What does Genesis 5:4 mean?

The previous verse tells us that Adam was 130 years old when he fathered Seth. Now we learn that Adam, having many other children, lived another 800 years after Seth was born. These long lifespans seem amazing, but make sense when one considers a world free from pollution, most diseases, and most genetic disorders.

It would have been necessary, of course, for Adam and Eve to have many, many children. With no other source of humanity yet on the planet, their children would have married each other, had more children, and then continued to intermarry with members of the ever-expanding family.

Later, God would restrict the practice of marrying close relatives. As humanity continued to suffer the consequences of the fall, genetic diseases would have become more likely, and more problematic. As a result, close intermarriage carries a much greater risk of birth defects for the offspring. At this time, however, before that genetic pool had become so diverse, that would not have been an issue.