The idea of using a durable light-weight substance as evidence of a promise to pay a bearer on demand originated in China during the Han Dynasty in 118 BC, and was made of leather. The first known banknote was first developed in China during the Tang and Song dynasties, starting in the 7th century. Its roots were in merchant receipts of deposit during the Tang Dynasty (618–907), as merchants and wholesalers desired to avoid the heavy bulk of copper coinage in large commercial transactions. During the Yuan Dynasty, banknotes were adopted by the Mongol Empire. In Europe, the concept of banknotes was first introduced during the 13th century by travelers such as Marco Polo, with European banknotes appearing in 1661 in Sweden.