The First Civilization is likely one of the most mysterious civilizations to modern date. So far, it is known that they lasted around 250 Million BCE to 1 Million BCE, before being killed off by unknown forces. It is still not known how they fell to this day.
Little is known of the First Civilization as they have become to be called. No one even knows what they look like, or even their nation's true name. All anyone knows about them is the few remaining installations left behind on a few worlds. The Confederacy has released many bits of information, which suggest that the First Civilization originated from the vicinity of the Carina Nebula. This is evidenced by the more concentrated amount of ruins discovered, as well as evidence of a few transplanted species from their planets. A notable example of a species-transplanted worlf is Eeyttir, a world full of the sapient descendants of prehistoric dinosaurs.
It is assumed they came from a low gravity world, as their installations were seemingly built for beings of taller stature than terrestrial humans. There is knowledge that they had four large fingers and were much thinner than Humans'.
Judging by carbon dating of the First Civilization installation at Eta Carina one of their galactic civilizations existed during the Triassic Era on Earth, making that civilization over 200 million years old. However, with the discovery of the much older Anera civilization plus the discovery of an ancient warship that didn't match the few remaining Anera warships, it was discovered that they had risen to power at around the same time as the Anera, over 1.69 billion years ago. It appeared as if they shared the galaxy with the Anera for the same amount of time before the two went to war, nearly annihilating each other in the process.
Their second civilization, if that is the case, lasted for 150 million years but surprisingly their only FTL was similar to Warp Drive, only much faster, as there have been a handful of artifacts found on the edge of Andromeda. They never learned how to generate artificial wormholes.
It is believed that a galaxy-wide plague killed nearly every one of their kind off at the end of the Second Era, but it isn't known if they were made extinct because of this. There are a few worlds with beings that could be descendants of the First Civilization but all those species are native to their worlds and their computer databanks have degraded too far to be recovered, plus there are no traces of First Civilization DNA.
They are one of the many mysteries of the Confederacy. It is unknown why another civilization couldn't take its place before humans came along.
However, in around 40000 CE, Gateway Transmissions were believed to come from the First Civilization. These transmissions were mysterious in origin and nature, and can be found here.
Evidence of the First Civilization was discovered during the Eta Carina Mission of 2674 when a small contingent of scientists discovered a Dyson Swarm around the massive star. The swarm of artifacts seemed to orient themselves to the Carina Nebula.
However, the significance of the discovery was lost on humanity for centuries due to the many wars and rebellions erupting in the URSS. However, that didn't stop a few daring explorers from exploring and discovering a couple other relics of this ancient civilization.
In 3008 when the first mission to the Carina Nebula arrived, they explored and discovered more numerous older relics within the nebula itself, confirming the theory that they originated within the nebula itself. When Neo-Norse people began exploring deeper into the nebula they discovered more relics on a few of their colonized worlds.
As humanity spread across the Milky Way, they discovered more recent but still ancient relics, including remains of a few Dyson Spheres and one intact one as well as dozens of ringworlds.
When evidence of the Anera was discovered, the moniker First Civilization was nearly changed but with the discovery of a non-Anera warship whose interior seemed to match the known dimensions of the First Civilization, it was discovered that they were, in fact, much older than previously thought. While it is unclear which civilization rose first, the name used by the Confederacy stuck.
After the Wormhole Drive was invented the first missions to Andromeda discovered a few ancient outposts on the edge of the galaxy as well as more evidence of the First Era version of the civilization having colonized many worlds of the nearby dwarf satellite galaxies, but nothing as they traveled inside the nearby spiral galaxy.