Trade gave people reasons to surmount or bypass those barriers to travel, and it has played an important role in facilitating human gene flow. Although its impact on gene flow was probably not as dramatic as that resulting from conquest and colonization, it was important, particularly after the development of sailing ships. Sailors and barmaids, like traveling salesmen and farmers' daughters, have played a crucial role in recent human evolution.
Trade—more exactly, traders—spread new alleles along every line of communication: along sea routes, up navigable rivers, between villages and market towns. In early times it often took the form of semi-military campaigns, as when Egypt sent trade expeditions to Punt in northern Ethiopia/ Eritrea in order to obtain gold, slaves, ebony, and ivory, or when Mesopotamian kings like Sargon of Akkad sought cedarwood in Lebanon.
Trade connected widely separated civilizations—some of them, some of the time—nearly as far back as we have records. There is evidence of trade between the Indus civilization and Mesopotamia during the Akkadian Empire more than 4,000
years ago—Mesopotamian cereals and wool in exchange for wood and ivory from "Meluhha," thought to be the Indus civilization. We have found Indus cylinder seals in Ur and Babylon, and references to a village made up of traders from Meluhha, who eventually became one more component of the local ethnic mix. This is exactly the sort of contact that could have played a significant role in the transmission of new beneficial alleles: The peoples of the Indus civilization probably faced similar selective pressures to those seen in Mesopotamia, and they were far enough away that gene flow through local marriage would have taken a long time to occur. However, the contact happened early in history, so that any good alleles introduced to the Middle East in this way have had 160 generations in which to spread.
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