 By 1100, the Song dynasty hosted a population of almost 100 million people while large cities had over 1 million inhabitants, a sophisticated medieval economic system boasting the use of paper money (written business contracts, mercantile credits, checks, promissory notes, bills of exchange), and was a maritime naval power with extensive and flourishing trade contacts with Southeast Asia.For much of East Asia's economic history, China was one of the most developed economies.After the Fall of the Western Roman Empire, for a thousand years from 500 AD to 1500 AD, China was the wealthiest country in East Asia in the aggregate total in addition to per capita income.