It is written that God spake to Moses in the tent. He held conversations with Moses. He could have easily dictated to Moses Genesis. No serious problem. And it is agreed that the books were written by Moses, or at least most.
Biological deformities
Today, brothers and sisters (and half-brothers and half-sisters, etc.) are not permitted by law to marry because their children have an unacceptably high risk of being deformed. The more closely the parents are related, the more likely it is that any offspring will be deformed.
There is a very sound genetic reason for such laws that is easy to understand. Every person has two sets of genes, there being some 130,000 pairs that specify how a person is put together and functions. Each person inherits one gene of each pair from each parent.
Unfortunately, genes today contain many mistakes (because of
sin and the
Curse), and these mistakes show up in a variety of ways. For instance, some people let their hair grow over their ears to hide the fact that one ear is lower than the other—or perhaps someone's nose is not quite in the middle of his or her face, or someone's jaw is a little out of shape—and so on. Let's face it, the main reason we call each other normal is because of our common agreement to do so!
The more distantly related parents are, the more likely it is that they will have
different mistakes in their genes. 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. Instead of having totally deformed ears, for instance, a person may only have crooked ones! (Overall, though, the human race is slowly degenerating as mistakes accumulate, generation after generation.)
However, the more closely related two people are, the more likely it is that they will have similar mistakes in their genes, since these have been inherited from the same parents. Therefore, a brother and a sister are more likely to have similar mistakes 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.
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.)
