Ancient similarities

I think that all the ancient stories from back then have the same similarties because, they were told in a way and passed on and while being past on people added little details here and there. I personally think that all ancients stories theres only one that is true and the other are just stories that people retold in there own words and changes. Like for example noahs ark and the gilgamesh story their both alike. But theres only one that is true and well i feel no one really knows which one or wo knows they can even be myths. All ancient have the same theme and ideas because their stories past on that along the way just got changed up. I learn that even stories today can be changed up adding details here and there to catch poeples attention. The truth behind a story can never really be told right or found out.