China, officially the People's Republic of China (PRC), is a sovereign state located in East Asia. It is the world’s most populous country, with a population of over 1.35 billion. Covering approximately 9.6 million square kilometers, China is the world's second-largest country by land area, and either the third or fourth-largest by total area, depending on the method of measurement. The history of China goes back to its ancient civilization – one of the world’s earliest – that flourished in the fertile basin of the Yellow River in the North China Plain. For millennia, China's political system was based on hereditary monarchies, known as dynasties, beginning with the semi-mythological Xia of the Yellow River basin (c. 2800 BCE). Since 221 BCE, when the Qin Dynasty first conquered several states to form a Chinese empire, the country has expanded, fractured and been reformed numerous times. The Republic of China (ROC) overthrew the last dynasty in 1911, and ruled the Chinese mainland until 1949. The word “China” is derived from the Persian word Chin, which in turn derives from the Sanskrit word Cīna. It is first recorded in 1516 in the journal of the Portuguese explorer Duarte Barbosa. The journal was translated and published in England in 1555. The traditional theory, proposed in the 17th century by Martino Martini, is that Cīna is derived from “Qin”, the westernmost of the Chinese kingdoms during the Zhou Dynasty.