text: Many known and unknown historical events have remained below detection thresholds 
  of genetic studies because subtle ancestry changes are challenging to reconstruct. 
  Methods based on shared haplotypes1,2 and rare variants3,4 improve power but are not 
  explicitly temporal and have not been possible to adopt in unbiased ancestry models. 
  Here we develop Twigstats, an approach of time-stratified ancestry analysis that can 
  improve statistical power by an order of magnitude by focusing on coalescences in 
  recent times, while remaining unbiased by population-specific drift. We apply this 
  framework to 1,556 available ancient whole genomes from Europe in the historical 
  period. We are able to model individual-level ancestry using preceding genomes to 
  provide high resolution. During the first half of the first millennium ce, we observe 
  at least two different streams of Scandinavian-related ancestry expanding across 
  western, central and eastern Europe. By contrast, during the second half of the first 
  millennium ce, ancestry patterns suggest the regional disappearance or substantial 
  admixture of these ancestries. In Scandinavia, we document a major ancestry influx 
  by approximately 800 ce, when a large proportion of Viking Age individuals carried 
  ancestry from groups related to central Europe not seen in individuals from the early 
  Iron Age. Our findings suggest that time-stratified ancestry analysis can provide a 
  higher-resolution lens for genetic history. 