It is only a the beginning of Gen. 4 that we hear that 'the human knew Eve, his woman, and she conceived and bore Cain, and she said "I have got me a man with the Lord", and she bore as well his brother Abel.' Most readers of the Bible will recognize the translation of the Hebrew term for having sexual intercourse as 'to know' implying sexual intimacy. Usually this term has a positive connotation but sometimes it refers to illegitimate intercourse, that is a sin, a violation of Biblical ethics.
One of the things that most often gets lost in translation is Hebrew word play, so for instance when Eve says 'I have got me a man'. the verb qanah is punned on in the naming of her first child. Cain which is qayin . Why is this important? Because in this ancient society names often connote something about the nature of the one named or about the significance of this person. So for instance the name Jacob means 'heel snatcher' and refers not only to the apparent struggle to get out of the womb first by dragging Esau out of the way, but later the attempt to snatch his birth rite, the rite of the first born. One thing that should already be apparent in the Genesis story is that the Biblical writer portrays characters with real flaws, real sins, sometimes very serious sins, as we are about to see. These are not characters in a fairy tale where 'never was heard a discouraging word and the skies are not cloudy all day'. These are genuinely self-centered and sinful persons.
The mention of Abel being a sheep herder, and Cain being a tiller of the soil is important because it makes clear that this is not a story about cave men, or Neanderthal man, but a story from much later in the development of human progress- agriculture in particular comes from about 7,000 B.C. or so at the earliest and was coupled with the rise of village life, so far as archaeology can tell us. Before that was a period of being hunter gatherers but neither Cain nor Abel are those sorts of folks, and further more, they both marry women which the evidence does not suggest came from Adam and Eve. In other words, the Biblical writer is telling the story of the origins of God's people, not all of humankind. Other peoples like the Hittites or the Moabites or the Philistines only come into the picture as they cross paths with God's people. There is nothing in Gen. 1-4 that suggests that homo sapiens could not have begun in various places on the earth, and that at some point interacted with, and even intermarried with God's people. This is what Joshua Swamidass, a biologist who teaches at Washington University in St. Louis argues in his fascinating book The Genealogical Adam and Eve. The Surprising Science of Universal Ancestry (IVP).
Scholars have long debated why Abel's offering to God was acceptable but Cain's was not, but perhaps the most basic clue is that Abel offered 'the choice firstlings of the flock' where as it simply says Cain brought from the fruit of the soil. Was there a difference in the quality of the gift, and only Abel offered 'first fruits'? Perhaps so. In any case, the text says Cain became incensed when God regarded Abel's but not Cain's offering. And then there is this famous speech by God to Cain rendered as follows by Alter: "Why are you incensed, and why is your face fallen? For whether you offer well, or whether you do not, at the tent slap sin crouches, and for you it is longing, but will you rule over it? (or you will rule over it).
And immediately after this Cain entices Abel to go out to the field, and there Cain slays Abel. Notice that twice the phrase 'his brother' comes up making clear the horror of the first murder among God's people involving fratricide. God asks Cain where is Abel, and Cain demurs, saying, as Tyndale (and Alter following him) put it -- 'I know not, am I my brother's keeper'? And yes, he should have been that kind of brother.
The Hebrew is very graphic at this point with the text reading "your brothers blood cries out to me from the soil" and as a consequence the soil thereafter was not going to co-operate with Cain's tilling of it. "And so cursed shall you be by the soil that drank with its mouth to take the blood of Abel from your hand". Cain complains his punishment is more than he can bear. Cain says he's being driven from the soil and from God's very presence, and is destined to be a restless wanderer upon the earth. There is a further word play with the word Nod, the land where he is going to go, as Nod is cognate with the noun wanderer. Cain fears he will be defenseless and will be killed but God promises sevenfold vengeance on whoever kills him "And the Lord set a mark upon Cain so that whoever found him would not slay him". Clearly the narrative implies there are other human beings on the earth that might find him and kill him, who are not members of his immediate family. So the story goes on to say Cain knew his wife, and she conceived Enoch, and Cain became a builder of a city, and called the name of the city like his son's name. There then follows Cain's genealogy which involves Lamech who had 2 wives, who had a son named Jabal, said to be the first tent dweller with livestock, and he had a brother named Jubal the first to play the lyre and the pipe. The reference to building a village, again points to a time well after homo erectus or the beginnings of homo sapiens, well after cave dwellers as well.
Lest we think the story of Adam and Eve is over, at 4.25 the human final gets his name- Adam, which comes from his origin in the soil. And we hear that Eve bears a son which she sees as a replacement for Abel, with his name being Seth. And what is interesting at this point we are told Seth had a son named Enosh, "and it was then that the name of the Lord was first invoked" by which is meant Yahweh. This has puzzled scholars forever because according to Exodus, that name was first revealed to Moses, perhaps at the burning bush. But we have to remember that there are various indicators in the text, that the text we have has undergone considerable editing over many years, and there are various retrospective comments like this one. The point perhaps would be that God only revealed his personal name to his chosen people, perhaps even before Moses, but it was forgotten when slavery happened and the Hebrews were in Egypt, and so Moses had to be told, in order to make a new start with God's people.
The way I would explain what is going on in Gen. 2-4 is: 1) this is the tale of the origins of God's chosen people, not the tale of the origins of all people: 2) however since God's people were supposed to be a light to all the other peoples, affecting them, the actions of God's people were of paramount importance. So, Adam, would be seen by the Biblical writers as the 'federal' head of all humanity, such that his actions affected all of humanity, as Rom. 5.12-21 says. as the Genesis story itself intimates, it did not mean he was literally the progenitor of the whole human race. So, from a theological point of view -- the actions of the first Adam affected the fallen character of the rest of the race, just as the actions of the last Adam, namely Christ, can potentially affect and save the whole human race if they will believe in him. None of us are literal descendants of Christ as he had no children, but by grace and through faith, we are 'in Christ' as Paul puts it, or as he also says, we are Christ's brothers and sisters, and co-heirs with him of the Kingdom.