Thank you for your question. This is a question that has been used in the past to try to disprove the Bible. They use this to "prove" that Adam and Eve were not the first or the only people (or race) on the earth in the beginning, even though the Bible explicitly says so.
So let us begin with a question...
If Adam and Eve are the only 2 people on the planet and they have kids, and their kids get married, who are the kids marrying? Well there is only ONE logical explanation....they are marrying each other. Yes, sisters marrying brothers etc. Now before everyone gets all grossed out with this concept of "incest" we need to go into some history.
In accordance with the Jewish historian Josephus,"The number of Adam's children, as says the old tradition, was thirty-three sons and twenty-three daughters." If they would have NOT married within the family there would quite simply have been no more generations of people.
Many people cry out when you mention this because in marrying your brother or sister you break God's law (Lev 18-20). But one must also remember that Abraham married his half-sister (Gen 20:12), and God blessed this by starting the whole Hebrew nation from this union.
In our current day an age brothers are not to marry sisters due to the genetic deformities that arise. The more closely that the parents are related, the greater the risk of the child being deformed in some way. The reason why this happens is because we (human race) after 6000 or so years have acquired many genetic "mistakes" thus creating and improving the chance of deformity. Yet back in Genesis where there was only 2 people (perfect people), there was no genetic "mistakes" as of yet. So any child of theirs would be able to intermarry and not have any risks what so ever of their children being deformed or having any genetic "mistakes".
When God gave the Israelites the laws, the human race had past the point of genetic "no-return" the tipping point had been reached. No longer could one safely intermarry and have a child without a chance of deformities and or birth defects.
You might ask well what about this land of Nod where the Bible states he got his wife? Where there really enough people by this time to form a city? What does Gen 4:16-17 really say....
Genesis 4:16-17 (King James Version)16And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden.17And 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.
As you read this verse its clear that Cain went east to a land called Nod. He than had "sexual relations" with his wife...now it doesn't say that this is where he GOT his wife but merely that she was there. So they bared a son and then started a city and named them both Enoch. The Hebrew word for "city" means a "walled town", or a protected encampment...not what we view a city today as being.
And one last thing...what about Genesis 4:14?
Genesis 4:14 (King James Version)14Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that every one that findeth me shall slay me.
So who are "all" these people? Well they were merely his brothers sisters nephews nieces etc. Since there was no real civil law brother would avenge brother so to speak. So he would live in fear of his own relatives since he killed Able.
I hope that this answer fulfills your question fully, I know its a bit choppy but I tried to limit "wordiness". :)
