Many Christians are baffled by the fact that the Bible tells us that Adam and Eve had only three sons, Cain, Abel and Seth. It then goes on to say that Cain murdered Abel and “…went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden.” (Gen 4:16) In Nod he not only found a wife but built a city, which presumably he did not do alone. “…and he builded a city, and called the name of the city, after the name of his son, Enoch”. (Gen 4:17). Furthermore Adam and Eve did not give birth to Seth until after Cain murdered Abel and was banished to Nod. (Gen 4:25).

One of the main reasons this question perplexes most people is because they do not actually read the words of the text and assume that Cain found his wife in the land of Nod. But read the text again!

"And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden. And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch." (Genesis 4:16-17).The Bible doesn't say that Cain found his wife in Nod. It says that he went to Nod and then “knew” his wife. . The word ‘knew’, means that Cain had sexual relations with his wife, who he could have been already married to when he left for Nod. (The word ‘knew’ is only used in some translations. The New American Standard Bible says “And Cain had relations with his wife…”) So… Where did Cain get his wife?

The Bible does tell us "after Seth was born, Adam lived 800 years," and it also tells us that "he had other sons and daughters." (Genesis 5:4 ).
In fact, the genealogy in Genesis 5 records that every descendant of Adam down to Lamech had "other sons and daughters," some born to men who were on the wrong side of 180 years. People up to this point in Biblical history had very long life spans thereby substantially increasing their childbearing years, which means there could have been a out-and-out population explosion in a relatively short period of time.
If Eve’s childbearing years were about 500 and they had their first child at about the age of 50 and another child every five years, it is not outside the realm of possibility that he had sisters from whom he picked a wife even before he left. However had he waited to marry until he was about 200 years old, he probably had more than a few women to choose from, some of whom could have left for Nod before he did. In fact the world's population could have approached a few billion by the time of Adam's death at the age of approximately 900, and at least 120,000 people could have been alive on earth within only a few hundred years. The math is simple….

Cain Built a City?
The Bible does not mention how old Cain was when he left for Nod nor how old he was when he started to build the city. Had Cain waited until he was 400 years old before starting his city he could have had several thousand people to help him. Additionally we should bear in mind that in the modern world we automatically associate the word ‘city’ with a huge bustling metropolis. However the Hebrew word translated as ‘city’ is not necessarily even similar to a ‘city’ of today. The word meant a "walled town" or a protected encampment (Strong's Concordance: "city, town, a place guarded by waking or a watch in the widest sense (even of a mere encampment or post)."
Inter-Family Marriage?

God’s laws at the time condemned sexual relations between children and their parents but said nothing about inter-marriage between brother and sister, which apparently did not bear any evil consequences. (Abraham married his half sister.) The more distantly related parents are, the more likely it is that they will have different genetic disorders. Children, inheriting one set of genes from each parent, are likely to end up with pairs of genes containing a maximum of one bad gene in each pair. The good gene tends to override the bad so that a deformity (a serious one, anyway) does not occur.

However, the more closely related two people are, the more likely it is that they will have similar genetic problems, since these have been inherited from the same parents. Therefore, a brother and a sister are more likely to have similar problems in their genes. A child of a union between such siblings could inherit the same bad gene on the same gene pair from both, resulting in two bad copies of the gene and serious defects. In the time of Adam and Eve the genetic pool was pristine and close marriages didn’t pose the problem they do today. However by the time of Moses (a few thousand years later), degenerative mistakes would have built up in the human race to such an extent that it was necessary for God to forbid close relative marriages, which makes perfect biological sense. Also, there were plenty of people on the earth by then, and there was no reason for close relations to marry.)

rite post...
so in sumary cains wife was one of Adam & eve descendant or grandchild born to an unknwn date

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