text: **Stepping-stone separation by distance simulation.**We adapt the 
  simulation model provided previously23 to simulate a stepping-stone 
  model of nine populations organized on a 1D grid, in which individu- 
  als are able to migrate between adjacent populations (Extended Data 
  Fig. 3d). We changed this script to use the human hotspot recombina- 
  tion map and simulate only chromosome 1. We simulate under migra- 
  tion rates of 0.001 and 0.005, corresponding to average*F*ST values of 
  0.01 and 0.002, respectively23. We sample 20 haploid sequences per 
  population. We then fitted population 4 using pairs of other popu- 
  lations as sources in a rotational qpAdm scheme such that unused 
  populations are assigned to the reference set. 