Extended Data Fig. 1: Mitochondrial haplogroup frequencies at British Iron Age sites.

Only sites with a sample size of six or higher are considered and first-degree relative pairs are removed. Of the 23 sites considered, 11 have a h value of 1 (no haplotype represented more than once). These are marked as black points on the map and are composed almost entirely of genetically unrelated individuals. Pie charts show haplogroup frequencies for the remaining 12 sites. Haplogroups with a count above one are emphasised with a black outline in the pie charts. Dotted lines are used to split haplogroups into downstream subclades based on additional mutations. Iron Age tribe names recorded by classical writers and their approximate geographic locations are shown. The haplotypes of all British Iron Age samples used in this analysis are given in Supplementary Table 12.