Genomic admixture tracks pulses of economic activity over 2,000 years in the Indian Ocean trading network

Abstract

The Indian Ocean has long been a hub of interacting human populations. Following land- and sea-based routes, trade drove cultural contacts between far-distant ethnic groups in Asia, India, the Middle East and Africa, creating one of the world's first proto-globalized environments. However, the extent to which population mixing was mediated by trade is poorly understood. Reconstructing admixture times from genomic data in 3,006 individuals from 187 regional populations reveals a close association between bouts of human migration and trade volumes during the last 2,000 years across the Indian Ocean trading system. Temporal oscillations in trading activity match phases of contraction and expansion in migration, with high water marks following the expansion of the Silk Roads in the 5^th century AD, the rise of maritime routes in the 11^th century and a drastic restructuring of the trade network following the arrival of Europeans in the 16^th century. The economic fluxes of the Indian Ocean trade network therefore directly shaped exchanges of genes, in addition to goods and concepts.

Figure 1

Association between human migration and trade volumes through time. The green line shows the number of migration events estimated by GLOBETROTTER per century. The red line shows the average volume of trade per century adapted from data detailed by Beaujard. The blue line shows the average distance of migration per century, with light blue shading showing the variance. Dashed vertical lines mark the four trade phases. Horizontal bars represent t-tests between successive phases with significance values shown for the variance of migration distances (light blue), the average migration distance (dark blue) and the number of migration events (green): **P < 0.01; *P < 0.05; -: not significant.

Figure 2

Maps showing gene flow during the four phases of Indian Ocean trade. Admixture events during two century periods are plotted for each phase: (A) Phase I, (B) Phase II, (C) Phase III and (D) the beginning of Phase IV. Arrow widths are proportional to the percentage of ancestry inherited from each source population. Colors are specific to each cluster, as defined by the fineSTRUCTURE results: green palette: sub-Saharan African Pygmies, Bantu, East African and Malagasy clusters; yellow palette: Middle East, Nilo-Saharan and Caucasus clusters; purple palette: Pakistan, India and Bengal clusters; blue palette: East and North Asian clusters; red palette: Indonesian clusters; brown and orange palettes: Negrito, Andaman and Papuan clusters. Blue dots illustrate the locations of sampled populations. Maps were generated using Global Mapper v.15. (http://www.bluemarblegeo.com/products/global-mapper.php).