When Cain received his punishment for murdering Abel, one of his greatest fears was that "anyone who finds me will kill me!" Old earthers love this verse. "See!" they cry. "This proves that Adam and Eve weren't the first humans, there were more that the Bible just doesn't mention!"
But in Genesis 6:1, we see that men only just began to multiply across the face of the earth a short time before the flood. Before that time, the population would have been "small". Exactly how small, we don't know, but to build a whole species from just 2 people isn't going to happen in a generation.
This actually makes a lot more sense of Cain's words. There are hundreds of fugitives in the world today. Terrorists, rapists, murderers, they roam our world. How many of them fear "anyone who finds me will kill me"? Not many. Even if, by some small chance, they got recognised, what are the chances someone would kill them? Would you kill a fugitive?
But Cain had two disadvantages. The people he would encounter would all be very close relatives of both him and Abel. Brothers, sisters, maybe nephews and nieces. It's logical to fear that anyone who found him would kill him because they would know his face and have reason to kill him.
It is far better to focus on what the Bible actually says than what we think the Bible could imply. We don't need to say "Cain feared being killed by humans, therefore Adam and Eve weren't the first and only two humans". We have multiple explicit statements that tell us the historical Adam and Eve were the first two humans who were created directly by God a mere 6 days after the heavens and the earth began, roughly 6,000 years ago. That we are all their descendants, and that this is both why we inherited their sin nature and why Jesus is capable of saving us all with one sacrifice. These explicit statements should not be set aside in favour of cherry picked verses which don't even imply what you want them to imply.