Fig. 2: Reduced mitochondrial diversity in British Iron Age communities.

Trends in mtDNA haplotype diversity (h) for archaeological sites with two or more individuals after pruning of first-degree pairs. Haplotype diversity is calculated as the probability that two randomly selected haplotypes are different (Methods). In the bottom panels, the h value is plotted against the normalized number of relative pairs seen for each site (1, all pairs are genetic relatives; 0, no pairs are genetic relatives; Supplementary Note 5.3). The shaded area represents the 95% confidence interval around the fitted line. There is a strong negative correlation between mtDNA diversity and the number of relatives present for Iron Age sites (Pearson correlation coefficient, P = 0.001, r = −0.449), which is not observed in previous periods of prehistory. When each period is further split into continental and insular (UK and Ireland) individuals (diamonds and circles), we find that the only significant correlation observed is for the British Iron Age (Pearson correlation coefficient, P = 5.853 × 10⁻⁷, r = −0.717). The top panels show the geographical distribution of these h values for sites with evidence of burial guided by kinship (at least one pair of genetic relatives present). Of the total 156 sites considered, 13 sites are less diverse than WBK: 12 from Britain and 1 from a Celtic La Tène period cemetery (320–180 bc) in Hungary¹⁷. The sample sizes for the h value and normalized relative pair estimation for all sites are presented in Supplementary Table 13.