Spanning five thousand years, the Asian civilizations on the Silk Road

Asia is the cradle of human civilization, having created the three great ancient civilizations: the Mesopotamian civilization in Western Asia, the Indus Valley civilization in South Asia, and the Chinese civilization in East Asia. The Indus Valley civilization had a widespread worship of animals, with the humped bull symbolizing male fertility and revered as a sacred beast. Seals were mostly square, made primarily of steatite, but also of clay and earth. They are believed to have been used to mark trade goods. In the 6th century BC, Western Asia saw the earliest artificial smelting of metal products. Coins of the Kushan and Sasanian Empires retained many characteristics of Greek coins and were an important settlement currency in Silk Road trade, gradually replaced by Arab coins after the 7th century. Chinese coins such as the Han Wuzhu, Tang Kaiyuan Tongbao, and Song dynasty era coins also served as a medium of exchange on the Silk Road, influencing the coin culture of East Asia and Southeast Asia. The hammering and chasing crafts of Iran, which appeared during the Elamite period, reached their zenith during the Sasanian dynasty. China had been utilizing these techniques since the Shang dynasty, which matured during the Tang dynasty and were widely applied in the making of gold and silver vessels. In the first half of the 1st millennium BC, most of Asia's early civilizations entered the Iron Age. Around 3500 BC, the Mesopotamian region began using copper-tin alloy technology to cast well-performing tin bronze, entering the Bronze Age. Early on, forging was used, while later periods often employed stone mold casting or the lost-wax casting method.