Ancient DNA sheds light on indigenous Britons and Anglo-Saxon migration impact

The history of the British population has been embodied by a series of immigrations, including the well known early Anglo-Saxon migrations after 400 CE. These events, along with prior and subsequent population movements, have led to a complex ancestry of the current British population. 

The scientists in this study used whole-genome sequences of 9 excavated ancient individuals within the context of extensive genome-wide data from modern populations: 

Seven ancient genomes are sampled from a cemetery in Roman York dated between the second and the fourth century AD, one from an earlier Yorkshire Iron-Age burial (210 BC–40 AD) and one from a later neighbouring Anglo-Saxon burial (650–910 AD).

Interestingly enough they found that: 

Six of the Roman genomes show affinity with modern British Celtic populations, particularly Welsh, but significantly diverge from populations from Yorkshire and other eastern English samples. They also show similarity with the earlier Iron-Age genome, suggesting population continuity, but differ from the later Anglo-Saxon genome. This pattern concords with profound impact of migrations in the Anglo-Saxon period. Strikingly, one Roman skeleton shows a clear signal of exogenous origin, with affinities pointing towards the Middle East, confirming the cosmopolitan character of the Empire, even at its northernmost fringes.

A common theme in this study was the genomic contrast between a southern and eastern lowland zone and a western and northern upland zone. This patterning resembles the geography of Anglo-Saxon settlements in the fifth to seventh centuries AD:

…inviting the conclusion that the cultural and linguistic change effected by these migration patterns also reflected major genetic change. 

It perhaps, in my opinion, explains why the ancient Roman genomes show close affinity with modern day Welsh populations. The mountainous terrains of Wales may have protected populations that are representative of earlier, more widely dispersed indigenous British genetic strata  and shielded against major genetic change in Eastern Britain that seems to have occurred within the last millennium and a half (i.e. Viking invasions and other admixture).