Adam and Eve were pure humans directly created by God and Abel the first fruit of thier human beings, but only Cain was not a pure human because he had the breeding of the serpent. Prior to the fall of Eve before the curse the serpent was human like-animal between apes and man form, the serpent was a highest form of a perfect animal created by God almost liken to human, proof that the serpent was liken to human because he has an ability to talk with Eve.
The serpent beguiled(sexually tempted)
Eve, and their fruit of their actions was Cain the firsborn(the fruit of the first ***).
After all the generations of Adam and Eve there were already an intermarriage between pure humans and the bloodlines of the serpent(an ape-form),
so all of us have the bloodlines of the serpent(Evolution calls it an ape origin)
that needs to be cut-off by the precious blood of Jesus.