The Giants mentioned in the Flood myths are hybrids.
In genetics, when two true breeding varieties are bred, they produce offspring with above average vigor and stature, intelligence, senses, all that.
As you can clearly discern while reading Enoch with a mind of there being a truth behind the old story, if you are a scientifically and archaeologically inclined or trained person, the "angels" are clearly a different, but interbreeding capable variety of human.
Just as it was recently discovered in china and several other places, that neanderthals indeed cohabited and interbred, thus also the haplogroup that gave us the red/blonde/white mutation, these were initially a differentiated group but not speciated.
The other thing is that the oldest human bones found, in caves in Morocco, were this haplogroup, the predecessors of the modern Berbers, and the hypothesis of Apocalypse is that they are the origin of the megalith building cultures that left pyramids and sphinxes (winged lion gatekeepers) all over the world. From Egypt, to china, to Bosnia, Serbia, Peru, Indonesia, even India.
The reason why the interbreeding was forbidden was because these ancient white skinned mutants saw themselves as guardians of the lesser homonid varieties, and practiced non-interference, a tradition you see still in Buddhism, Taoism and Christianity.
However. A faction of them disagreed, and this began the "War in Heaven" and the leader of the white people, known as Atlas by the Greeks, sought to prevent the breakaway faction, led by Chronus, also the Greek name, from continuing.
It was this breakaway grou that the Sumerians called "Annunaki" and the hybrids were called "Nephilim". I am not familiar enough with the stories yet, but intend to continue my own research and become a minor expert on the subject.
The reason why the Catholic priesthood purged Enoch's book from the apocrypha was because it explicitly makes it clear that, like Arthur C. Clarke's famous saying, "Any sufficiently advanced technology resembles magic". Though he omits, "To the primitive and childlike mind".
Just as the Polynesians worshipped the stranded American pilots who crashed on their islands. The ancient people of the middle east regarded their western cousins as creatures of magic, because of their weapons and transport devices.
They were just mutants with genetics that gave white skin, red and blonde hair, and bigger neocortexes. They rapidly advanced their knowledge of science and to the hunter gatherers, they were celestial beings of light in flaming chariots hurling thunderbolts.