I would like to put forth the theory that the humans came from Nalthis. The only real reasons I have to support this is that A) we know that Nalthis, and Warbreaker, happened long before the events on Roshar. B ) When Siri was teaching Susebron to read his book, the first thing she pointed out was the word Shash, which appears to mean the same thing to everyone on Nalthis and Roshar. Brandon has said that the languages on each planet were different and only understood by Worldhoppers by means of Connection. C) Everything from Nalthis seems to end up on or near Roshar (Nightblood, Washer, Vivenna, the painting from Lightsong's court). Also would maybe explain why Austere's monks and the Ardents both shave their heads.