When people first settled down into larger towns in Mesopotamia and Egypt, self-sufficiency – the idea that you had to produce absolutely everything that you wanted or needed – started to fade. A farmer could now trade grain for meat, or milk for a pot, at the local market, which was seldom too far away.
Cities started to work the same way, realizing that they could acquire goods they didn't have at hand from other cities far away, where the climate and natural resources produced different things. This longer-distance trade was slow and often dangerous, but was lucrative for the middlemen willing to make the journey.
Seems to be a series, with one from last week as well. This makes it seem like trade developed with civilization, but it had been going on for some time. Interestingly, the first quoted sentence made me think of the selection post from the other day: Wenke and Dunnell once posited that perhaps the shift from simple kin-based interaction systems to these sorts of functionally differentiated societies created a change in the unit of selection. Whereas before, the individual or the family was the basic unit of selection -- all individuals being pretty much functionally redundant -- afterwards individuals were functionally differentiated and could not survive outside of the larger society. Something akin to the development of multicellular organisms. Anyway, should be an interesting series to yak about.