I'm going to open up a can of worms here and say that there are theories that Adam was a pre-existing hominid who God infused with a soul, and that the people Cain was afraid of might possibly have been neanderthals.
I have heard this theory espoused even by Orthodox clergy, however, I have difficulty accepting this in that this would imply that the human body preceded the human soul. This particular belief does not appear to be consistent with the patristic assertion that the soul and body are created at the same moment. I can only see this interpreted as meaning that man is a "special creation". Also the fathers indicate that man changed bodily in some significant manner with the fall (this is the patristic interpretation of being clothed with the skins of animals). Whereas before man was bodily more akin to the angels after the fall he become more akin to the animals. If we can then trust the patristic teaching about the nature of man and the nature of the fall, it becomes difficult to say that a hominid animal was infused with a human soul. I find this "can of worms" much harder to believe and with many more complex contradictions to resolve than the simple belief that God created man in a unique and special act of creation.
I will admit the possibility that in being clothed with the skins of animals, man was given the form of a pre-existing hominid (neanderthal?) but superior in nature to that pre-existing hominid animal, thereby answering many questions or conflicts about the fossil/archeological record.
Fr David Moser