Climate and Prehistoric Migration

What factors drove human migration before modern states, markets, and borders? We develop a framework of ecological sorting 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 four main findings. First, migration flows decline with differences in growing degree days, precipitation, and soil characteristics between origins and destinations. Second, the dimensions of climate that bind vary across subsistence systems: farmers exhibit strong thermal and soil matching, while pastoralists match most strongly on precipitation---consistent with differential ecological constraints and limiting factors. 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. Fourth, genetic flow predicts subsequent convergence in destination vegetation toward migrants’ ecological profiles, consistent with migration shaping landscape change and the demic diffusion of subsistence practices.