Adam and Eve's son, after killing his brother, was marked by God for people to see. I've heard some argue that this does make sense with relation to special creation, because Adam and Eve may have had countless more children that we didn't here about. However, in light of modern science, I have a theory. Many people will claim that either Adam and Eve are purely allegorical or that they represent a group of people, because the human species was never just two people. These theories certainly make sense, but you have to understand the definition of human in a biblical sense and in a biological sense. There were probably never just two biological members of the human species, but the Bible defines humans as those created in his image. Because God has no body, he must mean their minds and ability to interact with Him. This may mean that Adam and Eve were simply the first people who God decided were ready for Him to reveal Himself to. They were the first "animals" that God would begin to hold acountable for their sin. And sure enough, Adam and Eve chose to create their own morality by eating the fruit.