The term civilization basically means the level of development at which people live together peacefully in communities. Ancient civilization refers specifically to the first settled and stable communities that became the basis for later states, nations, and empires. The study of ancient civilization is concerned with the earliest segments of the much broader subject called ancient history. The span of ancient history began with the invention of writing in about 3100 BC and lasted for more than 35 centuries. Mankind existed long before the written word, but writing made the keeping of a historical record possible. The first ancient societies arose in Mesopotamia and Egypt in the Middle East, in the Indus Valley region of modern Pakistan, in the Huang He (Yellow River) valley of China, on the island of Crete in the Aegean Sea, and in Central America. All of these civilizations had certain features in common. They built cities, invented forms of writing, learned to make pottery and use metals, domesticated animals, and created fairly complex social structures with class systems. Apart from written records and carved inscriptions, the knowledge about ancient peoples is derived from the work of archaeologists. Most of the significant archaeological findings have been made in the past 200 years. The Sumerian culture of Mesopotamia was discovered in the 1890s, and some of the most important archaeological digs in China were made after the late 1970s.