How did por and para come to mean so differently in Spanish while they both come from the same source, i.e., Latin?
I have been researching a bit because I found the question really interesting:
The preposition por comes from the latin pro (meaning forward, move forward; like probabilidad, progenitor or progreso) with the influence of per- (meaning through, complete; like perplejo). Both come from the same indoeuropean root per from where a lot of greek prefixes come from (para-, peri-, pro-, proto-).
Some say that the preposition para, previously used as pora, comes from latin pro ad (forward to), but others say that it comes from per ad (for).
I see that the sources are not exactly the same. We can say that one is related to the other, but it is not the same word. This difference between por and para happens also in Portuguese (they use the same words por and para), I'm not sure but also in French I think (par, and pour [some French-speaker check this please]), but it's not like that in Italian (per). I guess they lost that distinction somewhere, however other languages did not.