The Bible telling of Nephilim - angels coming to earth and mating with females may have been an interesting legend to explain a very real event that might have been seen around the world-
The Peruvian legends tell of the red-haired giants arriving on reed boats from long distances and settling in Peru without any females on board. A recent giant found in the Upper Midwest was buried with two human-sized females at his feet. These things make one wonder. Let's see what an early explorer reported -
From Pedro de Cieza de Leon:
As there is in Peru a story of some giants who landed on the coast at the point of Santa Elena, in the vicinity of the city of Puerto Viejo, I have resolved to mention what I was told about them, as I understood it, without taking into account the opinions of the common people and their various anecdotes, for they usually magnify events larger than life. The natives, repeating a story received from their forefathers from very remote times, say that there came from across the sea on reed rafts that were as large as big ships some men that were so big that an ordinary man of good size scarcely reached up to their knees: their members were in proportion to the size of their bodies, and it a monstrous thing to see their enormous heads and their hair hanging down about their shoulders. Their eyes were as large as small plates. They say they had no beards and that some of them were clad in the skins of animals, and others only in the dress nature gave them. There were no women with them. (*This red-haired population of giants were reported around the world, showing a tendency to be sea-based people who traveled, seashells were prominent in the dig sites of giants as ornamental and reed boats were described by the Paiutes of the red-haired giants in the Great Basin in Nevada. ) On reaching this point, they set up their camp like a village (and even in these times there is a memory of the site of their houses). As they found no supply of water they remedied the lack by making some very deep wells, a labor certainly worthy of record, being undertaken by such strong men as these must have been, to judge by their size. They dug these wells in the living rock until they came to the water, and afterwards they built the wells in stone from the water line upwards so that they would last for ages. In these wells the water is excellent and it is always so cold that it is very pleasant to drink. "When these great men or giants had thus made their settlement and dug these wells or cisterns, they destroyed and ate all the supplies they could find in the neighborhood. It is said that one of them ate more than fifty of the natives of the land; and as the supply of food was not sufficient for them to maintain themselves, they caught much fish with nets and gear that they had. They lived in continuous hostility with the natives, because they slew the latter's women in order to have them, and they also slew the men for other reasons. (yet more talk of mating up with local women and eating the Native People). But the Indians were not numerous enough to kill these newcomers who occupied their land and lorded it over them; and although they held great discussions about this, they never dared attack them.
"After some years the giants were still in this region, and as they had no women of their own and the Indian women of the neighborhood were too small for them, or else because the vice was habitual to them and inspired by the demon, they practiced the unspeakable and horrible sin of sodomy, committing it openly and in public without fear of God or personal shame. The natives say that our Lord God, unwilling to conceal so wicked a sin, sent them a punishment suited to the beastliness of the crime, and when all the giants were together engaged in this accursed practice there came a fearful fire from heaven to the accompaniment of a great noise, in the midst of which a shining angel appeared holding a sharp, bright sword with which he slew them all at a single stroke, and the fire consumed them leaving only a few bones and skulls, which God allowed to remain unconsumed as a token of the punishment. This is the account they give of the giants, and we believe that it happened, for it is said that very large bones have been found and still are found thereabouts and I have heard Spaniards say they have seen pieces of teeth which they thought must have weighed half a pound when whole, and who had also seen a piece of a shin-bone of wonderful size, all of which bears witness to the truth of the incident. In addition to this one can see the places where the sites of their villages were, and also the wells or cisterns they made. I cannot state whence or how these giants came there.
This account above would seem to jive with the "kernels of truth" in these legends. A tall race of red-haired people had been uncovered there. Well-fashioned water wells were discovered that were ancient.
It would appear likely that ancient giants launched from the South Pacific islands and tested out other lands for their resources. These launched ships arrived and these marauding cannibalistic, but advanced people would terrorize the locals. These repeated legends of giants coming and raping the women were no doubt reenacted in many lands around the world where stories such as nephilim might have come to light through these legends of actual events.
Were the giants angelic or alien? My firm belief is that they were of this earth, simply an earlier, perhaps more crude and brutal people, but also having more time to evolve technologies and language long before us. They were both devils and gods depending on how the locals felt about these advanced deadly people. And, ultimately the practice of sacrifices could have come from appeasing these gods with the blood they so craved, carrying on these devotions even after the giants were gone, in order to keep their spirits happy and to continue to have good crops and technology that they left them, sending a crude people into an advanced culture almost overnight from their exposure to the giant "gods."