When Vivaswan got married, he had two sons-- Manu, the father of mankind and Yamraj, the god of death. All the dynasties on the earth descended from Manu's sons. Brahmins, Kshatriyas, Vaisyas and Shudras, were his descendents. Manu had ten sons --- Ven, Dhrishnu, Narishyant, Naabhag, Ikshwaku, Kaarush, Sharyati, Ila Kanya, Prishadhra and Nabhagarishti.
Ila Kanya had a son called Pururava. She was both the father and the mother of Pururava. Pururava became the ruler of thirteen islands and even though he was a human being he indulged in hedonistic and animalistic pleasures. Intoxicated with his power and strength he looted the brahmins and took away their wealth. The sages became angry with him and their curses destroyed him.
Pururava had six sons by Urvashi whom he had brought from heaven. They were called Aayu, Dheeman, Amavasu, Dridhayu, Vanayu and Shatayu. Aayu's wife was called Swarbhanavi. She had five sons --- Nahush, Vridhasharma, Raji, Gaya and Anena. Aayu's eldest son Nahush was a great scholar and a very brave king.
He ruled his kingdom according to the Dharma. Once due to his immense arrogance he made the seven celestial saints, the Saptrishi, carry him in a palanquin. He insulted them and that ultimately became the cause of his downfall. He had six sons --- Yati, Yayati, Sanyati, Aayaati, Ayati and Dhruva. Yati became an ascetic and Yayati became the king.