text: **Ancient sample selection.**A full list of ancient genomes can be found 
  in Supplementary Table 1. Published ancient shotgun genomes pro- 
  vided by refs. 7,8 were only available aligned against the GRCh38 refer- 
  ence sequence. These data were realigned to the GRCh37d5 reference 
  sequence using bwa aln (v. 0.7.17-r1188). 
  
  We select genomes with average autosomal coverage above 0.5×, 
  except for VK518, which has previously been suggested to be of Saami 
  ancestry6 and which had a coverage of 0.438. We included VK518 in 
  our panel to capture this ancestry. Genomes above a coverage cut-off 
  of 0.5× have previously been shown to result in reliable imputation 
  results72. We exclude samples with evidence of contamination. We 
  remove any duplicate individuals, such as individuals who were rese- 
  quenced, choosing the file with the highest coverage. We then filter 
  out any relatives annotated in the Allen Ancient DNA Resource v. 54.127, 
  retaining the individual with the highest coverage in each family clade. 
  Our final dataset includes 1,556 ancient genomes. 