Cain's sacrifice was based on pride, and not a sacrifice.
Abel's was based on honesty, integrity, and love of God.
There is no where in genesis, where we are told sepecifically that Adam and Eve were the first two human beings ever.
In fact we are told that on the sixth day God created man and woman, and instructed them to go forth and multiply.
Then our attention is brought to the creation of one man, Adam, and eventually a mate, Eve.
It stands to reason that Eve could have played away with any one of those humans that were previously created, or the beguiler.
jan.