The question can be your Coup de grace with Christians if you wish it to be.
We don't know exactly. Organically, it was a gradual development into man. Interestingly, our ancestors were physically pretty much the way we are now for about the last two hundred thousand years, but intellectually and culturally they were not much more advanced than Neanderthals for most of that time period. Then, seemingly like magic, the full flower of humanity suddenly appeared on the scene about ten thousand years ago, including artistic and symbolic communication, society, religion, etc. I would estimate, therefore, that it was shortly before we observe this that God separated Adam from the homo sapien line he came from (which sprang "from the muck of the earth") and breathed into him a soul made in His image and likeness, and from his rib created his female clone, Eve.
I really don't have a problem with the above. Cain married into these established groups of modern humans.
