When God flooded the earth at the time of Noah, only Noah, his wife, their three sons and their three wives were saved—a total of eight people (I Pet. 3:20)! All other members of the human race—including giants—were destroyed (Gen. 7:22).
But giants are mentioned after the Flood. How is this?
God preserved the genetic diversity of the human race through Noah’s sons’ wives. The same was true with mother Eve. All of the genetic diversity that produced the races was within Eve’s womb at creation. Most of Noah’s descendants were of normal size. But through his grandson Canaan, giants were born (Num. 13:1-2, 32-33). Human nature in people generally tends to cause them to remain with those of similar likes, dislikes, and appearances. This is how ethnic groups and nations were formed. These larger sized people would intermarry, causing their offspring to be more like them—thus producing a family of giants, which eventually expanded into a nation, or race, of giants.
When Joshua conquered the Holy Land, God was directly involved in destroying the giants in that area. Why God allowed all of the giants to be killed is not clear. Perhaps it was allowed because those men always seemed to be against God and His people—the nation of Israel.