If you take the narrative literally, God was certainly free to create other humans. We know Adam and Eve were the first two, and they had two sons. God could have created many other humans and all of the above would still be true.
Could have, but if so, why doesn't the Bible give us that information?
Adam and Eve lived around 900 years, as most others did before the Flood. How many children could they have had from child bearing years on up, and how many children's children could have been born, and so on. If Cain killed Able after 100 years or so, there would have literally been thousands of people around for him to marry that were more than 2nd cousins twice removed. The Bible ONLY mentions the people that God wants you to know about and is important to the story of Jesus. There isn't enough paper and ink to tell all the stories, so you get the ones dictated by God to the prophet that wrote the particular book you are reading. If you read a history book, you may read about Washington, Jefferson, Lincoln, etc, but mysteriously, they leave out uncle Bill and his horse thievin.
When you get to Matthew, you get the lineage of Jesus back to Abraham, and thus to Adam. They only mention the names you should be familiar with in the Bible. If they came up with "so and so" begat "what's his name", you would say "where did that come from?" The Bible is the story of the Revelation of Jesus to mankind. All of the "begat's" in the Bible are for nothing more than to prove the line of Jesus. If Jesus was just a "nobody from nowhere", The Bible wouldn't be a prophetic book( aka the promises made to Abraham).
All names mentioned in the Bible are significant. Just because someone isn't named doesn't mean they didn't exist. By using the names givin to us in Luke 1:5," course of Abia:"( or Abija),( count 6 months from the course of Abia. John the Baptist was concieved in Abia and Jesus was concieved 6 months later and born 9 months after that.), we can research when the courses were laid out and through careful calculation, figure that Jesus was probably born on The Feast of Tabernacles. Dec 25th has more to do with paganism than anything Jewish. The Themes of Tabernacles are "The light of the World, swaddling rags are soaked and lit on top of a pole and they say "Behold, the Light of the World!". They build a hut in the yard and stay there( same hut the groom builds for his bride to be. Jesus speaks of it when He says He must go and prepare a place for us Jhn 14:3) there is gift giving and it is the only Jewish Holiday that has to do with the WHOLE world. Every other Feast is Jewish only. All that from a name that people pass over every time they read it.