what if the final five exist in the human genome, and every generation a new one arises(out of all the final five) so they repeat over and over again throughout eternity.
Thus, helo could of had a recessive cylon final five gene, and it was enough to make him able to reproduce with 8. and it hadn't work with any other human because they were so different. Thus Hera could be the final cylon.
I dont know just an idea...