Who did Cain marry, and who was he afraid of?

In Genesis 4:14-17, we see Cain in fear of retribution, marrying his wife and beginning to set up a city. Who did he marry and of whom was he afraid? Some theistic evolutionists suggest that Cain married a member of an evolved human race. Bible scholars suggest Cain married his sister or niece. Would this be a problem genetically as later these sort of relationships were banned?

The Bible only names 3 of Adam and Eves children (Cain , Abel and Seth), but Gen 5v4 says they had many sons and daughters. When Seth was born, after Abel’s murder, Adam was 130 years of age and probably had many other children besides Cain and Abel.
Other facts that need to be noted are :-
• The Bible clearly says Adam was the first man on earth (1 Cor 15:45-47)
• No other human was found as company for Adam (Gen 2:20)
• Eve was called the “mother of all living” (Gen 3:20)
• God says all people living on earth came from a one man (Acts 17:26)
• Adam lived 930 years and probably had many children.
• Death was rare so the population increased very rapidly. Estimated 2 billion by the Flood.
• Even a conservative estimate has the earth’s population at several million by Cain’s death.
• Cain was possibly already married when he lived in Eden (Gen 4:17)
• So Cain and his brothers had to have married sisters or nieces.
• Cain was concerned that his relatives would try to avenge the death of Abel.
• The Bible does not tell us over how many years Cain and his children built their cities (settlements), it could have taken hundreds of years .
• The land of Nod was probably the name used in the time of Moses for the place Cain had migrated to.

What about the biological issue? Marrying close relatives can cause deformities , and almost all human societies forbid it.

• Hereditary traits , and sometimes defects , are passed on by parents DNA to their children
• Genetic instructions work in pairs , one from each parent.
• As generations go by, copying errors do occur and can lead to disabilities.
• Generally the defective gene from one parent is overruled by the correct gene in the other parent.
• Over many generations defects in DNA accumulate in the population
• If close relatives procreate the chances of both parents contributing the same defective genes increases and serious defects can occur.
• But as Cain and his sisters (or nieces) were very close to the originally “ very good” condition of Adam, there was very little accumulated defective DNA in these years
• Abraham married his half sister, without problem.
• Only by the time of the law of Moses (2000years later), did God now ban close relatives from marrying. By now it was a real issue of concern.
• God banned incest on moral grounds as well as genetic reasons (Lev 20:11)

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