Anyone who believes that the Genesis account records actual history would believe that all of Mankind descend from Adam because it records that he was the first and only human being until the creation of Eve, his wife.
The Genesis account never claimed to be a complete record of all human history and other accounts of the same events claim that Adam and Eve had many many children. This stands to reason considering that they were recorded to have lived nearly a thousand years.
Cain married one of his sisters and he convinced others among his siblings and their children to leave Adam and build their city in the land of Nod.
I don't see any reason to assume that there were other human beings on the planet outside of the family of Adam and Eve.
If Cain had offered up a blood sacrifice as Abel did, God would have also accepted it.
However, Cain was prideful and did not want to go to Abel to obtain an offering, he also loved Satan more than God and offered up the first fruits of the field because Satan had commanded him to do it.
It's just as God said to Cain,
"If thou doest well, shalt thou not be
accepted? and if thou
doest not well,
sin lieth at the door. And unto thee shall be
his desire, and thou shalt rule over him." (Genesis 4:7)
God admonished Cain to do good and warned him about his relationship with Satan.
I believe that Joseph Smith Jr. was a prophet and servant of the Lord Jesus Christ and we regard him much like how the Jews regard Moses.
Moroni was a prophet/historian who sealed up the record of his people that would later be translated by Joseph Smith Jr. and it's called the Book or Mormon: Another Testament of Jesus Christ.
It was this same Moroni who visited Joseph Smith Jr. as an angel and directed him to where the ancient record had been deposited.