In Genesis 4 the Bible says that Cain, after being cursed by God to wander and be an exile on the earth, settled in the land of Nod. That simple statement, added with the assertion that Cain knew his wife, has caused some to speculate that the land of Nod was the dwelling place of another race of people not descended from Adam and Eve. If the people of Nod are not another race , where did they come from and how did Cain settle in the land of Nod?
As was already answered, nothing in Genesis 4 requires that Cain met people in the land and married into their clans. We don’t know if Cain and Abel had any other siblings at the time of Abel’s murder, but we do know from Genesis 5:4 that Adam and Even had other sons and daughters. It is not at all unreasonable to think that as the population of the world increased they spread out from around Eden into other lands. All the members of the first generation would have married siblings or children of siblings. The genetic mutations we are concerned about now would not have been a problem, only after generations would mutations have crept into the gene pool and the danger of close intermarriage (consanguinity) come into being. In the first generations after Adam and Eve, human population would only have increased by familial intermarriage.
After years of exile, Cain settled in a desolate land that came to be called Nod. Cain married, had children and his descendants populated the land first settled by their forefather. Instead of Cain joining the people of Nod, the people of Nod were descended from Cain. He settled in that region and become the father of the land of Nod.
Interestingly Nod means wandering. Since Cain was condemned by God to wander away from his parents and siblings as an exile, it makes perfect sense that the land where Cain eventually settled would be called the land of Wandering. It is not strange that Genesis gives the place of his settling a name in the fashion it does. We do this today when talking about historical figures. In the Upper Peninsula of Michigan, there are many places that have interesting historic significance. The early explorers that came through the area settled in places with names like Michilimackinac, Sault Ste. Marie and L’Anse. Many of these places were not given names until after they were settled, some were not given these names until much later, but it is very rare to hear any one say, “he settled in the place which would eventually be called . . .” We find the same thing in the Bible. Places were at times referred to by the names they would later be given in history. The fact that the land is called Nod does not require there to have been a people in the land that had already named it. Rather, the land Cain settled was eventually called Nod, the land of wandering.