The first cities formed when places along the most travelled pathways, trade routes or migration routes had the most desirable features: water supply, defensibility, and access. As civilizations developed, the most powerful leaders were able to accrue the resources and labor required to build more substantial fortifications and central structures that were essential to their rule. Whether isolated or along major waterways, trade routes, or migration routes, the people who served the leaders, or who found the opportunity to stay, built up their homes, hostels, and businesses close to the center of protection, wealth, trade, and power.
As civilizations developed further, permanently established populations expanded and built to support their growing numbers. Trade and migration routes may have even been greatly altered in order to get to the major cities, which were now developing into transport, trade, government and military hubs.
With further development, permanently established cities that survived wars began to plan for better management, hygiene, and control of the burgeoning populations. Centers of governmental authority and control were established. Military strategies were developed and staffed in order to provide more sophisticated defense against enemies, as well as control of disruptive elements of the population.
Throughout the history of human development, there has always been a trend toward leaving the wild lands or farms, and heading for the cities in order to seek opportunity. The biggest periods of urbanization occurred when job opportunities arose in the urban industrial centers. The industrial revolution of the 19th and 20th centuries is the most commonly cited period of mass urbanization as a result of technological breakthroughs.
Cities grew, prospered, and died, based on the era of political stability and instability. Changes in access to water as a mode of transportation and as a source of hygiene and sustenance, competition with more popular cities, flood, drought, disease, natural disaster, and other factors led to the continuation of the great ancient cities of the world, while lesser cities were built, died, and built again.
The universal themes of desirable location, location along major transportation, trade and migration routes, access to water for sustenance and hygiene, and defensibility continue to this day. But technology allows for other reasons for cities to spring up out of nowhere.
In countries where there are major highway infrastructures, cities can spring up whenever topography and careful planning and negotiation with local and regional governments allows. Advancements in city planning now allow for consideration and forecasting of so many factors that fairly large new cities, complete with highway and transportation interchanges, police and fire substations, adequate sewer, septic, water, and other utilities, shopping, schools and commerce areas, light and heavy industrial areas, and open natural space, can be laid out and built within five to ten years of inception.
And now, the information technology revolution includes technological advancement that is causing not only mass migration, but volatile changes in urbanization and deurbanization, as people may now work in places that are no longer centralized hubs of corporate or government activity. And this includes working at home. And “home” can be anywhere now.
Volatility in human migration is still increasing. Convenient transportation and ability to move possessions, leads to frequent and rapid relocation. Changes in job opportunities, where thousands of employees can quickly be relocated to job sites that are a thousand miles away, create the need for new housing in areas that have been rural, or urban, but in decline. In other cases, retirement migration occurs. As people retire or grow old, they may need to move to be closer to one or the other of their adult children. Retirees may tire of cold locales and move to more temperate places in the world.
As a result, cities grow, die, revive, and remain in a constant state of change throughout periods of natural and social stability, based on the need and the features that attract people to visit or live in them. Yet cities will never go away. They remain the hubs of commerce, culture, transport, migration, and opportunity for plenty of people.