Genesis 6:3-4

Then GOD said, “I’m not going to breathe life into men and women endlessly. Eventually they’re going to die; from now on they can expect a life span of 120 years.” This was back in the days (and also later) when there were giants in the land. The giants came from the union of the sons of God and the daughters of men. These were the mighty men of ancient lore, the famous ones.