Elohim had blessed the human beings He had created and allowed them to have children. That way A'dham became the father "of a son" in his own likeness, "after" his own image. We were not told this about Cain and Abel simply because the text did not wish to discuss them at length. In this chapter 5 Moses goes straight to the third son of Adam, Enosh. It is noted that from that time the people started to call upon the Name of Jehovah God.
The fifth chapter of the first book of the Pentateuch brings us the accounting of mankind. Moses gives an idea how the Children of God came one after the other on earth to play their role in history.
Man has to know he can connect to the Creator God as He reveals Himself within creation, but the only way we can connect to Jehovah God Himself is by likewise rising above the limits of rationality.