There are two different stories: the story of God creating humankind on the sixth day of creation, and the story of God forming Adam and Eve in the Garden of Eden. It seems traditional to match these two stories up as though they refer to the same events, and this is where the belief I’ve seen multiple people repeating that Adam and Eve were the first people, or at least the first people in the Image of God, seems to come from.
But does close examination of the timeline support this? First of all, where does the evening of day six fall, if it is after Gen 2? Is it before Gen 3, when they ate the fruit and were cursed? It seems it would have to be, if the curse is not to be included with Gen 1:31, “it was very good.”
And yet the end of Gen 3/beginning of Gen 4 is when they first speak of reproducing: Adam names his wife ‘mother of all life’ and she bears Cain after leaving Eden. (Before this, God says he will increase her pain in childbearing: increase from what? Was she meant to have children before the curse?)
But let’s go back and look at Gen 1:28: God blessed them, and God said to them, “Be fruitful and multiply, and fill the earth and subdue it…” Every other time in Genesis 1 that God gives an order like this, the literalists say it is carried out instantly, that day! And indeed, Gen 1:30 says “And it was so.” So by the end of the sixth day, humans—real ‘Image of God’ humans, like us—must have multiplied and filled the earth.
Do we see later Biblical evidence of the earth (that Adam and Eve are kicked out into) being filled with people? As others have mentioned, Cain is afraid of strangers not recognizing him, and he marries in a far land, and he builds a city. This is three separate points for the presence of other humans.
But Adam was the First Man! Paul said so! Well, Paul called Jesus the Second Man and the Last Man practically in the same breath, so he is clearly speaking figuratively, or at least with some unspoken qualifiers to the sense in which his claim is true.
Am I playing fast and loose with Scripture? Or is there something to this case? Feel free to poke holes in my balloons, or present other interpretations that make sense!