This is a rough draft, but I want to get it down while it’s on my mind.
It was all settled long before humans hit the galactic scene. Millenia ago, the wars were fought and the implications of the truth ripped cultures apart. Most of the galaxy is at peace with the findings from the Temporal Core.
The Temporal Core is an anomaly that serves as an “airlock” between linear and non-linear time. It is the heart of the Tourist incursion into linear time and space. When the first sentient species studied the Core and made contact with the echoes of the Tourist presence, they learned why there are so many similar humanoid races throughout the galaxy.
Once upon a time, there was a powerful interstellar race that existed transdimensionally, possibly outside linear time. Since they have no use for names, they are generally identified as “Tourists.” They played with matter the way humanity plays with computer programs. When they wanted to experience the lock-step of linear time, they required vessels to carry their consciousness. For them, it was like scuba diving, moving into an unfamiliar habitat that was inimical to their survival but surviving with a technological apparatus.
The Echo Sage is an interface the Tourists left behind at the Temporal Core to chat with anyone who had questions, and was capable of navigating the tricky defenses to get close enough to engage the interstitial being. According to the Echo Sage, the Tourists had internal regulations that ended up requiring them to make vehicles that were mostly four limbed, with a head and torso, and as compatible as possible for their biological needs.
The Tourists would create these biologically compatible vehicles using evolutionary processes, flitting in and out of “kitchens” on various worlds and adjusting their vehicles to taste. Some breathed different atmosphere, or refined the circulatory system in different ways, and all had various elaborations on their physical appearance. Still, the basic rules were the same. The access protocols for the dimension had something like stock car specs, and various Tourists could make minor adjustments but the chassis was to remain roughly the same.
The Tourists dipped in and out of these vehicles for some time uncounted, but then lost interest. Perhaps the dimensions fell out of alignment, or linear temporality was a fad that fell out of favor, or maybe they all died; no one knows. If the Echo Sage knows, it doesn’t tell. Maybe linear temporal beings have no way to understand the real answer.
Among these vehicles left behind, a residue of the genius of the creator remained. Their auto-piloting became self-determination. They discovered if they went deep inside, they felt a connection to all things, and when they reached out to the other they found a spark of themselves within.
Some of the cast-offs have porous skin, or scales, or feathers; some bleed red, or green, or black. Some of the more creative versions bleed light, or sound, and are organized around a circulatory system of resonance. Some of the more sketchy vehicles (probably either early or late in the development) have many more or fewer limbs. There is tremendous variety in what the Tourists left behind when they got back on the cruise ship.
Will they come back? Did they come back? Did they ever leave? Are they directing events? The answers are out of reach, and digging at the problem tends to trigger a spiral down into paranoia and nihilism (or religious extremism.) At best, it seems if there is a cosmic force looking after the fate of the universe, it has all the bemused affection and neglect of an uncle who only visits once every few years and never answers letters.