From all available evidence, the Precursors are the first species to evolve sentience and explore the known universe. 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, but there is no indication as to why.
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 still remains. Various artifacts of Precursor origin have been discovered on any number of worlds. The largest artifacts known to date 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 dumping grounds for their civilization.
From the recovered Precursor artifacts, it has been determined that as a species they were a race of "shaggy giants", perhaps 5-8 meters tall and twice as wide.
The only species that has been confirmed to have first-hand contact is the Slylandro. However, due to lack of writing implements, much of their knowledge of the Precursurs has been lost over the passing time. The Slylandro do mention that prior to the mysterious exodus, the Precursors were frantically searching for something and were obviously "worried" over the situation. 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.