Adam and Eve gave birth to Cain and Abel. Later they had a son named Seth, but the story of strife between these two brothers is the famous first example of murder among mankind, and they're really the only two people I think of as second generation human beings. I'm forced to realize that this is far from the truth - that there were likely hundreds of offspring from Adam and Eve in this second generation - when I get to verse 14 of chapter 4.
This is after Cain has killed Abel and God has banished him to wander the earth, which will also no longer produce fruit for him as punishment for his crime. This seems a pretty fitting, if not lenient, punishment for inventing murder. Of course, today we're all-too familiar with homicide and it's not a stretch to understand the implications of exile; separation from home, family, and familiarity. But this was more severe still, because cultivating the soil to bear fruit was Cain's livelihood. It was his work, his profession, his method of providing for his family, and the trade he learned from his father, Adam. This was among the very first learned behaviors between one of the very first father/son relationships. In only two generations of human existence there's hardly such a thing as heritage, but this was about as close as it gets. Fittingly, Cain has been deprived of it.
Anyway, at verse 14 Cain petitions God on his own behalf, claiming that this punishment to wander the earth would cause "whoever finds" him to kill him. God responds by putting a mark on him and announcing that whoever might kill Cain would be avenged seven times over. Needless to say we don't hear anything about Cain being harassed after that. But it was here that I realized Cain had other people to worry about on the earth.
At the time of Cain's sentencing, humanity consisted of much more than the happy little four-person nucleus of Adam, Eve, Cain, and Abel as I had always pictured. Adam lived a total of 930 years and he was 130 when Eve gave birth to their only other named son in the Bible, Seth - after Abel's murder (Gen. 4:25, 5:3). So Adam could have been 129 or 130 years old already at the time Cain's exile, meaning that it's physically possible for the first of his offspring to have been 128 or 129 years old as well. If Eve never delivered twins or multiples and had about one child per year until this time (remember, it was their job to multiply and populate the earth), then there could have been up to 127 other second-generation humans with Cain and Abel. Assuming the age of 14 before any of these offspring were fertile themselves, there could have been as many as 3348 third-generation human beings at this time (following the logic of a 50% female population and a reproduction rate of one child per year after age 14). These women could have turned out a total of 3477 first, second, and third generation humans for Cain to be concerned with at the time of his exile. Not to mention any fourth, fifth, or sixth generations in even larger populations, the oldest of which could have been 87 years old already. This means roughly tens of thousands of people at risk of judging or killing Cain as a wanderer on the earth.
All this to say that I was originally confused about why Cain was concerned for his safety, but there was obviously more happening on the earth than I originally pictured. Understanding this moderate estimate of reproduction rates and population growth also helps me with the picture of everybody "marrying" or "laying with" people who could not have been farther in blood relation than a first cousin. I guess when the numbers so quickly reach that of a moderately sized rural American county, kissing your sister isn't quite so much like kissing your sister anymore.
I'm still confused by the issue of gene pool diversity, but you've gotta start somewhere, right?