The Nephilim: Satan’s Fallen Angels
After the Fall of Adam and Eve human population exploded.  In Genesis 6:1-4 we learn two competing groups “the sons of God” and Nephilim (Gen. 6:4) battled each other for the hearts and minds of men during Satan’s war against the “seed” of the woman (Gen. 3:15), the birth of Christ (Rev. 12:1-4). From the presence of carnivorous dinosaurs in the fossil record we can infer corrupting the genome of life played a role in this warfare. God created herbivores (Gen. 1:29), but only 1,656 years later these were corrupted into violent carnivorous dinosaurs we today see frozen in time in the fossil record created by the Flood. As such a rapid mutation doesn’t occur naturally today, it likely didn’t in antediluvian times. Therefore, genome manipulation occurred, and we can safely rule out God as the culprit. He cites the violent change as the reason for destroying all life on dry land (Gen. 6:11-13). Consistent with this mutated life being something God hated, monstrous versions evidently were kept off Noah’s Ark (Gen. 6:11-13, 19-20; 7:21-23 cf. Gen. 6:19-20) .
Genesis 6:9 says Noah is without defect “among his contemporaries” implying the human genome was altered in others. 
Did antediluvian scientists have the knowledge to manipulate DNA? That seems likely as antediluvian life-spans were hundreds of years long and had about 1,656 years to exceed what scientists today have accomplished in less than 60 years. Confirming antediluvian technology was advanced, megalithic sites likely built by them often manifest advanced technology that doesn’t exist today.
Back to the war of the seeds. For the woman’s seed and Satan’s seed to physically harm each other (Gen. 3:15) both must be born of a woman. That explains Nephilim cohabitation with women, and “also after that” the angelic “sons of Elohim” fell from their “proper habitation” and joined them lusting after “strange flesh” taking human wives as they chose (Gen. 6:1-4. Jude 1:6-7. 2Pet. 2:4).