Many of the first cities were built on fertile land, as in ancient Mesopotamia, around 7500 BCE, Eridu, Uruk, and Ur were among the first to be made. Between Tigris and Euphrates are some settlements, among which these cities were located (the so-called Fertile Crescent). As people cultivated crops and settled in communities, cities grew along the Nile in Egypt, the Indus River Valley in India, and the Yellow (or Huang He) River in China. To take advantage of food surpluses, people could give up their nomadic lifestyle as hunter-gatherers in these fertile areas. In addition, settling near waterways provided a vital transportation system that aided trade.

As time passed, cities continued to expand in population, size, and importance. Cities in some of the world’s most populous regions have grown steadily for centuries, while others appear to have sprouted out of nowhere. There are no longer any of the world’s most populous cities, like those in Mesopotamia, and the population of others has declined. For example, Rome, Italy, had one million people in the first century B.C.E., making it the largest city.

By the Middle Ages, the population had fallen to just 20,000. But, as a rule, most cities are growing, experiencing unprecedented growth rates. Rome had nearly three million people in 2017 (in fact).

For centuries, people have sought out cities for trade, culture, education, and economic opportunity, but population growth in cities hasn’t always been consistent. Many people have been moving to urban areas for a relatively short period; this is a relatively new phenomenon in human history. In 1800 C.E., for example, more than 90% of the world’s population lived in rural areas. In 1800, more than 94 percent of the United States population resided in rural areas, according to the United States Census Bureau; by 1900, this figure had dropped to 60 percent. Industrialization, which started in England and spread to the United States and Europe in the middle of the eighteenth century, was a significant factor in expanding urban areas. The rise of factories during the Industrial Revolution resulted in an increased need for workers in urban areas. As a result, people in the United States and the United Kingdom moved to urban centers over the next century. Other parts of the world became more urban as they became more industrialized. One million people lived in London in 1800; a decade later, that number had grown to more than six million. By 1950, New York City had more than 12.5 million residents, making it the world’s most populous city at the time.

Additionally, new technologies have fueled urbanization, particularly those that allowed cities to grow upward. For example, skyscrapers were made possible by steel advancements, allowing for higher densities of people to live together.

Urbanization has resulted in ever-larger cities and a new type of city: the megacity, which has a population of 10 million or more. After New York City and Tokyo, Japan, which were the first two global megacities to emerge in the 1950s, a total of 37 such cities existed by 2018.

Due to industrialization, Asia and Africa are experiencing greater urbanization than North America, and Europe did during the Industrial Revolution. For example, Tokyo is the world’s largest urban area, while Delhi has nearly 30 million people living there in India. In addition, over 20 million people live in Shanghai, Mexico City, and So Paulo, Brazil, to name a few.

When looking to the future, it’s essential to look at the development of today’s cities as a guide. There will be 41 megacities by 2030, according to the United Nations. For example, India will have seven cities with populations exceeding 10 million by 2030, up from the current five.

Cities in Asia and Africa also struggle to keep up with population growth, just like rapidly expanding cities in England and North America did in the industrial era. For example, Lagos, Nigeria’s largest city, has 12.6 million people, with about two-thirds of them living in slums. Due to high birth rates, the city’s population grows, not due to migration. Despite the many drawbacks of rapid population growth, such as increased traffic and the spread of slums, the world’s megacities continue to see an increase in residents.