Some say the original vine bridges were the invention of the fleeing Heike Samurai, who came to the Iya Valley 800 years ago in order to hide. It is said that if their pursuers ever found them, they would be able to cut away the vines and have a better chance for escape.
And while the legends may indeed be plausible, what is sure is that the vine bridges of Iya did indeed serve another purpose (aside from being a way to cross the river). The original bridges weren't nearly as well built or as stable as they are today, and to cross the earlier constructions one needed to not just be sure of their balance, but also step in unison with the bridge as it swayed from side to side. People from outside the valley had difficulty with crossing the bridges, so when seeing someone having trouble, even from far up the hillsides in the surrounding villages, the local residents knew immediately that the person crossing was not from the area (people would often be suspicious not just because such outsiders could be a thief or even a con-artist, but more concerning was how the young women of Iya were prized for their beauty and fair skin, and they were often kidnapped).