Due to the presence of the Kurishio Current, a warm ocean current emerging from the northern equatorial zone, the environment of the Ryukyu archipelago is tropical despite its relatively high latitude. Numerous large, tropical shellfish species are found in the archipelago and were modified and used as symbolic items, personal ornaments, and/or craft items in prehistory. Exchange relationships based on these shell species developed in prehistory between the Ryukyus, China and Japan. The history of the shell exchange system can be subdivided into four phases. Phase I (2000-700 BC) saw exchange with the Yellow River area of China and resulted in the introduction of Chinese-like personal ornaments into the Rykyus. In Phase II (300 BC-AD 600), crops and iron tools were exchanged from western Japan. During Phase III (AD 650-800), Chinese coins were brought into this region from the Yellow River region. Finally, Phase IV (AD 800-1300) witnessed the development of agricultural and kiln technology, after which stratified societies emerged in the Ryukyu archipelago. The elites engaged in intensive exchange with China and mainland Japan in the 13th century and developed a maritime trading system in the 14th century. These "chiefdoms" were unified by the Ryukyu Kingdom. After 15th century, while shells were no longer major trade items, yellow cowrie shells and a green snail were still items of exchange.