Hebrew years 1 to 120 (3760-3640 BCE)
After the creation of the universe and of the world, and of all living creatures on earth, the man Adam was made (נעשה) on the 6th "day" of Creation. From his making, the Hebrew Calendar is started as day 1 month 1 (Tishri) year 1. The festival of Rosh Hashanah in Hebrew Calendar reminds the making of Adam. As of the five preceding days of Creation, they fall in year 0 (zero). Eve was made from Adam a bit later in that same day. Then, before the end of this day of making of Adam and Eve, they were entered by God in the Garden of Eden and, with it, came the the first Shabbat day, the 7th day since Creation.
But, as it also happened with other elements of the Creation, free-will prevailed and led to results that were not always according to the divine plan. Equally, from the very first generation, man erred. Adam and his companion, Eve, were expelled from the Garden of Eden and lost the divine protection. From then on, they were on their own.
The beginning of humanity, as we know it, started with Adam. It corresponded to the archaeological period called the Chalcolithic, which made used both of stones and or copper. This period is reckoned by the scientific community to have ended about 3600 BCE when the next period, the Bronze Age, started. In the Chalcolithic period, mankind used rudimentary habitations and tools, but they were the first to live in small villages rather than as nomadic clans. They were also the first to use agriculture as a mean to provide food for themselves, and also to domesticate some animals. This corresponds to the Biblical text which bears the order from God to Adam to work the soil and produce his own food.
They had two first sons: Cain and Abel. According to Tradition, both were born with twin sisters who became their respective spouse. Cain worked the earth while Abel was a shepherd. As previously mentioned, in the Chalcolithic period, mankind indeed used both activities to provide food to themselves: agriculture and breeding. In this era, the animals that were bred were: goats, lamb, cows, and probably also wild boars.And at the end of some days, Cain brought from the fruits of the earth an offering to God. And Abel brought, he as well, from the firstlings of his flock, and from their best choice. And God turned to Abel and his offering. And, as of Cain and his offering, He didn’t. This annoyed Cain very much and his countenance fell. God said to Cain: "Why are you annoyed, and why did your countenance fall? Surely if you improve yourself, you will be forgiven. But if you don’t improve yourself, sin rests at your door; its desire is towards you. And you will govern it." Cain said to his brother Abel. And it happened when they were in the field, Cain rose against his brother Abel and killed him.
--- Genesis 4:3-8
Cain gave an offering to God but not from the best of his production and surely not with his heart, as compared to Abel who gave from the best choice, and the expression he as well means that he himself accompanied his offering as if he was prepared to offer himself to God. Abel accompanied his offering with all his heart because Abel loved God, but Cain didn’t or not with the same spirit. The path of sin starts when man deviates from the love of God. Cain was fully aware of God’s presence, and didn't need a proof of His existence, because his countenance fell after God’s rebuke. Cain could have corrected his path but didn't because his mind was governed by sin. With his free will, man has such power because he has been created in the image of God. Through his soul (neshama), he knows to distinguish between good and evil. This is what makes man different from animals. Yet not everyone can overpower the animal instincts which govern the feelings of desire, jealousy, fear, etc. Cain was jealous of God's preference of Abel and this feeling governed his mind.
The first conflict occurred when Cain proved unable to speak out his own issues with Abel. This is why the text said Cain said to this brother Abel. Said what? The text says nothing else, because Cain had nothing to say. Instead he got overwhelmed by his feelings of jealousy and killed his brother.
The murder of Abel (Gustave Doré, 1868)
After the murder came the lie.God said to Cain: "Where is your brother Abel?" And he said: "I don’t know. Am I the keeper of my brother?"
--- Genesis 4:9
Cain is then cursed to become a wanderer on earth, but nobody could kill him. He established himself further east from Eden, in a land called Nod (נוֹד). This Hebrew name both means drifter and exclusion, reflecting his "separation" from society: this was to be the fate deserved by Cain. Presumably he established himself in what was to become Mesopotamia, where he came across the first two large rivers that created a natural "separation" from the land he came from.
The early males were born with the twin females that they will later take as companions. Thus the procreation could start to operate and the population to grow. But, after the murder of Abel, Eve did not want to procreate any more. So this first generation was largely left for Cain to populate. And his immediate descendants were hunters like him.
The conflict between Cain and Abel has been deeply anchored in early civilizations. It echoes the change of the life style of the early society, starting from being nomadic hunters as Cain was (and as the animal kingdom largely lives), until they settled down to become sedentary farmers and builders who would rather breed cattle and grow plants to regularly provide for their nutrition needs, such as bread, rather than rely on the success of a hunt. This change was surely not easy to make, but this was a necessary step to lead to civilization. Living day by day as a hunter, or a hunted, was not to be the fate of mankind. This early evolution has been found in Sumerian clay tablets which was appropriately titled the "Debate between sheep and grain":The people of those days did not know about eating bread. They did not know about wearing clothes; they went about with naked limbs in the land. Like sheep they ate grass with their mouths and drank water from the ditches.
--- Barton, George A., "Miscellaneous Babylonian inscriptions", Yale University Press, 1918, No.8 “A new Creation myth”, pp.54
So, the early humanity gradually moved from hunters to farmers.
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 Eve was extracted from the original Adam who had both genders when he was made; this was done after God placed the original Adam in the Garden of Eden; with the extraction of Eve, Adam was stripped of feminine gender.
 See for example Pirke de Rabbi Eliezer, chapter 21; this book also tells that Adam did not father Cain, but a fallen angel did (chapter 22), and this is why Cain was evil and different from Abel and Seth, the two real sons of Adam