Adapa or Adamu son of Ea (according to Sayce) was a Sumerian and Babylonian mythical figure who accidentally rejected the gift of immortality. The story is first attested in the Kassite period (14th century BC).
Adapa was a mortal from a godly lineage, a son of Ea (Enki in Sumerian), the god of wisdom and of the ancient city of Eridu, who brought the arts of civilization to that city (from Dilmun, according to some versions). He broke the wings of Ninlil the South Wind, who had overturned his fishing boat, and was called to account before Anu. Ea, his patron god, warned him to apologise humbly for his actions, but not to partake of food or drink while he was in heaven, as it would be the food of death. Anu, impressed by Adapa's sincerity, offered instead the food of immortality, but Adapa heeded Ea's advice, refused, and thus missed the chance for immortality that would have been his.
Adapa is often identified as advisor to the mythical first (antediluvian) king of Eridu, Alulim. In addition to his advisory duties, he served as a priest and exorcist, and upon his death took his place among the Seven Sages (Apkallū).
He is also merged with the Kassite-period apkal U-an, who is most familiar though Berossus' recounting of the myth of Oannes. (Apkal, "sage", comes from Sumerian Abgallu (Ab=water, Gal=Great, Lu=man) a reference to Adapa, the first sage's association with water.)
He was portrayed as a man wearing the skin of a fish.
Berossus describes Oannes as having the body of a fish but underneath the figure of a man. He is described as dwelling in the Persian Gulf, and rising out of the waters in the daytime and furnishing mankind instruction in writing, the arts and the various sciences.
Once thought to be based on the ancient Babylonian god Ea, it is now known that Oannes is in fact based on Uan (Adapa) - the first of the seven antediluvian sages or Abgallu (in Sumerian Ab=water, Gal=Great, Lu=man), who were sent by Ea to deliver the arts of civilization to mankind in ancient Sumerian mythology, at Eridu, the oldest city of Sumer.
Stephanie Dalley in her translation of Myths from Mesopotamia (Oxford World's Classics 1989) places Oannes as Greek formation of Uan and relates the god the name Adapa.
Some scholars suggest the Abgallu were seafarers aboard a ship from the Indus River Valley/Mohenjo Daro civilization comparable in age to ancient Sumer and has some identified parallels to the Sumerian knowledge base (like Base-6 mathematics, irrigation systems/water management engineering and mud-brick cities). Traders often organized trading ports in distant places to easily find both markets with goods accumulated and a safe place to resupply/refit their ships (similarly to the Vikings' development of Dublin and Moscow, the Conquistadors in the Americas, Portuguese Macao, Dutch New Amsterdam, British Hong Kong, Spanish Manila). An interesting theory on the fish appearance would be some sort of chain mail or sewn discs (wood, metal, bone, seashell?) worn as armor in most cultures.