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, 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.