Beautiful question!

There are several creation or Genesis stories in Puranas over several time cycles and eons. 

This is from Matsya Purana! (15th purana)

Manu at that time is Svayambhuva, I don't have the Brahma's name and this is a Shiva who was born as Brahma's son. In several creations, Shiva comes down as Brahma's son to help. 

Story begins:

Brahma married Shatarupa and they lived together as man and wife for a hundred years. Their son was named Svayambhuva Manu. Hearing the account, Vaivasvata Manu exclaimed. ―But what you have just said is truly amazing. How could Brahma have married his own daughter? Surely that is a sin.‖―Perhaps,‖ replied Vishnu, ―but it is not for humans to judge the actions of the gods. How could creation proceed if Brahma did not marry Shatarupa?‘

To continue with the account of the creation, Brahma created a sage named Sanatakumara and Shiva.

Brahma asked Shiva to help him in the act of creation. ―Why don‘t you create some beings as well?‘ asked Brahma. Shiva complied and started to create. But all the beings that he created were just like him in appearance. That is , they were all immortal. ―What are you doing?‖ asked Brahma. ―Don‘t create immortal beings. Create mortal ones instead.‖

―That I refuse to do,‖ retorted Shiva. ―If I am to create, I shall create only immortals.‖―Please do not create then,‖ requested Brahma. ―I will take care of creation myself.‖