The Book of Jubilees aka Lesser Genesis / Leptogenesis dates to circa 200 or 100 BC.
Let us consult the 1917 AD translation by R. H. Charles’s section “The Patriarchs from Adam to Noah (cf. Gen. v.); Life of Enoch; Death of Adam and Cain (iv. 13-33).”
We will pick up at the mention of Jared whose name means “descent” and who was named thus because, as the text states:
…for in his days the angels of the Lord descended on the earth , those who are named the Watchers, that they should instruct the children of men , and that they should do judgment and uprightness on the earth…Jared took to himself a wife…and she bare him a son…and he called his name Enoch…he testified to the Watchers, who had sinned with the daughters of men; for these had begun to unite themselves, so as to be defiled, with the daughters of men, and Enoch testified against (them) all.
Let us begin with  so that we may expand upon  below:
This statement is interesting. It describes what was probably the original commission by God to the angelic watchers, who, however, fell when they descended to the earth. According to 1 Enoch, Enoch acquired his supernatural knowledge from the instruction of angels.
This last line looks like a quotation from 1 Enoch vi. 6 (“who descended in the days of Jared”). Note the play in Hebrew on the name Jared (yāred) and “descended” (Heb. yārĕdû). The myth of the descent of the angels was based on Gen. vi. 1-4, but was rejected by the Rabbis, who rendered “sons of God” (i. e. angels) “sons of the judges.”
Latter Rabbinic commentaries misinterpret the sons of God as sons of judges but the earliest view of Jewish and Christian commentators is that they were Angels.
In fact, R. H. Charles’s The Book of Jubilees’ section “The Fall of the Angels and their Punishment; the Deluge foretold (v. 1-20; cf. Gen. vi. 1-12)” states:
And it came to pass when the children of men began to multiply on the face of the earth and daughters were born unto them, that the angels of God 4 saw them on a certain year of this jubilee, that they were beautiful to look upon; and they took themselves wives of all whom they chose, and they bare unto them sons and they were giants.
And lawlessness increased on the earth and all flesh corrupted its way, alike men and cattle and beasts and birds and everything that walketh on the earth-all of them corrupted their ways and their orders, and they began to devour 3 each other, and lawlessness increased on the earth and every imagination of the thoughts of all men (was) thus evil continually…
The footnote  here states, “This is the LXX rendering of Gen, vi. 2 (R.V. “sons of God”) and represents the older Jewish exegesis, which was later given up.”
Section “Noah offers Sacrifice; the Cursing of Canaan (cf. Gen. ix. 20-28): Noah’s Sons and Grandsons (cf. Gen. x.) and their Cities. Noah’s Admonitions (vii. 1-39)” states:
Noah began to enjoin upon his sons’ sons the ordinances and commandments, and all the judgments that he knew, and he exhorted his sons to observe righteousness, and to cover the shame of their flesh, and to bless their Creator, and honour father and mother, and love their neighbour, and guard their souls from fornication and uncleanness and all iniquity.
For owing to these three things came the flood upon the earth, namely, owing to the fornication wherein the Watchers against the law of their ordinances went a whoring after the daughters of men, and took themselves wives of all which they chose: and they made the beginning of uncleanness.
And they begat sons the Nâphîdîm, and they were all unlike, and they devoured one another: and the Giants slew the Nâphîl, and the Nâphîl slew the Eljô, and the Eljô mankind, and one man another. And every one sold himself to work iniquity and to shed much blood, and the earth was filled with iniquity.
And after this they sinned against the beasts and birds, and all that moveth and walketh on the earth: and much blood was shed on the earth, and every imagination and desire of men imagined vanity and evil continually.
And the Lord destroyed everything from off the face of the earth; because of the wickedness of their deeds, and because of the blood which they had shed in the midst of the earth He destroyed everything.
These statements mirror many made within the apocryphal Book of Enoch aka 1 Enoch aka Ethiopic Enoch (see here) chap 7:5 & 9:1, 9.
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