Where did the other peoples of the world come from? Even when Cain was sent to wander, God speaks of other peoples. Were they all still descended from Adam and Eve?

All the people on the earth came from the first two people—Adam and Eve. Once they were created, God blessed them and said to them, “Be fruitful and increase in number; fill the earth and subdue it—Genesis 1:28a. And, in that sense, they were obedient. We know of Cain, Abel and Seth but, according to the Scriptures, [a]fter Seth was born, Adam lived 800 years and had other sons and daughters—Genesis 5:4. Now of course, prior to the birth of Seth, we have the incredibly sad episode of the world’s first murder—Cain killing his brother Abel. As the book of Genesis tells it, In the course of time Cain brought some of the fruits of the soil as an offering to the Lord. But Abel brought fat portions from some of the firstborn of his flock. The Lord looked with favor on Abel and his offering, but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast. Then the Lord said to Cain, “Why are you angry? Why is your face downcast? If you do what is right, will you not be accepted? But if you do not do what is right, sin is crouching at your door; it desires to have you, but you must master it.” Now Cain said to his brother Abel, “Let’s go out to the field.” While they were in the field, Cain attacked his brother Abel and killed him—Genesis 4:3-8. Cain essentially “fled the scene” but, in the course of his escape from the area, God graciously placed a “mark” on him to prevent him from being killed in revenge by anyone who came upon him. Obviously the people coming upon him would have been family—seeing as only Adam and Eve and their children and children’s children and their children’s children were on the planet at the time. From there, the Bible tells us that Cain went out from the Lord’s presence and lived in the land of Nod, east of Eden. Cain lay with his wife, and she became pregnant and gave birth to Enoch. Cain was then building a city, and he named it after his son Enoch—Genesis 4:16-17. This would have been one of Cain’s sisters, or possibly a niece or even one more distant—but it still would have been family. We must remember that this was in the earliest days of the human race and, unlike today, there were not yet any genetic defects that had yet developed. Even by the time of Abraham, close family marriages had not yet been declared by God to be wrong—Sarah was Abraham’s half-sister (same father, different mother). The present-day circumstance of imperfect, defective genes does not apply to Adam and Eve. They were the most physically perfect human beings who ever lived, crafted personally by God Himself and as Genesis declares, God saw all that He had made, and it was very good—Genesis 1:31. But when sin entered the picture by Adam and Eve eating the forbidden fruit, a curse began to overshadow mankind; mankind became a species that experienced degeneration, decay and finally, death. Over a long enough period of time, that process would have resulted in all sorts of imperfections and defects beginning to appear, including within human genetic material. But Cain, the firstborn of Adam and Eve, and his brothers and sisters would have received virtually no imperfect genetic material, being in the first generation of children ever born. In that context, brothers and sisters could have (and would have) married with no reproductive complications. And certainly, God had commanded, Be fruitful and increase in number; fill the earth and subdue it—God speaking, Genesis 1:28a. God would not have deliberately put mankind in a harm’s way of His own devising. However, by the time of Moses, you would now have found sufficient time in the degeneration of the human gene pool for mistakes to accumulate, to such an extent that it would have been necessary for God to introduce into His law sanctions regarding the marriage of close relatives (see Leviticus 18-20). And of course, there would have been sufficient human population on the planet that there would no longer be the need for close family relations to marry. God obviously cares incredibly deeply about the family, so the laws that He gave to Moses would have had the aim of (1) trying to protect against the potential for deformed children, (2) keeping the nation of Israel strong and healthy, thus allowing them to remain strong and able within the plans that God had for their nation, and (3) protecting individuals, families and society at large from the destructive harm that incestuous relationships can cause.