After the departure of the Roman state administration in AD 409, England saw the arrival of numerous Germanic-speaking migrants during the 5th century. They brought with them a foreign language (Old English), a foreign religion (Scandinavian paganism), foreign social organisation (non-urban, decentralised), foreign material culture (south Scandinavian), and their genetics have been shown to survive in English people to this day.
These people later believed that they had originated in Angeln, Saxony and Jutland, the area around the mouth of the River Elbe. Therefore we call them Anglo-Saxons.* But linguistic and genetic data don’t point to that kind of origin.
The closest documented linguistic relative of Old English is Old Frisian, whose home was closer to the mouth of the River Rhine, west of Angeln and Sachsen. The closest genetic matches to Anglo-Saxon skeletons are also found in Frisia.
So was these people’s origin story erroneous? I’ve wondered about this for years, and I just learned something that suggests no. I’m at a conference in the Netherlands, and my colleagues here explain that parts of Frisia have very little 5th century settlement at all. This seems to have been due to over-exploitation of coastal peatland in the Late Roman era. People drained the peat for agriculture, it got compacted by gravity and microbial action, the ground level sank sharply, and the sea moved in, rendering the land useless to agriculture. And when eventually people re-colonised these areas… they were using pottery that my colleagues describe as Anglo-Saxon.
So the reason that the English immigrants’ language and genetics look Frisian is probably that both England and Frisia were colonised by the same people. Or possibly even, that Frisia was repopulated from A-S England.
* I am not interested in what this term means today to US right-wing hate groups.