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The conjectures about to this subject are many. Some think that Adam and Eve had children before the sin and that of those, Cain took woman. Others think that God created to another partner; like these, many other ideas arise more on the matter.
THE FIRST CHILDREN
The first order that our parents received was: “...Be fruitful, and multiply and fill the earth...” (Genesis 1:28). The first children were Cain and Abel according Genesis 4.
WERE THERE MORE PEOPLE?
The dialogue maintained between GOD and Cain can make us think that others people existed (men and women) on the Earth: “And Cain went out from the presence of the LORD and lived in the land of Nod, on the east of Eden. And Cain knew his wife, and she conceived and bore Enoch. And he built a city, and called the name of the city after the name of his son, Enoch.” (Genesis 4:16-17).
However, if we analyze with more care, we see clearly that there are not more people. We read thus in Genesis 4:14 “Behold! You have driven me out from the face of the earth today, and I shall be hidden from Your face. And I shall be a fugitive and a vagabond in the earth, and it shall be that anyone who finds me shall kill me.” The expression “...that anyone who finds me shall kill me...”, it does not mean that it happened immediately but anticipating future events, when the humans were multiplied.
Many years, Cain was alone, as he says himself in the verse 14, VAGABOND, but later we read when his first son was born, he built a city, meanings that he stopped and was not a vagabond longer (verse 17).
EVEN WHEN DID HE KNOW HIS WOMAN?
Genesis 4:17 says thus: “...Cain knew his wife, and she conceived and bore Enoch...” According to this verse, it seems that immediately after he went away from the residence of his parents, and knew his woman, but already we have seen that there was not more people, meaning that it was a lapse of several years until it happened.
The book of Genesis regularly relates distant events as if these had been immediate; the test we can find it reading the verses 17 and 18: “And Cain knew his wife, and she conceived and bore Enoch. And he built a city, and called the name of the city after the name of his son, Enoch. And Irad was born to Enoch. And Irad fathered Mehujael. And Mehujael fathered Methusael. And Methusael fathered Lamech.” In the verse 17 says that Enoch born and in the verse 18 says that his son born, but this could not happen immediately; this means that between the verses 17 and 18 there is a difference of several years, even though the verses describe the followed event.
This same happens between the verses 16 and 17. In the verse 16 says that Cain went out from the presence of the LORD and in the verse 17 says that he knew his wife, so it seems that were united events, but the reality is that some years happened, like in the verses 17 and 18.
WHAT IS HER ORIGIN?
After death of Abel and the expulsion of Cain, Adam knew his wife again, and she bore a son, and called his name Seth (Genesis 4:25). Seth was instead of Abel: “...God has appointed me another seed instead of Abel, because Cain killed him.” (Genesis 4:25).
After birth of Seth, Adam fathered more children and daughters, as Genesis 5:4 says: “And the days of Adam after he had fathered Seth were eight hundred years. And he fathered sons and daughters.”
When the children of Adam were multiplied, they populated the Earth, having taken Cain from these women. This means that Cain took woman from his own family.
The marital union between brothers and sisters, in the beginning did not have anything of bad, because being the order of GOD to be multiplied and to fill the Earth, it was necessary that there were marriages between brothers and sisters. God allowed this so that the humanity was multiplied.
Later, such unions were prohibited, as we read in Leviticus 18:6 - 18. So that when people already were organized, the LORD described such union as SIN.