The Caucasus, Himalaya, Karakum Desert, and Gobi Desert formed barriers that the steppe horsemen could only cross with difficulty. The centre and periphery were kept separate by mountains and deserts. The northern part of the continent was not accessible to the steppe nomads due to the dense forests and the tundra. The earliest known expansion out of the steppe is that of the Indo-Europeans which spread their languages into the Middle East, the Indian subcontinent, and in the Tocharians to the borders of today's China. The steppe region had long been inhabited by nomads, and from the central steppes they could reach all areas of the Asian continent.
Cities, states and then empires developed in these lowlands. Other aspects such as that of writing developed individually in each area. Therefore it is likely that they exchanged technologies and ideas such as mathematics and the wheel. The civilizations in Mesopotamia, the Indus Valley, and China had much in common. 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. The history of Asia can be seen as the history of several distinct regions, East Asia, South Asia, and the Middle East that have more or less context depending of the situation in the central Eurasian steppe.