Story of Yam & Yamuna

Initially, the Purusha, the supreme form of godhood created the universe as we know it. After he created the Earth, it was time for creating the humans. So he created a man and a woman. The man was named Yama and the woman was named Yamuna. They both knew that for life to prosper in earth, their intercourse was necessary. Yamuna forced Yama to complete the task but Yama refused to do so as it was against the Dharma/principals of a brother and sister, so Yama descended himself to Patala or the netherworld. Yamuna couldn't bare the agony and pain of separation from her brother and she cried endlessly till she herself was transformed into a river.

In high heavens, the Gods appreciated both of them, and thus declared Yama as Dharma, as he had upholded Dharma and Yamuna as the mother goddess of river who shall take away the agony and distress of a person who takes a dip in her.

But to my knowledge Manu and Satrupa were the first humans created by Brahma.Then , what is the true story?😕