The history of orcs goes way back beyond any written record. As writing was invented by other races, this makes perfect sense. Orcs were amongst the first-born races, primeval and unspoilt. They were fertile and spread across all known lands (and beyond, I assume). Therefore orcs cannot be limited to a specific area or region. The sheer thought is simply ridiculous. They exist in deep forests, endless steppes and on the wild seas as naturally as in the mountains and deserts. But where they first appeared only the gods know. There are quite a few areas that claim this privilege for themselves but none of them is any more likely than the next.
Which brings us straight to the question of the origin of the species:
Several theories or doctrines are considered legitimate (as long as you hold the bigger axe in your hands).
The predominant theory states that all orcs go back to one ancestor, the so-called First Orc of Divine Blood. He got himself a female of a lesser species (ape, blob, whatever, opinions vary on this matter) and off they went, spreading the seed. This First Orc is considered as the pride of creation, naturally.
Others believe that in every possible world orcs have to exist as a consequence of divine harmony. Meaning, that wherever one of the primary people exists (primary being defined as: races that use artificial means to battle, i.e. weapons, strategy, unnecessary violence), all primary races must exist due to biological preconditions. In other words: “From every primeval sludge it’s always the same amino acid that pops up” (Original quote from a she-human of the author’s acquaintance). This phenomenon is usually called Divine Balance and proves that even the evolution has a tendency towards political correctness.
A third doctrine states that other primary races can only come into existence as long as orcs are not around. The moment a world appears into which orcs could expand, they will (the so-called orc-vacuum), so the Divine Balance will be established. Whatever theory is preferable, in the end it always comes down to one consequence: orcs are the superior race.
Even in this world, where the author dwells as he writes, the origin of orcs can be traced back to a god called Orcus (which immediately answers the question whether orcs are an original race in this world), a god of the underworld, naturally, because that’s where the fun is. A self-fulfilling prophecy, if ever there was one.
Wherever orcs meet other races, they mix up (after battling passionately). It is important to recognize that orc genes always have a dominant heritability. If someone had orcish ancestors somewhere in the long forgotten past, you can tell.
Now take a look into the mirror.