Missing link. There were other humans or hominids already here on earth when God placed Adam in the garden of Eden (also a metaphor). Adam was different in that he was more spiritual with a soul (God breathed into his nostrils the breath of life and man came to be a living soul). He was created in the likeness of his Father. There was none found among all that God created suitable as a mate for Adam. God provided a woman in the image of Adam for a help mate.
Eve partook of something forbidden and contaminated the pure blood and Adam followed suit. Based on God's just recompense law, he would have covered their mouths rather than their genitals had the "sin" been just disobeying a command not to eat of a certain fruit tree.
She introduced into the DNA --- the blood line --- the gene pool --- however you want to say it, a contamination that introduced death like the animals and hominids.
This is just another hypothesis that makes SOME sense. The "death gene" is in the BLOOD because of going outside God's mandate to procreate with one's own kind.
That is the all point. Nice post. Human history shows that humanoids existed millions of years. We have been evolved, and that is a fact. Homoerectus, neantertal, homosapiens. they were part of the human race.
Adam was chosen at that time by God, and he was giving the free gift of the Holy Spirit, and the soul. That is all. God he was going to use Adam to glorify nature, and human race, but Adam failed to do that.
So that is why He sent Jesus to fullfill that purpose.