The states of North Africa (with writing) and West Africa (without writing) were separated from each other by Saharan desert unsuitable for agriculture and cities. The deserts of northern Mexico similarly separated the urban centers of southern Mexico from the chiefdoms of the Mississippi Valley.
Is there evidence that the strength of the trading network between two societies mediates the amount and nature of information flowing between them? Can "higher-order" information such as writing systems be transmitted only via strong trading links? Seems intuitive, but may not be true.
