So humans on Scadrial were created by Ruin + Preservation, and Preservation got to put a little extra in them in accordance with "the deal".
(At least, according to epigraphs in Hero of Ages.)
But we know that there were humans around pre-shattering, as Hoid used to be human and Hoid was around when Adonalsium was shattered.
So does this mean Ruin & Preservation just happened to create humans that are pretty much identical to the existing humans in the cosmere?
Or are all humans somehow created by them across all the worlds? (That doesn't make sense to me, as it implies these shards were distinct entities pre-shattering.)
Are there any good theories or write-ups about this?
(BTW I haven't read Self yet, doing a reread and am 2/3rds of the way through HoA, got to thinking about this.)
EDIT: http://www.17thshard.com/forum/topic/13733-about-humans-on-scadrial/
Sounds like the theory (or fact, I didn't research heavily) is that pre-shattering humans were on Adonalsium's world, and post-shattering the shards or whatnot all ended up creating life on their own shardworlds, and used the life from Yolen as a template (thus the similarities).
Though in that case, what about Nalthis... didn't Endowment only arrive on Nalthis recently? How did humans get there?
EDIT 2: After more digging (#19 here: http://www.theoryland.com/intvmain.php?i=977),it appears that humanity had some way to travel to other worlds. (Yolen was the original world that humanity sprang from, but also humanity existed on some other worlds pre-shattering.)