15k years isn't so short a period. Suppose a generation every 20 years or so. 50 per millennium. 750 generations in 15k years. If one person per generation sett

15k years isn't so short a period. Suppose a generation every 20 years or so. 50 per millennium. 750 generations in 15k years. If one person per generation settles 120 km further east than he/she was born (and odds are there would be more than just one) then 750 generations would be long enough to circle the planet twice. Genes travel slowly, but in many directions at once. And there may be some advantage to the offspring of two people genetically "far" from each other, reinforcing via evolution whatever instinct lead to them travel. 15k years is more than enough time for all of us to share an ancestry.