The archives at Memory Alpha record the names of 2,558 spacefaring races known throughout history. Solid evidence for interstellar travel-identical artifacts on different planets-goes back as far as 500,000 years. Traces of habitations on worlds incapable of supporting carbon-based life suggests that races with stardrives existed over one-million years ago. The largest single source for information on these lost races is Rigel, where tiny fragments of nearly all these lost cultures have been found. Starfaring races with advanced civilizations now less than atoms once traded inside the Rigel system.
The earliest known interstellar civilization for which coherent records exist is that of the world of Arret. Although civil war destroyed their world half a million standard years ago, Federation scientists found and interviewed three survivors shortly before their deaths. They spoke broadly of far-flung settlements and their original humanoid natures, and suggested that the Federation's races may be descendants of theirs (unlikely, based upon the archaeological record).
From 500,000 to 100,000 years ago, the historical record contains only ambiguous references to starfaring civilizations. Ancient Vulcan research on these lost civilizations is impossible now, for their records refer to sites and structures that have long been plundered, lost, or simply destroyed. This is the so-called Forerunner Era, characterized chiefly by nearly complete ignorance of who these races were, what they did, or even what they looked like. In this era, archaeology slides into paleoanthropology and geology. Remains are most likely to be found on dead planets, and lifeforms are likely to be only fossils.
Of all the species known to have existed during this time, the two most common are the Fett, or Forerunner A, and the Bursen, Forerunner B (both named after the scientists who established them as spacefaring peoples). A chitinous race, perhaps like lobsters, the Fett lived perhaps 200,000 years before the Bursen, whose skin imprints suggest a reptillian origin. The last Bursen may have died only 150,000 years ago, and their decline-like that of their origin and their homeworld-remain a mystery.
From 100,000 to 40,000 years ago was the more complete Emergent Era. Romantic novelists to the contrary, these ancient cultures contributed nothing to modern Federation civilization until long after they were dead. Though there are many gaps in the record, the Emergent Era seems to have consisted of three periods of large expansionistic reaces separated by three intervals of retrogression of primitivism when major races or at least starflight were not present at all.
The Ena lived between 100,000 and 90,000 years ago. Although they left no physical remains, a scattering of images and the dimensions of the ruins suggest a blocky, even clumsy kind of body structure. The heart of their space nation lay on the rimward side of the Orion Arm, in what is now Klingon space, but there are reports of at least two minor sites in the Triangle area, and possibly two other minor sites somewhere in Orion space. After the 15,000 year First Interregnum, they were succeeded by the djKen, a race of slender near-humanoids who left numerous works of art in the tumbled ruins of their spired cities. Most djKen site lie in Gorn space near the Romulan Neutral Zone, with a few minor sites on either side of the Neutral Zone in Romulan and Federation space respectively. Because of their fragility, few djKen artifacts have survived intact, though the race itself seemed to have a relatively long run on the galactic stage-fully 20,000 standard years. The Second Interregnum that followed lasted less than 10,000 years when the Querat, already a multi-world society far to coreward, began expanding. Eventually, they would come very near to the Federation hearthworlds, but they left few artifacts, and never visited Rigel themselves, and exist in ancient Rigellian records entirely from secondhand reports from ancient merchants. Most archaeological evidence for the Querat also tends to come second hand, and to date only 2 ruin sites are known, both near the center of Federation space. Save for the Preservers, the Querat remain the least know ancient interstellar race.
With the decline of the Querat, the Third Interregnum began, but precisely when it began or ended is unknown. Sometime between 40,000 and 20,000 years ago, the shadowy Preservers lived. The near-complete absence of reliable data on the existence of the Preservers infers that the Preservers deliberately obliterated all traces of themselves in galactic history. This puppet-master race, or group of races, are alleged to be responsible for the preponderance of humanoid races on both sides of the Orion Arm. The historical record clearly shows that at no prior time were humanoids of any species so numerous; perhaps the Preservers were paving the way for humanoid mastery of the Orion Arm. As no records exist, whether the Preservers themselves were humanoid or chose the form for other reasons cannot be determined.
During the Preserver Era, the galaxy was in a quiet period with only a handful of races in space, many of them planet-bound even though technologically capable of space travel. If the Preservers had a sizable empire then, it met no oppositioin coreward or rimward of the Arm. Their influence seems to have been most pronounced around the Arm itself; they surely must have been active near Rigel, though they left no trace. What precious few of their works that have survived are still functional-and there is every indication that the Preservers intended things to be that way.
It is an interesting coincidence that, just as the Preservers vanished, the Orions first appeared.