The history of Asia can be seen as the collective history of several distinct peripheral coastal regions such as East Asia, South Asia, Southeast Asia, and the Middle East associated with the inland mass of the Eurasian steppes. The coastal ocean was home to some of the world's oldest known civilizations and religions, with each of the three regions developing early civilizations around fertile river valleys. These valleys were fertile because the soil in them was rich and could bear many root crops. Civilizations in Mesopotamia, India, and China share many similarities and likely exchanged techniques and ideas such as mathematics and the wheel. Other concepts such as the idea of ​​writing individually are likely to develop in each field. Cities, states, and then empires developed in these lowlands. The steppe has always been inhabited by nomads, from whom they can reach all parts of the Asian continent. The northern part of the continent, covering much of Siberia, was inaccessible to the nomadic steppes due to the dense forests and tundra. These regions of Siberia were very sparsely populated. The center and the periphery were separated by mountains and deserts. The Caucasus, the Himalayas, the Karakum Desert and the Gobi Desert formed barriers that the knights could only cross with difficulty. While the city's population was more technologically and culturally advanced, they could do little militarily to defend against the steppe hordes. However, in the lowlands there was not enough open pasture to support a large force of horses. Thus the Bedouins who soon occupied countries in the Middle East had to adapt to the local communities. The spread of Islam appeared in the golden age of Islam and the Timurid Renaissance, which later affected Islamic firearms. The history of Asia is marked by major developments seen in other parts of the world, as well as events that affected those other regions. This trade includes the Silk Road, which spread cultures, languages, religions, and diseases throughout Afro-Asian trade. Another major advance was the invention of gunpowder in medieval China, which was later developed by the gunpowder empires, mainly by the Mughals and Safavids, and led to advanced warfare through the use of guns.