A link to the past: mapping co-ancestry connections between a Medieval and modern European genomes. Esquerda individual

Historical genetic links among similar populations can be difficult to establish. Identity by descent (IBD) analyses can identify genomic blocks that confirm direct genealogical relationships among individuals. However, this method has rarely been applied to ancient genomes because IBD stretches are progressively fragmented by recombination and thus not recognizable after few tens of generations. To explore such genealogical relationships, we estimated all IBD blocks longer than 6 centimorgans (cM) among modern Europeans, applying principles of graph theory to uncover the genetic structures. We found that Basques, Sardinians, Icelanders and Orcadians form highly interconnected sub-clusters in a European network, indicating dense genealogical links across small, isolated populations. This approach also exposes individual genealogical links -such as the connection between one Estonian and four Icelandic individuals- that cannot be uncovered with standard population genetics methods. Moreover, using ancient DNA technology we sequenced a Late Medieval individual (Barcelona, Spain) to high genomic coverage and identified IBD blocks between her and modern Europeans. Compared to the geographically structured pattern seen in modern Iberians and Europeans, the genomic blocks of the Late Medieval genome are shorter and more dispersed across the continent, which is consistent with 600-years of recombination breaking down the blocks. The longest IBD blocks tend to be shared with geographically close populations such as Iberians and French. When repeating the analysis with one random modern individual from the same region, the genomic network reveals a higher degree of clustering, again with predominant links to Iberian and French populations. With an increasing number of modern and ancient genomes becoming available, this method can be used to produce a fine-scale reflection of shared ancestry across different populations of the world, offering a direct genetic link from the past to the present.