1 Enoch 7:1

1 And all the others together with them took unto themselves wives, and each chose for himself one, and they began to go in unto them and to defile themselves with them, and they taught them charms and enchantments, and the cutting of roots, and made them acquainted with plants. 2 And they became pregnant, and they bare great giants, whose height was three thousand ells: 3 Who consumed all the acquisitions of men. And when men could no longer sustain them, 4 the giants turned against them and devoured mankind. 5 And they began to sin against birds, and beasts, and reptiles, and fish, and to devour one another's flesh, and drink the blood. 6 Then the earth laid accusation against the lawless ones.

Date: 200-50 B.C.E.
* Dates are based on scholarly estimates

Lactantius Divine Institutes 2.15

When, therefore, the number of men had begun to increase, God in His forethought, lest the devil, to whom from the beginning He had given power over the earth, should by his subtlety either corrupt or destroy men, as he had done at first, sent angels for the protection and improvement of the human race; and inasmuch as He had given these a free will, He enjoined them above all things not to defile themselves with contamination from the earth, and thus lose the dignity of their heavenly nature. He plainly prohibited them from doing that which He knew that they would do, that they might entertain no hope of pardon. Therefore, while they abode among men, that most deceitful ruler of the earth, by his very association, gradually enticed them to vices, and polluted them by intercourse with women. Then, not being admitted into heaven on account of the sins into which they had plunged themselves, they fell to the earth. Thus from angels the devil makes them to become his satellites and attendants. But they who were born from these, because they were neither angels nor men, but bearing a kind of mixed nature, were not admitted into hell, as their fathers were not into heaven. Thus there came to be two kinds of demons; one of heaven, the other of the earth.

Date: 300-310 C.E.
* Dates are based on scholarly estimates