"There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them" (Genesis 6:4).
Few passages in all of Scripture have yielded so many interpretations as Genesis 6:1-5. Who were the "sons of God"? Who were the "daughters of men"? Why would their union produce giants and be associated with great wickedness?
Some have proposed that the "sons of God" were the descendents of Adam's son Seth, and the "daughters of men" were from the line of rebellious Cain. An interesting thought, but in Noah's day there were essentially no righteous people, from any line.
Others have rightly noted that the term "sons of God" in the Old Testament refers only to angels, thus these must be the fallen angels. Satan had launched an aggressive campaign against Adam and his descendents in an attempt to so pollute mankind that the promised Redeemer, the "seed of the woman" (Genesis 3:15) could not come.
But angels in Scripture are spiritual beings having no permanent body, and in heaven, at least, they do not "marry" (Matthew 22:30). And just what would this half angel/half man be? Would it be a giant, and more importantly, would it have an eternal spirit?
Let me suggest another alternative from our modern-day understanding of genomics, one which Bible scholars of yesteryear could not have suggested.
I suspect that if today's geneticists and molecular biologists can accomplish such technical wonders as gene splicing and cloning, that the much greater intelligence of Satan could potentially have done it too. The inner workings of the DNA molecule would not have been hidden from the prying eyes of Satan and his henchmen. If today animal genes can be inserted into human DNA, could not it have been accomplished by malevolent spiritual beings bent on destruction of the "image of God"?
Obviously we cannot speak with certainty, for the Bible gives little detail. At the very least, Satan's demons could have selected and indwelt certain men and women, and performed selective breeding experiments to produce over the generations a race of giants. (He could have done the same with animals too, and maybe that's where some of the unthinkable features we see in the fossil record come from. This could represent Satan's rage in trying to fully destroy any vestige of God's once "very good" creation.) Could he not have inserted genes and fabricated clones, mocking and ruining God's majestic handiwork? Perhaps this is why God had to send the Flood, eradicating a civilization beyond repair and starting anew with Noah, preserving the true seed of Adam.
One more thought. Christ compared the days of Noah to the days prior to His return. Are the actions which produced "giants," whatever they were, considered in this comparison? Certainly at no prior time of history have humans been able to "play God" with the genome as they do now. The rush to embrace the ghoulish possibilities of cloning and embryonic stem cell manipulation may be reminiscent of that long ago time. God's utter condemnation of that effort may help us evaluate modern experiments.