I mean trade as an institution, not just sporadic interpersonal bartering and sharing; more like merchants, trade routes, and the like. Exchange is not unique t

I mean trade as an institution, not just sporadic interpersonal bartering and sharing; more like merchants, trade routes, and the like. Exchange is not unique to man; trade, as such, is. Trade becomes more worthwhile when communities begin to specialise. When people are nomadic, they cross many lands, and can simply find what they want on the way (obsidian from the mountains, shells by the sea, berries, roots, and game in forests). When people begin to stay in one place, long distance travel for trade becomes more valuable. As soon as agriculture develops, we discover that obsidian shards from deposits in the Caucasus are found thousands of miles away in central Europe. You begin to have towns focusing on a specific resource, or serving a trade route as goods and people criss-cross the land (Jericho, among the earliest known cities, for example, was possibly involved in the trade of salt; few things can grow in the soil near it).