Climate and Prehistoric Migration

What factors drove human migration before modern states, markets, and borders? We develop a sorting framework in which climate-specific subsistence knowledge depreciates with ecological distance. To test this, we use ancient DNA identity-by-descent segments to construct bilateral migration flows across Western Eurasia over the last 10,000 years. We document three main findings. First, migration flows decline with differences in growing degree days, precipitation, and soil characteristics between origins and destinations. Second, the binding factor varies across subsistence systems: farmers exhibit strong thermal and soil matching, while pastoralists match most strongly on precipitation. Third, periods of warming increase farmer expansion while cooling increases pastoral expansion in patterns that recover known archaeological migration episodes. Migration also acts as a margin of climate adaptation: populations exposed to temperature change move to destinations that partly offset the shift.