Many of the details of the early population of the earth are not known. At some point in time Cain, the first recorded child was born to Adam and Eve. Afterward, his brother Abel came into the world. We don’t know how many other people had been born at this time, nor do we know how long after the creation these two boys were born.
After a period of time, Cain and Abel brought offerings to present to God. Cain brought from the fruit of the ground (probably some type of grain) and Abel offered from the firstborn of his flock. God received the offering of Abel, but rejected Cain’s offering.
A casual observer could think that God was unfair in His dealings with the two brothers. Cain’s offering was probably from the best of his harvest and it was rejected. Why was his sacrifice not pleasing to God?
God has always had a pattern for man to worship Him. The writer of Genesis had not recorded the plan that was in effect at that time. We learn from later writings, that faith comes by hearing God’s word and that Abel offered his sacrifice by faith. We must infer from this that God had indeed commanded an animal offering of the firstborn of his flock.
Jealousy caused Cain to murder his brother, Abel. After being confronted by God, Cain’s punishment caused him to become a fugitive and vagabond. However, he did marry and become the father of at least one son, Enoch.
As God’s plan for redeeming man began to form, only the important genealogies were recorded. Among the other children of Adam and Eve, Seth was born. It would be through his descendants that Jesus would be born thousands of years later.