I think this has purposefully hidden from us. Too many answers, I think, if a dunyain such as Kellhus looked into the faces/souls of Nonmen and told us what he say. The idea of the Darkness that Comes Before affecting all things is very central, and the nonman preceded the men of Eanna more completely than anything else.
The Nonmen preceding the Eannean Men does not necessarily mean that they themselves would not be just as susceptible to the Principles of Causality and the vagaries of history and evolution, though {even though they claim they can decide to leave at any time}. Anyone as adept as Kellhus at peering into the Darkness would likely be able to peer into the Darkness that precedes the Nonmen, it would just be a slightly more strenuous effort perhaps {lacking a Nonman equivalent to the Unmasking Room in the Ishual and other things}. But this doesn't mean such Sight would be impossible.
The "What Came Before" segments in the books always mention Nonmen having reached a pinnacle of civilization while men roamed the wilds wielding stone tools and dressing in furs. But any cultural zenith implies a gradual / historical development from preceding eras. The institutions / relationships of Kings and Ishroi and Quya mages
must have their roots somewhere before the apogee of Nonman Civilization, just as much as the caste-system and jnan must have roots before the height of Three Seas Civilization...
Even though the Nonmen could have witnessed the evolution of Men from Beasts throughout the ages,
this doesn't mean that they are any less subject to the Principle of Before and After... i.e., the "modern Nonmen" of the Cuno-Inchoroi Wars
must have been built upon preceding generations of Nonmen back into the depths of history...
It does not make sense for Nonmen to simply incarnate, or appear, at the summit of their civilizational development.
Unless they did not, in fact, ever evolve and are artificial in some sense.In short: Nonmen and Nonmen Society must be derived from something that Came Before, due to the Causality Principle. If not, this has wide reaching implications for the Causality Principle and Earwa as a whole. Although it seems that the Nonmen are as oblivious to their ultimate origin as humans are to theirs.
Not sure I understand where these questions are coming from. How would you answer these?
I'd say it wouldn't be too terribly unlikely that two unrelated species evolved similarly. Fish and dolphins, for example. Personally, I hope there is some kind of not too distant link between them (go back far enough and everything has a common ancestor. I'd prefer a somewhat more recent ancestor )
The questions, they come from the darkness...
But yeah, there may be indications in the text that we've overlooked that Nonmen could be evolved from a more bestial ancestor... This ancestor could be a close relative to the ancestor of Men or something else entirely {in which case their superficial similarities to Men would be incidental, as opposed to ancestral}.
Think of it this way... could be a species of proto-ape from which Nonmen and Men both branched off and evolved from. Or Men could be derived from an ape-ancestral with Nonmen somehow being evolved from a hairless, subterranean creature. I know that's probably a stretch but it does throw the difference between the species in a new light.
But if neither is the case, then it would seem to indicate that the Nonmen had somehow been engineered or designed instead of evolved...
Which would, of course, really change the scope of the setting and the relationships between the races / gods.
I prefer my aliens to be alien. I'd be extremely surprised if the humaniod form was dominant on other planets, in the Earwaverse or IRL.
The Humanoid form appears to have cropped up at least twice in two separate examples on Earwa, Men and Nonmen...
"Seem to". I think this notion comes from the fact that our view of Nonmen comes from the men who came in after they where already defeated. I think the young nonmen of eons past worshiped Gods i a manner similar to men.
Good point.
Its mentioned in the books that Nonmen cannot see paintings, which is why they only do carvings. Take from that what you will.
I feel like their colors are largely muted or near-grayscale, with perhaps some exceptions with bronze or copper {the Copper Tree of Siol, etc.}. Color has significant implications for language and communication though, at least for Men. So for Nonmen their muted world's characteristics would have to be read in entirely alien ways.... i.e.. a mineral would not reveal itself to be copper or bronze just by noting its color, they would have to deduce its mineral properties in some other fashion.
I'd have to look up this part and read it again to answer this. Or you could provide a quote and/or reference so I can be as lazy as possible 
Upraised palms braced his every step. Blank eyes studied his every angle. The Nonmen who had authored this place possessed more than a fascination with the living form; it had been their obsession. Everywhere, they had cut their image into the dead stone about them, transforming the suffocating weights that hemmed them in into extensions of themselves. And Kellhus realized: the mansion itself had been their devotional work--their Temple. Unlike Men, these Nonmen had not rationed their worship. They did not distinguish between prayer and speech, idol and statue ...
Which spoke to their terror.
{TTT, p. 316-317, RSB}
Bakker's emphasis in italics, mine in bold.The Nonmen do not sequester religion / worship into a certain societal niche, all of their activities are religious / worshipful in nature. They are obsessed with the living form, beyond mere fascination, and this indicates their terror.... But terror of what? Terror of being flesh and blood? Or terror of the living form's connection with the Outside, and the potential of Damnation?
