Over the course of human history, the ranking of the most populous cities has changed many times over. Jericho was the most populous city back in 9000 BCE. Now it is Tokyo, thousands of miles away. Population growth, climate change, and political shifts are largely responsible for moving the world's biggest urban centers, but there are truly countless reasons as to why populations move and fluctuate.
When evaluating the most populous cities throughout history, archaeologists look at the total estimated global population to determine the cultural hubs of the period. Before the widespread use of recorded history, many cultures relied on oral traditions to help keep their chronicles alive. Because of this, it is challenging to calculate how many people lived in cities before recorded history.
But historians have done their best to determine where populations converged throughout history. These cities were at one point considered to be the biggest in the world.
Jericho, West Bank

Population in 9000 BCE: 2,000; current population: 14,674
Most academics agree that Jericho is among the world’s oldest continuously inhabited places, as settlements have been uncovered dating back to 9000 BCE. Jericho is considered the oldest and most populous city throughout history. It is located near Mt. Nebo and the Dead Sea in what is now the West Bank. The plentiful natural irrigation from the Jordan River makes it an ideal ancient city for long-term habitation.
Uruk, Iraq

Population in 3500 BCE: 4,000; current population: Uninhabited
Uruk was once an agricultural hub that lay the foundation of Mesopotamia. However, Uruk is no longer inhabited. Nestled between the Tigris and the Euphrates rivers, Uruk was once a thriving trade center that specialized in local crafts, writing, and grain.
Mari, Syria

Population in 2400 BCE: 50,000; current population: Uninhabited
Researchers discovered a large population migration from Uruk to Mari, indicating a flourishing trade and livelihood in that region of Mesopotamia. Estimates place the population of Mari, which is located in what is now Syria, at 50,000 people in 2400 BCE. It was the trade capital of the region and had a fully functioning government and recorded history.
Ur, Iraq

Population in 2100 BCE: 100,000; current population: Uninhabited
Ur was a very rich city in 2100 BCE, with a huge amount of luxury items made from precious metal and semi precious stones. After 500 BCE, Ur was no longer inhabited due to drought and changing river patterns. Today, the Iraqi city of Tell el-Muqayyar is at the site of Ur.
Yinxu, China

Population in 1300 BCE: 120,000; current population: uninhabited
Eventually, the world's biggest population centers shifted away from the Middle East. The earliest forms of Chinese writing can be found in the modern day ruins at Yinxu, sometimes written as two words (Yin Xu). At its height, this city was the academic center of the Chinese world.
Carthage, Tunisia

Population in 300 BCE: 500,000; current population: 20,715
Located in present-day Tunisia, Carthage was an enlightened civilization until drought and famine sped up the decline of this ancient city. It was not until 1985 that the mayors of Carthage and Rome officially ended their 2,000-year-old conflict.
Rome, Italy

Population in 200 CE: 1,200,000; current population: 2,754,440
What started as a small village a thousand years ago is now a bustling metropolis. In 200 CE, Rome was the most populated city in the world. It is no secret that Rome has been one of the longest occupied settlements and for a good reason. As a center for government, politics, religion, fashion, ancient history, archaeological sites and culture, it is still a top travel destination for millions of people.
Beijing, China

Population in 1500: 1,000,000; current population: 22,000,000
Still one of the world's most populous cities, Beijing broke out around 1500, when it relied on grain and monetary taxes from the population to feed and supply the city. However, that was not enough. The population was so large that commerce destroyed all of the forests in the region. This irrevocably changed the ecosystem in the area.
London, England

Population in 1825: 1,335,000; current population: 13,945,000
During the pinnacle of the British Empire, crime and terror in London ran rampant. The city was considered unsafe. However, this did not stop people from finding their way in the Empire’s capital. Today, it remains a global capital that welcomes millions of visitors every year.
Tokyo, Japan

Population in 2000: 20,500,000; current population: 36,000,000
After this trip through history, we arrive at the present day. Tokyo is the most populous city in the modern world, home to an astounding 36 million people in its metropolitan area. There was a brief interlude following World War II until Tokyo recovered economically. Prosperity and a strong bond to Japanese tradition, family, and history maintain Tokyo’s high population today.
The draw and allure of cities continue to bring human civilization closer and closer together. Currently, more than half of the world’s population lives in urban centers, and this number is expected to climb. The current practice of census-taking will undoubtedly help future historians.