Hello, Mr. and Mrs. Johnston,
Your question to Fr. Spitzer has been forwarded to me. I help out from time to time when Fr. is on overload. I'm a member of the Board of Academic Fellows for Magis Center. Here are your questions: "In Genesis, Who were the people Cain was afraid of after slaying Abel? Weren't Cain and Abel the first offspring of the first couple? Where did the others come from?"
I'd like first to set some general considerations, then I'll try to address your specific questions.
First, consider the doctrine / dogma of monogenesis, that man originated from one pair of humans, man and woman, Adam and Eve. This is fundamental to the dogma of Original Sin. Now, how is the reconciled with all the evidence-fossils, genetics-that humans developed from a population of primates? There is one clear answer to this: that it was the implantation of a soul in Adam and Eve that made them human. A soul is what defines us as human: the capacity for rational thought, for abstraction, for wonder about where we came from and what will happen to us when we die. It is not the physical form. This has been expressed much better than I could by Pope St. John Paul II:
"Pius XII underlined the essential point: if the origin of the human body comes through living matter which existed previously, the spiritual soul is created directly by God..." Pope St. John Paul II, Address to Pontifical Academy of Sciences: "On Evolution".
Now to your question about other "humans" being present when Cain committed murder. We can suppose that other homo sapiens might have been present, but they were not "human" in the sense that they did not have a soul. It is the implantation of a soul by the Holy Spirit at the moment of conception that makes us truly human. The other point to keep in mind is that Scripture is not a science text, it is a book about God and his people, about ethical and moral behavior. We have to take Scripture seriously, but not necessarily literally. Some 1600 years ago St. Augustine of Hippo put this very well:
"Often a non-Christian knows something about the earth, the heavens, and the other parts of the world, about the motions and orbits of the stars and even their sizes and distances,... and this knowledge he holds with certainty from reason and experience. It is thus offensive and disgraceful for an unbeliever to hear a Christian talk nonsense about such things, claiming that what he is saying is based in Scripture. We should do all that we can to avoid such an embarrassing situation, lest the unbeliever see only ignorance in the Christian and laugh to scorn." --De Genesi ad litteram; the Literal Meaning of Genesis.
I've written about evolution and monogenesis at greater length in several blog posts:
In His Service,
Bob Kurland, Ph.D.