[SALADIÉ, Òscar; OLIVERAS, Josep (2010). Desenvolupament sostenible. Tarragona, Universitat Rovira i Virgili, p. 78-79]
It is commonly believed that the first cities appeared a little more than 5000 years ago, when after the Neolithic Revolution allows the development of the agriculture, a subsequent population increase, which causes the growth of the ancient rural settlements.
These rural settlements were created accordingly to the needs of a quantitatively very small community, strongly linked to the agriculture and the natural environment phenomena. The progressive population growth allowed the transformation of these settlements into urban centres, which involved the expansion of the cultivated land at the expense of the vegetation. The young cities need to have a good agrarian economic base in order to feed a social group that has been freed from the agricultural activities so as to devote themselves to manage and administer the city, to legislate, to keep order and safety, to trade or to guide people spiritually. Some examples of these early cities are Uruk (Sumer), Damascus (Syria), Memphis (Egyptian Empire), Jericho (Palestine) or Tikal (Mayan Empire).
From that moment on, new cities have appeared involving a continuous population and surface growth, and a subsequent needs increase of the resources, so as to maintain them with food, water and a great diversity of material for building, for tools and weapons manufacture, for ornamentation, etc. This resources need involves a larger exploitation of the territory and the creation of conflicts with other cities for controlling these resources. At the same time, these large population concentrations involve the first environmental problems, as water contamination and the subsequent fecundity loss of many agricultural areas, as a result of the overexploitation, or, in some cases, salinisation, as a result of an inadequate use of the irrigation, which brought lands to standstill.
This height of cities creation can be still found in Classical Greece (the polis) and in Roman Empire (Rome has reached 1 million inhabitants at its greatest height). After the Roman Empire disintegration, in the Middle Ages, the Western European cities suffered a decline, while Byzantium and other Muslim cities as Bagdad or Cordoba were enjoying their greatest moments of splendour.
Later, the regeneration of the trade stimulated the redevelopment of the Western European countries, which become again centres of products exchange with markets and fairs. However, the cities were the centres that most suffered from food crisis and epidemics which periodically hit the preindustrial societies. Black Death, for instance, was one of the deadliest pandemics that struck Europe in the second half of the XIV century.
Cities have always received rural population. This fact has been speeded up by the Industrial Revolution and it caused a great agglomeration, since cities were not planned for fitting such a great number of people, nor the industrial activities they were developing. Now, a part from the water pollution, there is also air pollution, which, together with the lack of hygiene, causes great public health problems. There is a German saying from the XI or XII century that says ‘stadluft macht frei’, that is, ‘city air makes you free’. Within the context of the XI-XII century, migrating to the city and breathing its air was a revolution for the rural population, since it released people from the feudal slavery. But later, the Industrial Revolution turned the air of many cities unbreathable. The urban environment has been degraded by its own growth, which has been often excessive and has entailed the continuously increasing land occupation, air and water pollution, climate modification, noise and light pollution, waste generation. The final result is the contamination of the entire environment, which brings serious public health problems.