There's no evidence that the first cities were mercantile. Settlements like Jericho and Çatal Hüyük were probably more defensive in nature- in a world where some have crops and domestic cattle, and others live as gatherers and hunters it makes sense to concentrate strongly enough to frighten or fight off marauding bands. The Mesopotamian cities developed around the need to organise massive communal works for irrigation- based agriculture. The need for organisation came first, and it's the power behind the organisation that needed a legitimisation- hence priesthoods and religions. Trading in the earlier days seems to have been more ritual than economic, though as the elites became more successful at monopolising surpluses, trade in luxury goods became established.