in Cambridgeshire found first-generation Continental migrants in the sixth century, and again in the eighth. Migration was a normal fact of life in the Roman empire, including migration of German barbarians into the empire to serve in the military. The studies that have claimed to measure Anglo-Saxon or Viking invasions are looking for a single event, but their data could be easily explained through much longer, lower-intensity processes of movement around the North Sea.

The one thing that no one is talking about (to my knowledge -- there may be something I've missed?) is the genetic impact of the Norman Conquest. I'm honestly not sure if this is because there' s little evidence of an impact (there just wasn't much there), or if there are further assumptions that are clouding our ability to spot that event with any clarity.

At all events, this is far from an exact science. Or more precisely: the science is exact, but the results depend a lot on the assumptions we feed in. We've a lot to discover still before we finish untangling the data.