His actual origin isnt ever declared, but it points to Turkey.
I like the idea that he's a reincarnation of ancients, defined as shamans/sorcerers etc... but being the most powerful psyker known in recorded history, not to mention immortal, or a perpetual, I like to think he has remnants of the old ones in him
I read the master of mankind recently, and he does talk about his time as a boy and leaving for one of the major first human city states, must be Summeria?
I love the lore so much! I'm just onto tje Siege of Terra books now. No spoilers if you know lol