| Abstract: |
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. |