The history of Asia can be seen as the collective history of several distinct peripheral coastal regions such as, East Asia, South Asia, and the Middle East linked by the interior mass of the Eurasian steppe.The coastal periphery was the home to some of the world's earliest known civilizations, with each of the three regions developing early civilizations around fertile river valleys. These valleys were fertile because the soil there was rich and could bear lots of root crops. The civilizations in Mesopotamia, the Indus Valley, and China shared many similarities and likely exchanged technologies and ideas such as mathematics and the wheel. Other notions such as that of writing likely developed individually in each area. Cities, states and then empires developed in these lowlands.The steppe region had long been inhabited by mounted nomads, and from the central steppes they could reach all areas of the Asian continent. The northern part of the continent, covering much of Siberia was also inaccessible to the steppe nomads due to the dense forests and the tundra. These areas in Siberia were very sparsely populated.The centre and periphery were kept separate by mountains and deserts. The Caucasus, Himalaya, Karakum Desert, and Gobi Desert formed barriers that the steppe horsemen could only cross with difficulty. While technologically and culturally the city dwellers were more advanced, they could do little militarily to defend against the mounted hordes of the steppe. However, the lowlands did not have enough open grasslands to support a large horsebound force. Thus the nomads who conquered states in the Middle East were soon forced to adapt to the local societies.
Asia has the second largest nominal GDP of all continents, after Europe, but the largest when measured in purchasing power parity. As of 2010, the largest economies in Asia are China, Japan, India, South Korea and Indonesia. Based on Global Office Locations 2011, Asia dominated the office locations with 4 of top 5 were in Asia, Hong Kong, Singapore, Tokyo, London and Shanghai. Around 68 percent of international firms have office in Hong Kong. According to Citigroup in The Wealth Report 2012 stated that Asian centa-millionaire overtook North America's wealth for the first time as the world's "economic center of gravity" continued moving east. At the end of 2011, there were 18,000 Asian people mainly in Southeast Asia, China and Japan who have at least $100 million in disposable assets, while North America with 17,000 people and Western Europe with 14,000 people.