Well, how do you explain reincarnation without something like a soul, huh???
No, but seriously, soul itself comes from matter/physical universe as far as I can tell.
To me the soul is the summation of yourself, starting with the physical all the way to the spiritual. I got a theory of sorts about this I actually took up to writing about a bit ago:
Hastily made intro to my “Theory of Aspects”
Not a perfect explanation, but maybe explains a little bit where I'm coming from.
TL;DR from matter, instincts grow for life, then it creates more layers of complexity, in humans the next one is base emotions, then our thinking minds then our spiritual side. "Soul" in this system is the total sum of the distinct entity. (I plan to explain in a future, fuller article, that explains what parts survive and how and how that works into my beliefs about reincarnation).
Sorry to mostly drop a link like that. No point in me putting in huge effort to basically rewrite the darn thing for just one post.
Essentially my argument is that a soul is inevitable as a series of rising complexity to a point where streams of consciousness can find ways to survive death and possibly reincarnate. I think this is an eventuality and inevitable so long as complex life is around.