According to Chinese tradition, the first dynasty was the Xia, which emerged around 2100 BCE. The Xia dynasty marked the beginning of China's political system based on hereditary monarchies, or dynasties, which lasted for a millennium. The dynasty was considered mythical by historians until scientific excavations found early Bronze Age sites at Erlitou, Henan in 1959. It remains unclear whether these sites are the remains of the Xia dynasty or of another culture from the same period. The succeeding Shang dynasty is the earliest to be confirmed by contemporary records. The Shang ruled the plain of the Yellow River in eastern China from the 17th to the 11th century BCE. Their oracle bone script (from  BCE) represents the oldest form of Chinese writing yet found and is a direct ancestor of modern Chinese characters.