(Gen. 3:13) Satan brought the spiritual enmity between God and Satan into the physical world when the serpent beguiled (to lead astray, deceive) the first woman and man to choose to know good and evil. (Gen. 3:15 also Gal. 3:16) God told the serpent; “I will put enmity (hostility, hatred) between thee and the woman, and between thy seed and her seed; IT (the seed) shall bruise thy head and thou shall bruise HIS heel.” IT identifies the seed as one and HIS identifies the seed as male so this “Promised Seed” will be one man.
(Gen. 3:1-24 esp. V.7 & 21) Adam and Eve had covered their nakedness (the knowledge of which was brought about by sin) with fig leaves (plants) sewn together but God hid their nakedness with the skins of animals (plural), of course He had to kill the animals to take their skins. It was the only recorded shedding of blood and deaths in the Garden of Eden. Adam and Eve must have been horrified at what their sin had caused. Surely they had taught their sons the story and what God had done. It would have taken three sheep because they are small; one for Adam's loins, one for Eve's loins and one for Eve's torso. Three- the father, the Son and the Holy Spirit.
(Gen. 4:1-7) In the process of time Cain and Able brought offerings to God. (Heb. 11:4) "By faith Abel offered unto God a more excellent sacrifice than Cain by which he obtained witness that he was righteous,---." Cain had brought to God the offering that man tried to hide his own sin (plants-fig leaves). Able had the faith that what God had done was what worked so he did it, he offered a blood offering. It was the first prophesy that a blood sacrifice would cleanse man of his sin! AGAIN:
(Gen. 4:1-10) This “spiritual enmity” (hostility, hatred) was made immediately apparent in the “physical enmity” between Adam and Eve’s first two sons. (V.4) Abel worshipped God the correct way by offering blood (life- Abel had to shed blood to obtain the fat and it was mingled with the fat) and fat (representing God's blessing upon Abel's work) for a sacrifice and God respected it. (Gen. 9:4) Blood is life. (V.3 & 5) Cain worshipped God the wrong way by offering fruit (earthly) for a sacrifice; (Gen. 3:17) The ground is cursed; and God did not respect Cain's offering and (V.7) God counted it as sin. Cain was jealous and angry and started a “family fight” in which he murdered Abel. SO:
Time went on: (Gen. 6:13) Exercising their right of choice mankind became so corrupt God destroyed all of mankind with the flood except Noah, his wife, his three sons, Shem, Ham and Japheth and their wives. God started over again with those eight people. (Gen. 9:18-27) The “spiritual enmity” centered on Noah’s family and the “family fight” started again when Noah’s son Ham offended Noah and Noah placed a curse on Ham’s son Canaan. Canaan means humiliated. (Gen. 10:1-5) Japheth’s descendants founded the isles of the Gentiles. (Gen. 10:6-18) Ham's descendants were the Canaanites.
(Gen. 11:10-31) Shem's descendants were the Israelites through Abram (Abraham). The "sides" in the "family fight" had been chosen and nobody knew where it was going but God. (Gen. 11:10-26) Ten generations from Noah’s son Shem to Abram’s (Abraham) father Terah had passed when God involved himself with Abram.
If you wish to read the above Biblical process of the progression of time and the people involved in exacting detail please read certain articles in the commentary God, Satan and Man: The Relationship. The article titles will tell you where to look BUT be sure and read the INTRODUCTION and Part 1 THE BEGINNING because they explain the sources, the scripture, the reasoning and purpose of that commentary and the life lived, semi-story, novelistic style of writing.