Capturing the fusion of two ancestries and kinship structures in Merovingian Flanders
In this study we report the whole-genome shotgun sequence data of 30 human skeletal remains from a coastal Late Merovingian site of Koksijde (675-750 AD), alongside 18 remains from two Early to Late Medieval sites in present-day Flanders, Belgium. We find two distinct ancestries in Koksijde, one shared with Early Medieval genomes from England and the Netherlands while the other, minor component, reflecting likely continental Gaulish ancestry. Kinship analyses identified no large pedigrees characteristic to -elite burials revealing instead a high modularity of distant relationships among individuals of the main ancestry group. The isotopic and genetic evidence combined support a model by which the burials, representing an established coastal non-elite community, had incorporated migrants from inland populations. The main group of burials shows abundance of >5cM long shared allelic intervals with the High Medieval site near Koksijde, implying continuity and suggesting that, similarly to Britain, the Early Medieval ancestry shifts left a significant and long-lasting impact on the genetic makeup of the Flemish population. We find substantial allele frequency differences between the two ancestry groups in pigmentation and diet associated variants, including those linked with lactase persistence, likely reflecting ancestry change rather than local adaptation.