The Bible only records those significant events which God determines will be helpful to us. We know Adam and Eve had more children than Cain, Abel, and Seth. When Cain was punished, Gen. 4:16, 17 tells us, “Then Cain went away from the presence of the LORD and settled in the land of Nod, east of Eden. Cain knew his wife, and she conceived…” This proves Adam had more children who were living in Nod; their births were simply not recorded.
Adam and Eve were created perfect in God’s image. That’s why they and most of early mankind could live hundreds of years. Consequently, Adam’s children, sons and daughters, were very, very healthy. They married each other and their offspring were also healthy. By the time the Law was given (about 2,600 years later), God prohibited family intermarriage. Lev. 18: 6-17 explains in detail the family relationships which God declared wicked. Today, intermarriage within family members produces children who have very severe health problems.
There were no other human fathers of our race. The scriptures plainly teach that death came to us through Adam because of his sin. We are all dying because we are Adam’s children. 1 Corinthians 15:22, “As in Adam all die.” Jesus took Adam’s punishment and thereby redeemed the entire race of mankind. 1 Cor. 15:22, “So also in Christ shall all be made alive.” Romans 5:18 explains this as follows, “Therefore, as one trespass led to condemnation for all men, so one act of righteousness leads to justification and life for all men.”
In summary, Adam and Eve were the sole parents of the whole human race. When they had nearly perfect children, their children married each other. But as the genetic health of the race degraded, family intermarriage became prohibited. The ransom sacrifice of Jesus redeems all mankind because Jesus bore the punishment for Adam (and all of his progeny).