"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 many root crops. The civilizations in Mesopotamia, India, 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's history would feature major developments seen in other parts of the world, as well as events that would affect those other regions. These include the trade of the Silk Road, which spread cultures, languages, religion, and disease throughout Afro-Eurasian trade. Another major advancement was the innovation of gunpowder in medieval China, which led to advanced warfare through the use of guns."