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In the Hebrew Bible and several non-canonical Jewish and early Christian writings, nephilim (in Hebrew הנּפלים means the fallen [ones]) are a people created by the cross-breeding of the "sons of God" (beney ha’elohim, בני האלהים) and the "daughters of men". (See Genesis 6:1.) The word nephilim is loosely translated as giants or titans in some Bibles, and is left untranslated in others.

...The sons of God saw that the daughters of man were good, and they took themselves wives from whomever they chose. God said, ’My spirit will not continue to judge man forever, since he is nothing but flesh. His days shall be 120 years’. The Nephilim were on the earth in those days and also later. The sons of God had come to the daughters of man and had fathered them. They were the mightiest ones who ever existed, men of renown.

The traditional Jewish view, deriving from the Book of Enoch, is that the fathers of the nephilim, the "sons of God", were the Grigori (a class of fallen angels also called the Watchers); however, there is some controversy on this point (Targum Jonathan) [1]. Some commentators suggest the nephilim were believed to have been fathered by members of a proto-Hebrew pantheon, and are a brief glimpse of early Hebrew religion, most of the details of which was later edited out from the torah. Others, especially some Christians, suggest the "sons of God" were fully human. It is sometimes suggested that ridding the Earth of these nephilim was one of God’s purposes for flooding the Earth in Noah’s time.

Despite the literal text of the Bible and its traditional interpretation, the idea that heavenly beings mated with humans is controversial, particularly among Christians, who cite the teaching of Jesus in the Book of Matthew that angels do not marry. Others who find the idea of angels mating with humans as distasteful have suggested more figurative interpretations of the nephilim, such as the idea that they were the offspring of men possessed by demons, or of aliens.

Still others believe that the most reasonable view of Genesis 6:1 is that the allusion refers to the fact that some men, from the Godly lineage of Seth, called sons of God (an expression denoting those in covenant relationship with YHWH - cf. Deuteronomy 14:1; 32:5), began to pursue fleshly interests, and so took wives of the daughters of men, i.e., those who were unbelievers.

Rephaim
Rephaim is a general title that the Joshua states was given to the aborigines of Palestine, which were afterwards conquered and dispossessed by the Canaanite tribes. They were known to the Moabites as Emim, i.e., "fearful", (Deut. 2:11), and to the Ammonites as Zamzummim. In the books of Samuel, it states that some of them found refuge among the Philistines, and were still existing in the days of David. We know nothing of their origin.

Anakim
In the torah, the Anakim are the descendants of Anak, and dwelt in the south of Palestine, in the neighborhood of Hebron. In the days of Abraham they inhabited the region afterwards known as Edom and Moab, east of the Jordan river. They were probably a remnant of the original inhabitants of Palestine before the Canaanites, a Cushite tribe from Babel. They are mentioned during the report of the spies about the inhabitants of the land of Canaan. The book of Joshua states that Joshua finally expelled them from the land, excepting a remnant that found a refuge in the cities of Gaza, Gath, and Ashdod. The Philistine giants whom David encountered were supposedly descendants of the Anakim.

...We also saw ha’anak descendants there .... All the men we saw there were huge men. While we were there, we saw the nephilim, they were sons of anak, who descended from the nephilim, and we saw ourselves as tiny grasshoppers and that’s what we were in their eyes...

(Numbers 13:21-33, abridged)

Note that it is more commonly suggested by traditional Jewish sources that the spies saw large and powerful inhabitants in Canaan and because of their own fears, cowardice, and inadequate faith in YHVH, saw themselves as grasshoppers in the eyes of the Canaanites, whether they were actual ’giants’ or not.

Nephilim in other works
The story of the nephilim is chronicled more fully in the Book of Enoch (part of Ethiopian biblical canon).

There are also allusions to these descendants in the deuterocanonical books of Judith, Sirach, Baruch, and Wisdom of Solomon.

Nephilim in parahistory
There have been many interesting attempts to reconcile science with evolution, the theory being that mythology often contains grains of truth in the form of a highly distorted "folk memory" of events in the remote evolution of the human past.

In this context, the nephilim have been associated with everything from Atlantis to extraterrestrials in efforts to rationalize their literal existence. One theory is that nephilim were actually surviving Neanderthals, or a Homo sapiens-Neanderthal hybrid.

It is believed by such people that modern man shared several thousand years of history with Neanderthals, and also that the Middle-Eastern region was home to some of the last surviving pockets of Homo sapiens neandertalensis or H. neandertalensis. Therefore, it is conceivable that a Folk Memory of these creatures survived by way of mythology- but not necessarily in the way that is is imagined. In addition, it appears that the very last Neanderthals adopted some of the technological and cultural innovations of their contemporaries. So, the theory goes, surviving Neanderthals or hybrids might have been very large, powerful men, possessing the intellect and societal characteristics of our species, explaining their identification as: "mightiest ones" and "men of renown," which could be a great exaggeration based on some level of prowess.

 

One major flaw in this "theory" some have suggested is that neanderthalensis were in fact slightly shorter in height than H. sapiens and unlikely to have been named giants. On the other hand, they were giants relative to their even shorter predecessors, Australopithecus and Homo Habilis. Of course, this flaw is easily resolved when one remembers that the word Nephilim earlier described means "fallen ones" and not giants, the word being translated in the King James Bible as "Giants", recognized now as somewhat of a mistranslation.