The most populous capital cities in the Middle East include Baghdad, Iraq; Riyadh, Saudi Arabia and Tehran, Iran, as of 2011. Riyadh had a population of 4.9 million, while Baghdad had 7.2 million people. The largest of the three, Tehran, had 13.8 million people.
The epicenter center of knowledge and power during the Golden Age of Islam, Bahgdad was not only the largest city in the Middle East, but the largest city in the entire world in the year 900. With a population of around 900,000, traders descended upon the city to learn revolutionary farming techniques. As a result, the Arab Agricultural Revolution was born, bringing with it the scientific approach to agriculture that farmers use in modern times.