Genesis 4:2

And again she bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.

The speculation has long prevailed that Cain and Abel were twins, based on the omission of a second statement that Adam knew his wife. This may "very well be the meaning,"[7] but it should not be pressed. Also, it appears that the names of these two brothers were "bestowed by the mother,"[8] which is another hint of the matriarchate, when a man left his father and mother and went to live with his wife, at a time long prior to later customs when the right of naming children was the prerogative of the father. This is another indication of the extreme antiquity of the events of this chapter.

"Keeper of the sheep ... tiller of the ground ..." Both of these occupations were shown to Adam by the Lord, the tilling of the ground by direct commandment, and the keeping of sheep through the provision of clothing by the slaying of animals. It was natural that one of the sons would choose one department, and another the other.

It should be particularly noted that nothing in this chapter indicates either that Cain was the firstborn of Adam and Eve, or that these two were the only children they had. Commentators who speak as if such conclusions were true are ignoring the fact of this entire section of Genesis being an extremely condensed and abbreviated narrative. Adam and Eve lived many centuries and had "sons and daughters" (Genesis 5:4); and the total number of their children could well have been fantastic. Furthermore, the arbitrary placement of this episode in close proximity to the expulsion from Eden is forbidden by the words, "In process of time," in the very next verse. Right here is the true explanation of why Cain was afraid that he would be killed, following the murder of his brother, and also the true explanation of where he got his wife.