Ichi no Mi or First Fruit in english, is the very first devil fruit ever known to One Piece World history. Also named Eternity and or Devil, because of it's permanent bad effect even after the death of it's owner. Ichi no Mi doesn't follow regular law of Devil Fruits because of it's prehistorical nature.
Ichi no Mi -according to legend- was eaten by Ariel Eve (japanese: Ibu Arieru), a prehistoric era giant fishman (or fishwoman? or mermaid princess?). Because of this fruit, it is said, that eve became a permanent surface dweller, unable to swim or return to the ocean. The fruit changed her tail into a pair of legs. While bearing child of her beloved fishman husband, she -acompanied by her husband- is the first to learn how to live in the surface.
It is also told that Ichi no Mi was the cause of every humankind today. Originating from the first human-turned fishman (or fishwoman?) who gave birth to four (or more?) prehistoric human twin. The "cannot swim" effect of the Ichi no Mi at some degrees was then inherited until modern humanity today.
The fruit power was eternal to the owner, even after death. It makes the fruit reincarnation impossible. It also makes the dead body of the owner cannot touch water -or the body will be instantly dispersed to the water-. The descendants of this fruit owner decided to bury her -for the first time in history- in the surface's ground. The legend says that a leaf sprouted from her burial ground and grew into a monstrous-sized tree, which became the originator of many Devil Fruits today.