For example, at the Liangzhu Site in Hangzhou, Zhejiang province, which dates from 5,300 to 4,500 years ago, archaeologists unearthed a huge dam 3.5 kilometers long and more than 10 meters thick. Ruins of the city cover 3 million square meters, and the city's walls are 40 to 60 meters thick. The discovery was made possible with remote sensing technology.

"In the city center, we found a terrace that is 630 meters by 450 meters," he said. "Some of the foundations are as large as 900 square meters."

He estimated that the entire city would have taken 10,000 people a decade or longer to build.

"It hardly would have been possible for a tribe or a league of tribes to mobilize so many people," Wang said. "That indicated a time of early-stage states with wide influence."

Division of labor, hierarchical social classes and inequality in wealth were also observed when studying the Taosi Site (from 4,300 to 4,100 years ago) in Xiangfen county, Shanxi province, and several other key sites of "huge city ruins", he said.

The relics are from what is believed to have been the capital of Emperor Yao, a legendary ancient ruler. Some markings were discovered that are believed to be a prototype of written characters, but they appear to have been used exclusively within a small group of ruling elites.

"Around 5,000 years ago, some regions along the Yangtze River, the Yellow River and the western bank of Liaohe River (in Northeast China) stepped into the age of civilization," Wang said. Those ancient states, he said, "controlled certain territory, but had economic, political and cultural connections with each other".