The Precursors are, from all available evidence, the first species to evolve sentience and explore the galaxy. They moved quickly to become masters of the galaxy, leaving evidence of their presence virtually everywhere. However, it appears that the Precursors vanished from the galactic scene quite suddenly. There has been no evidence discovered that explains the disappearance of the Precursors. The height of Precursor culture seems to have been approximately 200,000-250,000 Earth years ago. The Precursors reigned for only a few thousand years before they vanished.
While the species itself appears to be long gone, evidence of their existence remains. Various artifacts of Precursor origin have been discovered on any number of worlds. The largest artifacts that have been discovered are the Sa-Matra battle platform, discovered by the Ur-Quan, the Precursor factory discovered underneath the surface of Vela 1 (Unzervalt), and the 10 Rainbow worlds found throughout this region of space, which seem to have been used as garbage dumps for their civilization.
From the Precursor artifacts, it has been determined that the Precursors, as a species was a race of "shaggy giants", perhaps 5-8 meters tall and twice as wide.
The only species that we know of that was a contemporary of the Precursors is the Slylandro. Due to their lack of writing implements, much of their knowledge has been lost. However, they mention that, prior to the Precursor's disappearance, that they were frantically searching for something, and that they could be described as worried. Their last transmission seemed to indicate that the Precursors had found the answer they were searching for, and that the arrangement of Rainbow worlds would give a hint as to where they went.