Or you don't have to accept either - authors don't need to canonize a theory of biogenesis within the world they build to tell compelling story. "How can Vulca

Or you don't have to accept either - authors don't need to canonize a theory of biogenesis within the world they build to tell compelling story. "How can Vulcans mate with Humans" doesn't need to be explained if the compelling part of the story is "how does the child of a human and Vulcan navigate both worlds and what does that tell us about ourselves." Of course, you could write a story where a shared or divergent biological/geological past was a part of the metaphor/allegory - but Star Trek doesn't need that. And if Star Wars replaced the "galaxy" part of that sentence with "kingdom" and all the space ships with wooden ones no one would even need to ask the question, because Star Wars is as much fantasy as science fiction. There are certainly some creatives who go that far, but I personally take issue with the necessity of detail and depth in scifi worldbuilding. It's really a recent phenomenon that audiences expect so much out of creators and I think it hurts the stories we tell.