M odern historiography has not fully appreciated the ecological complexity of the Silk Roads. As a result, it has failed to understand their antiquity, or to grasp their full importance in Eurasian history. The role played by the Silk Roads in exchanging goods, technologies, and ideas between regions of agrarian civilization is well understood. Less well understood is the trans-ecological role of the Silk Roads-the fact that they also exchanged goods and ideas between the pastoralist and

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... n worlds. The second of these systems of exchange, though less well known, predated the more familiar "transcivilizational" exchanges, and was equally integral to the functioning of the entire system. A clear awareness of this system of trans-ecological exchanges should force us to revise our understanding of the age, the significance, and the geography of the Silk Roads. Further, an appreciation of the double role of the Silk Roads affects our understanding of the history of the entire Afro-Eurasian region. The many trans-ecological exchanges mediated by the Silk Roads linked all regions of the Afro-Eurasian landmass, from its agrarian civilizations to its many stateless communities of woodland foragers and steppe pastoralists, into a single system of exchanges that is several millennia old. As a result, despite its great diversity, the history of Afro-Eurasia has always preserved an underlying unity, which was expressed in common technologies, styles, cultures, and religions, even disease patterns. The extent of this unity can best be appreciated by contrasting the history of Afro-Eurasia with that of pre-Columbian America. World historians are becoming increasingly aware of the underlying unity of Afro-Eurasian history.