Although there is no proof that other humans existed, it is reasonable to suppose that Adam, Eve, and their three sons were not the only humans during the early post creation years. One of the biblical writing methods, is not to reveal everything, to leave it to the reader to figure it out.
We can investigate the issue by looking at some possible indicators.
- The Bible does not list all the people whenever it has a list of people. It generally excludes females. Also, in Genesis 5, for example, where the ten generations from Adam to Noah are listed, only a single person from each generation is named. Similarly, in chapter 10, where the Bible lists the descendants of Noah, it does not include the females. Thus, listing only Adam, Eve, and their three sons without mentioning the other people who may have existed at the time, is consistent with the general biblical style and should not dispose us to think they did not exist.
- Cain, Abel, and Seth had wives who are not listed. When were they created? A midrash suggests that a female twin was born with each son. This would mean that they bedded their twin sisters. This explanation is not in the text. It is contrary to the moral stand of the later Torah which forbids a man marrying a sister. It is arguably more reasonable to suppose that other people existed at the time.
- Similarly, the Torah states that Adam and Eve did not have their son Seth until Adam was 130- years-old. What happened during the 130 years? It is possible that he abstained from sex during these many years. It is more likely, that he had many children, male and female, during this period of time, but true to its style, the Bible does not mention everything that occurs. Seth is mentioned since he was an ancestor of Abraham.
- In Genesis 4:14, Cain tells God that he is afraid “whoever finds me will kill me” (perhaps in revenge for killing Abel). Who is he afraid of? If we accept the idea that only he and his parents and, perhaps, his and Abel’s wife, were alive, should we interpret “whoever” to refer to them? If he meant his relatives, why doesn’t he say so? Is it reasonable that he is afraid of his relatives? Wouldn’t it be more reasonable to say he feared the non-relatives who also existed at that time?
- In response to Cain’s fear, God placed a sign for Cain. If only the family existed, why would a sign be necessary and why does the Bible say that God placed the sign “lest any finding him should slay him”? These words “any finding him” does not seem to fit the small family.
- Cain goes out in 4:16 and “dwelt in the land of Nod.” If no other people lived at the time, why is there a land of Nod?
- In verse 7, Cain builds a city and names it after his son. If only he, his wife, son, and possibly Abel’s widow lived at the time, and he left his parents, why build a city?
- Chapter 4 states that Abel was a keeper of sheep and Cain a tiller of the ground. If only the small family existed, why would the brothers need to engage in these vocations? Surely, for example, if they were meat eaters, they could easily find an animal. There was no need for Abel to raise sheep unless he was doing so for other people.
- True, 1:26 has God state “Let us make man in our image, after our likeness” and in 2:7, the Torah states, “Then the Lord God formed man of the dust of the ground,” and it is usually understood that “man in these verses denotes a single individual. However, “man” could mean “humans” in the plural. Also, there is no explicit statement in scripture that there were no other humans that God created. Yes, later in chapter 5, the Bible only gives us the genealogy of Adam and Eve and mentions no other humans existing. But, as previously noted, this genealogy is not complete. Also, it focuses of the descendants of Adam and Eve simply because these are the people who are important to the story, they are the ancestors of Abraham.
These ideas seem to
suggest the possibility of the existence of more than the small Adam and Eve
 Adam and Eve bore Cain and Abel soon after creation. They had Seth when Adam was 130 years old.