robardin

Actually, this detail has crossed my mind before.
It's stated or implied a few times in Mistborn that Preservation and Ruin had to work together to create Scadrial and all the life on it ex nihilo, from nothing, because separately, their Intents would not allow for it. OK that makes sense - you can't Ruin what's not already there, nor Preserve it.
And the reason given for Preservation's "weakening" versus Ruin was not because of what he did to imprison Ruin, but because Preservation "gave of himself" just a bit more power into creating humanity on Scadrial - patterned after "that which we'd seen before", Yolen-based humans, which both Leras and Ati originally were - so that they'd exist not just physically, but spiritually, as given by the epigraph to Ch. 54 of The Hero of Ages (in the voice of Harmony, who ought to know):
That certainly implies to me that had Preservation NOT given "a tiny bit" of himself more to Scadrian humans, that they would NOT have had "awareness and independent thought" - that is to say, had "souls". Or, read another way (perhaps more likely), that humans would not have been created on Scadrial at all!
For had Preservation not done so, Scadrial would not even have been at risk, as he would have been able to "cancel out" Ruin's attempts at destroying it. As Ruin also monologues to Vin in Ch. 76 of HoA,
Ergo, the "extra bit of Preservation" either (a) enabled the humans created on Scadrial to be more than soulless and mindless simulacra of Yolen-formed people, or (b) having independent thought/will is part and parcel of creating Yolenish humans at all, and also required that "extra bit".
Now, Scadrial is the only world we've seen (so far) that has actually been created, and peopled, by Shards of Adonalsium, rather than dating to before the Shattering. That would further imply that all Yolen humans were either created by Adonalsium - the way the singers on Roshar were, per linked WoBs stating that Roshar was specifically crafted by Adonalsium and that the singers were an integral part of that designed ecosystem - or, in fact, pre-dated Adonalsium!
If creating humans from nothing required "a bit more Preservation than Ruin", which represented two of sixteen Shards of Adonalsium, what does that imply about all other humans? Do they all have "a bit extra" of one or more of the Shardic essences to throw them out of balance?
Perhaps it's moot because the Shattering happened after the creation of humans - in fact, was largely perpetrated by Yolenish humans, on Yolen. So each Shard is 1/16 of whatever power Adonalsium retained AFTER imparting consciousness to mankind and singers and other sentient species/races in the Cosmere.
But would it matter for Scadrians to have an admixture of just Preservation and Ruin, slightly tilted to Preservation, while all other humans have some "pre-Shattering blend of Spiritual Investiture"?
I mean yes, we know this is the reason that Preservation/Ruin/Harmony can hear thoughts/project thoughts into Scadrians, and that it somehow forms the Spirtual Connection basis for enabling Allomancy and Feruchemy (Hemalurgy is actually usable anywhere and by anyone in the Cosmere who knows the right metals and binding points). But in terms of their "essential nature"?

Also of interest to the how "light eyes" is part of the Rosharan sDNA, possibly inherited from the original Radiants, is that we've seen several instances of a lighteyes/darkeyes union resulting in a "one-eye", i.e., someone with one dark and one light eye. Moash mentions that his friend Graves (who he evidently valued over Kaladin... Grrr...) is a lighteyes "who doesn't care about eye color", marrying a darkeyes and who has a son who's a one-eye, and we also saw a Highprince with a "one-eyed" bastard son (presumably gotten on a darkeyed woman) who was sent to the Davar household to investigate the charge of murder.
Just so you know, that's not something that happens in the real world, or any other Cosmere world we've seen. There's something about eye color on Roshar, indeed.