It doesn't involve common origins or ancient astronauts, but there was originally an explanation for why the people of Krypton seemed to be biologically almost identical to Earthly humans. In Krypton's earliest appearances in The Adventures of Superman radio series (which was extremely important to the development of Superman's mass appeal in the 1940s), the planet was supposed to be a Counter-Earth, a planet identical to the Earth, except orbiting on the exact opposite side of the sun.
Counter-Earths were a common mechanism used in science fiction stories to explain how an alien planet could be almost exactly like Earth. It is obvious from simply symmetry arguments that there is a solutions of the gravitational equations of motion that has two planets of equal mass orbiting half an orbit apart around a central star. The fact that the planets are equal in size and equally far from the sun could mean that their environments (and the life they develop) would be extremely similar. Finally, by having Counter-Earth positioned behind the sun, there is a natural explanation for why it would not have been detected.
(There are problems with these ideas, naturally. The Earth-Sun-Counter-Earth system is unstable; perturbations due to the presence of other planets would break up the system, sending one planet in closer to the sun and pulling the other farther out. Moreover, we understand now that the role of randomness in evolution is extremely pronounced, and you would never expect to have the Counter-Earth populated by beings almost identical to those on Earth.)
However, if you are unaware of those problems, or willing to gloss over them, the Counter-Earth hypothesis could seem useful for world-building purposes. In the 1940s Superman cosmology, Krypton the Counter-Earth was peopled by a race of Kryptonains who were supposed to be just a bit more evolved than the humans on Earth. Other than a bit of luck, which had enabled them to advance further, Superman's race was essentially just like humans. This is the origin of Superman's relatively little-remembered nickname the "Man of Tomorrow"; he was supposed to be an example of humanity would evolve into in the future. This is also behind the notion that Superman's powers include essentially anything a human can do, only at a super- level.