Cain wished to be accepted by God by the agricultural offering he brought. Abel's offering was an animal sacrifice. He believed and obeyed God. Throughout Scripture two truths remain constant no matter what age or economy is present. God gave Cain every opportunity to change his mind about the proper way to approach God.
Today is no different.
Tragically, most people, like Cain, believe that they can come to God through ways that seem reasonable and good to them. God's only means of acceptance of a person today is through a person's trusting in the accomplished work of Christ on the cross and his resurrection. In the early chapters of Genesis we read about a sophisticated civilization and extreme life spans for mankind. The offspring of these unions were giants, the Nephilim. From these unions and offspring come the myths of the gods of ancient people.