Economic Systems Throughout history there has been a common theme of progression towards more complex societies. The advent of agriculture allowed the size of the population to increase and the creation of permanent settlements. As the extensive cultivation of agricultural land progressed, a surplus of food was created that allowed some people within a society to be non-food producers. These people who were no longer required to farm to survive were able to develop marketable goods that they could exchange for food. This transition to interdependence (artisans depend on the exchange of their goods for food), brought with it the opportunity for each society's government to demand tribute to monitor the economy and ensure that exchanges occurred smoothly . This led to societies with a well-defined hierarchy and slowly led to the abolition of egalitarian societies where everyone uses the same means of production. Within these egalitarian societies mentioned above there is some professional specialization, but it is based on the skill of the craftsmen and no one is a full-time specialist. The differences in the economies of Copan, Teotihuacan, and ancient Rome can illustrate why and how economies increase in complexity and what criteria are necessary for large-scale economic specialization. More specifically, what factors limited Copan and Teotihuacan, preventing them from reaching the population size and economic complexity of ancient Rome. Teotihuacan was a huge metropolis in what is now southern Mexico. It became a large city before 100 AD and reached its peak size around 600-650 AD. At its peak it had around 125,000 inhabitants. In the vicinity of the ancient city there are permanent springs and satellite photos have indicated the presence of a possible irrigation system with canals used to irrigate agricultural sites. Although the age of the irrigation canals is yet to be established, it seems very likely that this canal system was created in conjunction with the development of the city. This conclusion is also supported by the lack of rural population, which would have been necessary to provide food for such a large population if there had been no agriculture within the city. Since the food was not brought from a great distance, the travel costs to bring the food to the population would have been low. This allowed there to be a dense concentration of population within the urban area. The expense of moving goods to the new world appears to have been a factor in limiting the size of cities.