Jericho was the biggest city in the world in 7000 B.C. with 2,000 citizens.
Jericho may be the oldest continually occupied spot in the world, with settlements dating to 9000 BC.
The city, nestled between the Dead Sea and Mt. Nebo, had natural irrigation from the Jordan River and the best known oasis in the region. The springs allowed residents to grow the highly lucrative opobalsamum plant, which produced the most expensive oil in the ancient world.
It is described in the Old Testament as the "City of Palm Trees."
Mari took the lead in 2400 B.C. with 50,000 citizens.
Mari was the robust trade capital of Mesopotamia, central in moving stone, timber, agricultural goods and pottery throughout the region.
The city was home first to the Sumerite kings, then the Amorite kings, one of which built a massive 300-room palace.
Mari was sacked in 1759 B.C. by Hammurabi of Babylon and then abandoned.
In the 1930s, a French archaeologist discovered 25,000 tablets written in an extinct language called Akkadian. Most were municipal documents, economic reports and census rolls -- a third were personal letters. The find changed our understanding of the ancient Near East.
Ur took the lead in 2100 B.C. with 100,000 citizens.
Ur was the most important port on the Persian Gulf. It was also a rich city, which held huge amounts of luxury items crafted from precious metals and semi-precious stones imported from throughout the known world.
Because of possible drought, or changing river patterns, Ur was no longer inhabited after 500 B.C.
It remained a holy site, however, and a burial site for people around the region. When archaeologists began sincere excavations in the mid 1850s, they discovered an immense necropolis, or city of the dead.
Yinxu took the lead in ,300 B.C. with 120,000 citizens.
An old village on the Huan River, Yinxu was reborn as the capital of the Shang Dynasty. It would be abandoned with the beginning of the Zhou Dynasty.
The city is a major archaeological site for its immense deposits of Oracle Bones, which contain the earliest form of Chinese writing. Pieces of ox bone and tortoise shell were inscribed using a bronze pin, heated until the bone cracked and then presented for divination. Later, when the tradition changed to ink and brush, entire genealogies and city histories were written on the fragments and deposited in central pits.
Babylon took the lead in 700 B.C. with 100,000 citizens.
Just the word Babylon today conjures up images of decadence and hubris. It was here, the Bible says, that residents believed so fully in themselves that they tried to build a structure into the heavens.
God was not impressed with the Tower of Babel, and the narrative holds that he assigned every resident a different language to confound any future teamwork.
Babylon was an epicenter of wealth, power and prestige from 2000 B.C. to about 538 B.C. That year Cyrus of Persia is said to have re-routed the Euphrates and sent his army into the city on the bare riverbed and routed the Babylonian forces.
Rome took the lead in 200 A.D. with 1,200,000 citizens.
From its humble roots as a small Italian village 1,100 years earlier, Rome in the second century A.D. was enjoying the pinnacle of its influence and achievement.
At this time, the city was a military dictatorship under Septimius Severus; a move the people welcomed to correct the corruption instilled by Emperor Commodus. Recall Joaquin Phoenix in "Gladiator" -- that Commodus.
Rome reached this size because it could draw food and taxes from most of Europe and the Mediterranean, but it proved an untenable position. By 273 A.D., Rome had fewer than 500,000 inhabitants and the Dark Ages were looming on the horizon.
Constantinople was in a fight for its survival in the year 600.
The nomadic Avars and Eastern Europe's Bulgarians were crushing the city from the west, and the Persians had completely overwhelmed the city's defenses in the east.
The metropolis was spared through a combination of impenetrable walls, its navy, and the arrival of soon-to-be emperor Flavius Heraclius Augustus, who eventually routed the Persians from Asia Minor.
The city is now known as Istanbul. By 618, however, as the Persian Wars dragged on and decimated the city's supply of grain from Egypt, Constantinople's population dwindled to one-tenth of what it was 18 years before.
Beijing took the lead in 1500 A.D. with 1,000,000 citizens.
To feed its growing population and vast number of troops in 1400 A.D., Beijing officials constructed the Jingtong storehouses to hold the grain it received as tax from the region.
The practice helped control prices and prevent inflation until the city grew to the largest in the world and the demand outgrew supply.
The population was then forced to harvest the regional forests for housing material and firewood -- leaving only coal, mined from the Western Hills, for heat and fuel. The resulting pollution changed the ecological makeup of the entire region.