The Forerunner Era was the name given by historians of the 23rd century to the period of roughly 500,000 to 100,000 years before their era.
There was incredibly little knowledge available on the civilizations of this period, with only vague and uncertain references in ancient Vulcan historical records referring to long-lost, destroyed, or looted sites and structures. Research was practically impossible, and archaeology crossed over into palaeontology and geology. There was almost complete ignorance on these races, their deeds, and even what they looked like.
There was however reliable evidence for interstellar travel dating back over 500,000 years, in the form of identical artifacts found on different planets, suggesting it was already old and commonplace. The largest source for this evidence was in the Rigel system, containing fragments of most of the ancient cultures of the time, who all traded through the worlds of Rigel, particularly at Rigel IV.
The earliest known civilization for which knowledge was extant was that of Arret, at its zenith, and destroyed, around 498,000 BCE (reference stardate −5,000/00). Two of the most common, however, were the Fett, or Forerunner A, (around 398,000 BCE, stardate −4,000/00), and the Bursen, or Forerunner B, (around 198,000 BCE, stardate −2,000/00), both named after their discoverers and known only from the fossil record.