How do we know that humans are actually based off of Gallifreyans?
An audio drama called Zagreus. Since it’s 4 hours long and immediately follows a 2.5 hour audio called Neverland, and since those two make up the finale to a 11 audio arc, I’ll give you the short version.
Rassilon founded the Time Lords*. Since he’s actually a massive dick with an inferiority complex, he engineered the Time Lords in a manner best suited to him.
As the audio then puts it: “Rassilon believes humanian life, the basic physical form exemplified by Gallifreyans, to be the only essential form of life. [unnecessary plot things] All life not conforming to Gallifreyan standard should be destroyed. To this end, Cardinal Rassilon researched life on two hundred and seventy-eight thousand planets. Of those, sixty-nine thousand contained sentient life. To those, Cardinal Rassilon sent, via manipulation of the space-time vortex, a self-eradicating set of biogenic molecules with enough power to rearrange the cellular structure of each planet’s dominant life form, so it would take on Gallifreyan physiology and would continue to evolve along those lines. All other life forms will become extinct within six thousand spans.”
So we’re based off Gallifreyans. (The explanation for non-humanoid sentient aliens is that Rassilon missed a few planets.)
*essentially. It’s a bit more complicated than that, but this is the short version.