The GAE scenario builds from a set of premises:
1] A population of Pre-Adam humans evolve as per the current state of evolutionary theory.
2] these pre-Adamites would be seen as inadequately competent in spiritual matters.
3] at some point in the timeline, say 6000 to 10,000 years ago, God sets up Eden as nursery/lab for cultivating 2 special humans.
4] God creates Adam & Eve de novo.
5] When the couple is inevitably expelled, they and their offspring begin to mix and mate with the pre-Adamite population.
6] Only a handful of genealogical lineages are able to persist from the time of Eden to the birth of Christ. Perhaps only the Adam lineage?