When the Greek gods were adopted by the people of Rome, they sometimes underwent some changes. Mars was gentler than Ares, Latona was more prominent than Leto, and Vesta had more major temples than Hestia.
Sometimes, though, the mythology and personality of a given god remained the same and all that changed was their name.
One seemingly minor detail remained, however, that showed how different Neptune and Poseidon’s origins were. While Poseidon was a god of the sea, Neptune ruled over both the Mediterannean and fresh water.
This singular difference, easily ignored in the retelling of ancient myths, hints at the pre-Roman origins of their sea god and how he had a very different beginning than the Greek Poseidon.
Roman mythology is known for the obvious ways in which it adopted the legends and pantheon of the Greeks.
The people of Rome considered themselves to be descendants of Greece, both literally and culturally. While their language was different, they adopted the religion of Greece as their own from a relatively early time.
The Roman gods, however, were never exact duplicates of their Greek predecessors. While the majority of Roman religion was based in that of Greece, there was still influence from earlier traditions of Italy.
While the Romans themselves claimed to be descendants of Greek gods and heroes, their origins were in the Latin tribe of Italy. Through much of its early history, the land of the Latins had been controlled by their northern neighbors, the Etruscans.
When later Romans began to incorporate the mythology of Greece into their own, they did not take the names of most Greek deities. Instead, they found counterparts within the earlier beliefs of the Latins and Etruscans.
Over time the Romans adopted Greek religion almost entirely, but certain details remained uniquely Italian. One of these was that most of the gods had names taken from Etruscan, rather than Greek, tradition.
The Greek god Poseidon was given a name that likely originated from an Italian language. He came to be called Neptune by the people of Rome.
It is unclear exactly where the name Neptune came from. While modern linguists point to a possible Indo-European root, the Romans themselves claimed the name came from the word nuptus, or “covering.”
Neptune was one of the earlier Greek gods to be assimilated into the Roman pantheon. In legend, the first gods to sit together and dine in Rome were Neptune, Mercury (Hermes), Hercules (Heracles), Diana (Artemis), Latona (Leto), and Apollo.
Their mythologies were almost identical between the two cultures. Like Poseidon, Neptune was the brother of Jove (Zeus) and Dis pater or Pluto (Hades).
Both were sea gods who wielded their tridents, caused earthquakes, and were associated with horses. But there was one key difference between the two.
The Greek Poseidon was almost exclusively associated with the sea. Neptune, however, was a god of fresh water as well.
The association with inland freshwater is one of the few indicators in Roman mythology that Neptune was not taken exclusively from Greek mythology.
Both the Greeks and the Etruscans were descended from Indo-European peoples. While the Greeks came into contact with Semitic cultures and borrowed some aspects of their mythology from them, much of their tradition was rooted in prehistoric Indo-European ancestry.
The early Proto-Indo-European religion is believed by some to have developed inland. As early Europeans spread out and encountered coastal regions, their religions developed gods to fit this new landscape.
The sea gods of Indo-European cultures developed from either the gods of inland water, such as lakes or springs, or from chthonic Underworld types. For this reason, sea gods in Indo-European cultures tend to have more variation than other, more ancient, types.
While pre-Greek people moved along the eastern Mediterannean, the pre-Italic people originated in central Europe. Far from the sea, these people had no god to correspond to this unknown natural feature.
While the people who would become the Greeks developed Poseidon relatively early because of their increased exposure to the sea, the ancestors of the Etruscans did know of the sea for much longer. Their god was one of the water they knew, which was fresh.
In fact, some etymologists believe that the closest linguistic relative of Neptune is a god who was known far from the Mediterannean and never developed an association with saltwater. The Irish Nechtan was the source of all the world’s rivers but had no connection to the sea.
By the time the Greek religion reached the boot of Italy, of course, the residents there lived along its coasts. But their sea god had not developed in the same was as Poseidon.
The Etruscan water god was associated with fresh water as much as the sea because of the priorities and experiences of the people who believed in him. When then Romans adopted Greek mythology, they retained the land-locked aspect of their previous water god.
Neptune, therefore, continued to be closely associated with lakes, streams, and springs as well as the open sea. While the rest of his mythology and characterization was Greek, this detail hints at his northern predecessor.
In early Roman writing, it was Portunus, the god of ports, rather than Neptune who was associated with the navy. Even after the Romans, who were never as strong of a seafaring power as the Greeks, adopted Neptune as a sea god, it took time for the former Etruscan river god to assume all the responsibilities of Poseidon.
Neptune was the Roman name for Poseidon, the god of the sea. When the people of Rome adopted Greek traditions, the characterization and mythology of the sea god remained virtually unchanged.
The only notable difference between the two was that Neptune, unlike Poseidon, was associated with fresh water as well as the sea.
This is a minor detail in most stories, but it shows the different origins of the Greek Poseidon and the early Italian Neptune.
Proto-Indo-European religion had no archetype for a god of the sea, so cultures developed their own deities based on the importance of the sea in their own experiences.
The coastal Greeks developed Poseidon as an exclusively sea-based god, while the more inland Etruscans and their ancestors did not place as much emphasis on the sea as they did fresh water.