If Adam and Eve had children who had to mate with each other?

The Bible never says that they interbred. It does clearly say that Cain ran off out east and took wives and built a city, indicating that there were other people out there. After he killed his brother, he was worried that people would persecute him for it, but who was he afraid of? It is obvious even from the Bible that Adam and Eve had neighbors. The first part of Genesis 6 tells us that the children of Adam mated with the children of natural men (evolved). I interpret this as meaning that God took notice of a developing species on Earth and interjected just a little upgrade (Adam and Eve), to produce the modern human race. My views are not shared by your typical fundies, though.