To learn, read the Chronograph[y] - the oldest book started to be written at the time of Moses...
The short history:
Adam and Eve had two sons, Cain and Abel.
Cain killed Abel.
Adam asked the Lord to have another son to "give" to Him.
Adam and Eve have been blessed and the third son was born - Seth.
Cain had children and these were "the children of the Man".
Seth had children and were called "the children of the Lord".
Cain told to Seth that his children never to marry to the children of Cain.
In other words, "the children of the Lord" never to marry with "the children of the Man".
There was no angel to breed with humans.
Simply, the angels do not have physical body and have no sexual distinction.
After Adam died and Seth too, died... the girls of Seth family forget to obey the rule and married the men of the Cain family.
For this non-obedience, their children grew tall, and tall... over 100 ft tall...!
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Until this breeding to happen, everybody was not a meat-eater.
Since, these giants started to eat meat, beginning with the normal people, too small to defend themselves.
For such crimes the Great flood did happen, to remove them all from the Earthly life, I mean the giants.
This is the reason their skeletons were found about everywhere and hidden, people to have no clue about the truth of Creation.
Same happened with the material proof of the life of Jesus Christ - people to be confused and not to believe He was really a man, a living one.
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Any other story is just a false description, is a fantasy, fiction of the writer.