Cain was the first child of Adam and Eve recorded in Scripture (Genesis 4:1). His brothers, Abel (Genesis 4:2) and Seth (Genesis 4:25), were part of the first generation of children ever born on this earth.
Even though only these three males are mentioned by name, Adam and Eve had other children. In Genesis 5:4 a statement sums up the life of Adam and Eve—“And the days of Adam after he had fathered Seth were eight hundred years. And he fathered sons
and daughters.” This does not say when they were born. Many could have been born in the 130 years (Genesis 5:3) before Seth was born.
During their lives, Adam and Eve had a number of male and female children. The Jewish historian Josephus wrote that, “The number of Adam's children, as says the old tradition, was thirty-three sons and twenty-three daughters.”[11]
The Bible does not tell us how many children were born to Adam and Eve. However, considering their long life spans (Adam lived for 930 years—Genesis 5:5), it would seem reasonable to suggest there were many! Remember, They were commanded to “Be fruitful, and multiply” (Genesis 1:28).
If we now work totally from Scripture, without any personal prejudices or other extra-biblical ideas, then back at the beginning, when there was only the first generation, brothers would have had to have married sisters or there would be no more generations!
We are not told when Cain married or any of the details of other marriages and children, but we can say for certain that some brothers had to marry their sisters at the beginning of human history.Many people immediately reject the conclusion that Adam and Eve's sons and daughters married each other by appealing to the law against brother-sister intermarriage. Some say that you cannot marry your relation. Actually, if you don't marry your relation, you don't marry a human! A wife is related to her husband even before they marry because all people are descendants of Adam and Eve—all are of “one blood.” The law forbidding marriage between close relatives was not given until the time of Moses (Leviticus 18-20). Provided marriage was one man to one woman for life (based on Genesis 1 and 2), there was no disobedience to God's law originally when close relatives (even brothers and sisters) married each other.
Remember that Abraham married his half-sister (Genesis 20:12). God blessed this union to produce the Hebrew people through Isaac and Jacob.
It was not until some 400 years later that God gave Moses laws that forbade such marriages.
Adam and Eve did not have accumulated genetic mistakes. When the first two people were created, they were physically perfect. Everything God made was “very good” (Genesis 1:31), so their genes were perfect—no mistakes! But, when sin entered the world (because of Adam—Genesis 3:6, Romans 5:12), God cursed the world so that the perfect creation then began to degenerate, that is, suffer death and decay (Romans 8:22). Over thousands of years, this degeneration has produced all sorts of genetic mistakes in living things.
Cain was in the first generation of children ever born. He (as well as his brothers and sisters) would have have received virtually no imperfect genes from Adam or Eve, since the effects of sin and the Curse would have been minimal to start with (it takes time for these copying errors to accumulate).
In that situation, brother and sister could have married with God's approval, without any potential to produce deformed offspring.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 brother-sister (and close relative) marriage (Leviticus 18-20).[12] (Also, there were plenty of people on the earth by then, and there was no reason for close relations to marry.)
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