In 7000 BC, the world's largest city was Jericho with a population of just 2,000. By 300 BC, Carthage had claimed the title with 500,000. Fast-foward to 1968 -- the year Tokyo topped them all with 20,500,000 people. From Constantinople to Baghdad, Business Insider offers an interesting look at the greatest cities the world has ever known -- then and now. What will the world's next great city be? Click here to see 12 "megacities" vying for the title.