Again, check what the scripture actually says - you are reading your own assumptions into the text. You are assuming there were other people, and assuming that's what he is talking about. Let's just look at the wording itself.
"And Cain said unto the LORD, My punishment
is greater than I can bear. Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass,
that every one that findeth me shall slay me."
In an ancient society, when there was a murder, who would try to kill the murderer? The family of the victim. They would be out for vengeance. Other people have no problem with the murderer - he hasn't harmed them, they probably don't even know who he is. But the victim's family will kill him on sight.
Cain says that "every one" who finds him will kill him. If there were other people in the world who were NOT from his family, this would not be true! There would be other tribes who knew nothing about the murder, or didn't care, who he could take refuge with and who would NOT kill him.
The only reason for him to fear that "everyone" would kill him would be because everyone who could possibly find him was a member of Abel's family and would be out for vengeance.
So Cain does not say he is worried he will be killed by "others" not of his family. He just says he is worried everyone will kill him. And that only makes sense if there are no others.
You're reading your own assumptions into the text because you have presuppositions like the above, and are trying to make the Bible fit what you already "know" is true. There is actually a lot of uncertainty in the scientific world regarding human evolution, which does not come through in what you are taught in school, where all the uncertainty is brushed over and you are told a "just-so" story, most of which is actually speculative on further examination. I would highly recommend you
buy a copy of the book "Bones of Contention". When I was going through high school, we spent much of our final year of biology class studying human evolution. I took Bones of Contention as a companion textbook. It was far more detailed than the school textbook, and included far more information - most crucially, it shows you the actual scientific evidence that lies behind the hominids you are learning about. For instance, the school textbook would tell us all about some hominid, and Bones of Contention would clarify that they only actually found a tooth and a toebone, and show which of what we were taught was based on fact and which was speculation. It prompted many very useful discussions in our class also, and several years later one of my old classmates tracked me down to ask about it and I gave my copy to them. If you're interested in human evolution,
this is essential reading.