The Gap Theory shows evidence of the dinosaurs becoming extinct as the result of Satan's fall and explores the origin of demons as being disembodied spirits of a pre-Adamic race, who were judged by God, as the result of Satan's sin.
Why was man commissioned by God in Genesis 1:28 to "replenish" the earth? Why did God use that word? Could this be a reference to a pre-Adamic race that was created in Genesis 1:1 and destroyed in Genesis 1:2 due to Satan's fall, and which the human was to "replace" or "replenish"?
Read where angels that sinned left their heavenly habitation and came down to earth to procreate with humans and whose children were called "Mighty men, which were of old, men of renown," offspring that are the source of all the ancient secular myths that claim the "gods" came down to earth and married human women.