"OK. Fair point. So let’s look at Oannes to see just why this Babylonian fish-man is not an ancient astronaut. Fair warning: Because Coppens identified this issue as essential to the ancient alien hypothesis, this post is very long.
The story of Oannes is told only by Berossus, a late Babylonian priest who related the tale in his Babylonian History, which does not survive. Summaries were made by Apollodorus, Abydenus, and Alexander Polyhistor, but of course none of these survive either. Extracts from these Greek summaries were recorded in Late Antiquity by Eusebius of Caesaria and in the Middle Ages by George Syncellus, whose books are the sole surviving record of Berossus’ work. We know Berossus existed because he is mentioned by other writers, such as Pliny, whose work survives. (Unrelated fragments of Berossus’ astronomical works were also preserved by Seneca.)
But this isn’t the end of the story. The Greek fragments of Berossus are known to modern readers in the form given them in the early 1800s by I. P. Cory, whose Ancient Fragments (an edition of which I recently edited) freely ran together material from Eusebius and Syncellus while excising the presumed contributions of the Greek authors to produce relatively linear narratives. (I have made this text available online here.) These fragments were further adapted by Robert Temple, who published them in the appendix to his Sirius Mystery from Richard Hodge’s 1876 revision of Cory’s Fragments. This is the form of Berossus’ work ancient astronaut hypothesizers know.
Now, Berossus is generally an accurate writer, but the form of his work that comes down to us does not perfectly match cuneiform records where such records exist. For example, the Greek summarizers make Berossus state that Belus (Marduk) “cut off his own head, upon which the other gods mixed the blood, as it gushed out, with the earth; and from thence men were formed.” However, the Babylonian creation epic, the Enuma Elish, differs on this detail in the cuneiform text. In tablet six, Marduk decrees that the god Kingu must be beheaded and his blood used by Ea to create man. Now, had the work of Berossus—a priest of Marduk—come down to us perfectly, it is very unlikely we should see such a profound mischaracterization of a sacred act of the god himself. As a result of such mistakes, we simply cannot be certain that the Oannes passage is uncorrupted.
Nevertheless, reading the passage on Berossus as it currently stands gives us no confidence that it describes an extraterrestrial. In fact, it says nothing about outer space at all:
At Babylon there was (in these times) a great resort of people of various nations, who inhabited ChaldŠa, and lived in a lawless manner like the beasts of the field. In the first year there appeared, from that part of the ErythrŠan sea which borders upon Babylonia, an animal destitute of reason, by name Oannes, whose whole body (according to the account of Apollodorus) was that of a fish; that under the fish's head he had another head, with feet also below, similar to those of a man, subjoined to the fish's tail. His voice too, and language, was articulate and human; and a representation of him is preserved even to this day.
This Being was accustomed to pass the day among men; but took no food at that season; and he gave them an insight into letters and sciences, and arts of every kind. He taught them to construct cities, to found temples, to compile laws, and explained to them the principles of geometrical knowledge. He made them distinguish the seeds of the earth, and shewed them how to collect the fruits; in short, he instructed them in every thing which could tend to soften manners and humanize their lives. From that time, nothing material has been added by way of improvement to his instructions. And when the sun had set, this Being Oannes, retired again into the sea, and passed the night in the deep; for he was amphibious. After this there appeared other animals like Oannes, of which Berossus proposes to give an account when he comes to the history of the kings.
Such legends prompted Carl Sagan to write in the 1960s that “stories like the Oannes legend, and representations especially of the earliest civilizations on Earth, deserve much more critical studies than have been performed heretofore, with the possibility of direct contact with an extraterrestrial civilization as one of many possible alternative explanations.” Sagan later discounted this when he learned more about myths and legends and why they are unreliable.
Note that contra Coppens, Berossus clearly states that this event happened at Babylon (not Sumer), which was only founded in 1894 BCE, many centuries after the arts and sciences the creature claimed to bring with him were already in use at Sumer, Eridu, and Ur. (You can claim Berossus is wrong here, but if so, why trust anything else?) Note, too, that Oannes is described as a fish-man (and depicted in “literal” ancient art as a man in a giant fish suit) who lives in and returns to the sea. This is not outer space, and the only reason anyone ever thought it had anything to do with space is because at one particular moment in history—the 1960s and ’70s, when Sagan and Temple wrote—spacecraft routinely “splashed down” in the ocean, thus leading to an erroneous—and artificial—assumption of a connection between space and water."