While Cain and Abel were among Adam and Eve’s first children, they were not their only children. Adam and Eve had many other children, grandchildren, great-grandchildren, etc. In Genesis 5:4 a statement sums up the life of Adam and Eve— “And the days of Adam after he had fathered Seth were eight hundred years. And he fathered sons and daughters.”
Adam lived 930 years during which he fulfilled the divine command, “Be fruitful and multiply; fill the earth and subdue it; have dominion over the fish of the sea, over the birds of the air, and over every living thing that moves on the earth” (Genesis 1:28). He lived long enough to see eight successive generations reach maturity. His life spanned more than half the time to the Flood.
And the fact that Cain was scared for his own life after he killed Abel, indicates that there were many other children and perhaps even grandchildren of Adam and Eve already living at that time. Cain feared that the curse of God meant the withdrawal of His restraint from those who might seek to avenge Abel’s blood. And in effort to seek justice, his other brothers may pursue him and end his life.
In His service,