The basic astronomical symbols of the Four Spirits can be found in "Dragon-tigershaped Shell Pile" in Tombs no. 4 and 5 in Xi-shui Po, Puyang, Henan, dating back to six thousand years ago. In the northern side of the male tomb owner, there was a shell triangle; on its eastern side, two human tibia bones were crossed to represent the Big Dipper, an asterism of seven stars. This astronomical imagery system can be found in later burial sites of Han and Tang dynasties. According to the archeological data, there were four combinations of these Four Spirits in Han and Tang burial sites. In the Xin Mang period, the theory of Five Spirits began to circulate. These Five Spirits correspond to the four cardinal directions plus the earth element of the center, represented by the imaginary animal Qiling. They formed the foundation for the theory of five elements in later times.