La biblia se puede leer literal; o sea que después que Dios creo al hombre creemos que el pecado sucedió rápido y no; tuvo que haber pasado mucho tiempo después de ellos haber sido creados para que cuando Cain matara a su hermano ya hubiera gente en otros pueblos

Adam and Eve had children, who in turn had children themselves. When God created Adam and Eve, God knew that it was a necessity that the children that Adam and Eve bore, had to intermarry within the family. Today God forbids incestuous relationships, but in the time of Adam and Eve, it was a necessity. Also, after Adam and Eve sinned against God, God had to withdraw his presence from sinful people, and a sinful world because God is Holy and can have nothing to do with sin. As a result, God's hand, that was holding all things together in perfect harmony, was now removed from the world, and as a result, sin has had a destructive effect not just on man, but on all of creation as a whole. That is why the natural order of things fell into disorder and chaos. The destructive effect of sin on the world has steadily gotten worse over time, but in the time of Adam and Eve, although sin had a negative effect on the natural order, it was much much less so than it is today. Therefore, children born from close family relationships didn't have the same effect as it would today. Therefore, there was less chance that children born out of incestuous relationships in the time of Adam and Eve were not likely to born with physical defects. But because the natural world has mutated over the centuries, it is much more likely that children born of incestuous relationships would be born with physical defects today. This is one reason, but not the only reason , that God forbids incestuous relationships....but he allowed it in the beginning of the world out of necessity, otherwise how else could the world be populated?

Adam and Eve populated the world. Remember God made Adam a help meet. Eve is the mother of all mothers