Chu Ju traces ancestors of state of Chu to Ji Lian, grandson of Zhu Rong. As mentioned above, Zhurong is a tribe, so it has existed for a long time, by time of Jilian it was already in middle of Shang Dynasty. "Chu Ju" reports that he "went up Xi River and saw Pan Geng's son, who was in Fangshan." This shows that ancestors of Chu people migrated to Han River basin.
The Yin Wu Songbook says: "Attack Yin Wu, fight Jing and Chu. Go deep into his obstacles and embark on journey of Jing. There is a thread to cut off place, Tang Song." This is Song Dynasty. People go back to praising great martial arts of Shang Dynasty, which mentioned that Shang Dynasty army attacked all way to Jingchu area. Therefore, it is quite normal for Chu people to meet Pan Geng's son.
In 1954, archaeologists discovered ruins of Panlong City in Wuhan City, Hubei Province. A city, palaces, and many noble tombs were found here. Hundreds of bronze, pottery, jade and stone tools have been found here, and their style is almost same as that of Ying ruins. Some people believe that it is a military stronghold founded in Jianghan after Shang Dynasty moved south. Of course, I think it is early capital of state of Chu.
Map of Shang Dynasty
The Book of Songs mentions that after Southern Expedition of Shang Dynasty, "the country was ordered to move on to next country, and feudalism was blessed", i.e. state of Chu must be under enfeoffment. "Chu Ju" writes: "Ji Lian heard that he was hired, and from Ji Zhipan, Yuan Shengbo and Yuan Zhong", it can be seen that State of Chu and Shang Dynasty got married, and Ji Lian got married on Pan Geng's daughter. The concubine is his wife. Later, Ji Lian's grandson Xue Xiong remarried his concubine Li. This shows that two races have intermarried for some time.
When Concubine Li gave birth to Xiong Li, she died of dystocia, and wizard wrapped spikes around her sides, so they began to call themselves "Jing people" or "Chu people." In other words, people of Chu are actually a nation formed by intermarriage of merchants and Zhurong family. In this case, it is possible that predecessor of State of Chu was State of Fang of Shang Dynasty, with support of Shang Dynasty, a city similar to ruins of Yin was built.
Then why did Chu people move south en masse during Shang Dynasty? It is also associated with Shang Dynasty. At beginning of Shang Dynasty, Xia Dynasty was conquered on a large scale, including a crusade against princes of Xia Dynasty. The ancestors of Chu people participated in Xia Dynasty in early days, so they were naturally targets of conquest of Shang Dynasty. Among Zhuanxu clan is Kungwu. Some people think that Gongwu is in Puyang, Henan Province only because of Zhuanxu, and it may be a tribe of Zhuanxu clan that has not yet moved. Others think it's in Zhengzhou area. Kunwu is Fang Country of the Xia Dynasty, "Guoyu Zhengyu" says, "Kunwu is Xia Bo."
The Shang Dynasty had already destroyed Gongwu before they destroyed Xia. Under such circumstances, Zhu Rong's family was forced to move south. In middle of Shang Dynasty, descendants of Zhu Rong decided to submit to Shang Dynasty and became Fang Kingdom of Shang Dynasty, so war came to an end. But at end of Shang Dynasty, Chu people decided to cooperate with Zhou people and were called "Chuzi". However, Chu was never truly controlled by Western Zhou dynasty. During era of King Zhao of Zhou, State of Chu and Zhou Dynasty officially collapsed, and a large-scale war broke out. Until fall of Western Zhou Dynasty, State of Chu never surrendered to Zhou Dynasty and even proclaimed himself king and finally became largest country in Spring and Autumn period.